xref: /frameworks/support/v7/recyclerview/src/android/support/v7/widget/helper/ItemTouchHelper.java

/*
 * Copyright (C) 2015 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.support.v7.widget.helper;

import android.content.res.Resources;
import android.graphics.Canvas;
import android.graphics.Rect;
import android.os.Build;
import android.support.v4.animation.ValueAnimatorCompat;
import android.support.v4.animation.AnimatorCompatHelper;
import android.support.v4.animation.AnimatorListenerCompat;
import android.support.v4.animation.AnimatorUpdateListenerCompat;
import android.support.v4.view.GestureDetectorCompat;
import android.support.v4.view.MotionEventCompat;
import android.support.v4.view.VelocityTrackerCompat;
import android.support.v4.view.ViewCompat;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.GestureDetector;
import android.view.HapticFeedbackConstants;
import android.view.MotionEvent;
import android.view.VelocityTracker;
import android.view.View;
import android.view.ViewConfiguration;
import android.view.ViewParent;
import android.support.v7.recyclerview.R;

import java.util.ArrayList;
import java.util.List;

import android.support.v7.widget.RecyclerView.OnItemTouchListener;
import android.support.v7.widget.RecyclerView.ViewHolder;
import android.view.animation.Interpolator;

/**
 * This is a utility class to add swipe to dismiss and drag & drop support to RecyclerView.
 * <p>
 * It works with a RecyclerView and a Callback class, which configures what type of interactions
 * are enabled and also receives events when user performs these actions.
 * <p>
 * Depending on which functionality you support, you should override
 * {@link Callback#onMove(RecyclerView, ViewHolder, ViewHolder)} and / or
 * {@link Callback#onSwiped(ViewHolder, int)}.
 * <p>
 * This class is designed to work with any LayoutManager but for certain situations, it can be
 * optimized for your custom LayoutManager by extending methods in the
 * {@link ItemTouchHelper.Callback} class or implementing {@link ItemTouchHelper.ViewDropHandler}
 * interface in your LayoutManager.
 * <p>
 * By default, ItemTouchHelper moves the items' translateX/Y properties to reposition them. On
 * platforms older than Honeycomb, ItemTouchHelper uses canvas translations and View's visibility
 * property to move items in response to touch events. You can customize these behaviors by
 * overriding {@link Callback#onChildDraw(Canvas, RecyclerView, ViewHolder, float, float, int,
 * boolean)}
 * or {@link Callback#onChildDrawOver(Canvas, RecyclerView, ViewHolder, float, float, int,
 * boolean)}.
 * <p/>
 * Most of the time, you only need to override <code>onChildDraw</code> but due to limitations of
 * platform prior to Honeycomb, you may need to implement <code>onChildDrawOver</code> as well.
 */
public class ItemTouchHelper extends RecyclerView.ItemDecoration
        implements RecyclerView.OnChildAttachStateChangeListener {

    /**
     * Up direction, used for swipe & drag control.
     */
    public static final int UP = 1;

    /**
     * Down direction, used for swipe & drag control.
     */
    public static final int DOWN = 1 << 1;

    /**
     * Left direction, used for swipe & drag control.
     */
    public static final int LEFT = 1 << 2;

    /**
     * Right direction, used for swipe & drag control.
     */
    public static final int RIGHT = 1 << 3;

    // If you change these relative direction values, update Callback#convertToAbsoluteDirection,
    // Callback#convertToRelativeDirection.
    /**
     * Horizontal start direction. Resolved to LEFT or RIGHT depending on RecyclerView's layout
     * direction. Used for swipe & drag control.
     */
    public static final int START = LEFT << 2;

    /**
     * Horizontal end direction. Resolved to LEFT or RIGHT depending on RecyclerView's layout
     * direction. Used for swipe & drag control.
     */
    public static final int END = RIGHT << 2;

    /**
     * ItemTouchHelper is in idle state. At this state, either there is no related motion event by
     * the user or latest motion events have not yet triggered a swipe or drag.
     */
    public static final int ACTION_STATE_IDLE = 0;

    /**
     * A View is currently being swiped.
     */
    public static final int ACTION_STATE_SWIPE = 1;

    /**
     * A View is currently being dragged.
     */
    public static final int ACTION_STATE_DRAG = 2;

    /**
     * Animation type for views which are swiped successfully.
     */
    public static final int ANIMATION_TYPE_SWIPE_SUCCESS = 1 << 1;

    /**
     * Animation type for views which are not completely swiped thus will animate back to their
     * original position.
     */
    public static final int ANIMATION_TYPE_SWIPE_CANCEL = 1 << 2;

    /**
     * Animation type for views that were dragged and now will animate to their final position.
     */
    public static final int ANIMATION_TYPE_DRAG = 1 << 3;

    private static final String TAG = "ItemTouchHelper";

    private static final boolean DEBUG = false;

    private static final int ACTIVE_POINTER_ID_NONE = -1;

    private static final int DIRECTION_FLAG_COUNT = 8;

    private static final int ACTION_MODE_IDLE_MASK = (1 << DIRECTION_FLAG_COUNT) - 1;

    private static final int ACTION_MODE_SWIPE_MASK = ACTION_MODE_IDLE_MASK << DIRECTION_FLAG_COUNT;

    private static final int ACTION_MODE_DRAG_MASK = ACTION_MODE_SWIPE_MASK << DIRECTION_FLAG_COUNT;

    /**
     * The unit we are using to track velocity
     */
    private static final int PIXELS_PER_SECOND = 1000;

    /**
     * Views, whose state should be cleared after they are detached from RecyclerView.
     * This is necessary after swipe dismissing an item. We wait until animator finishes its job
     * to clean these views.
     */
    final List<View> mPendingCleanup = new ArrayList<View>();

    /**
     * Re-use array to calculate dx dy for a ViewHolder
     */
    private final float[] mTmpPosition = new float[2];

    /**
     * Currently selected view holder
     */
    ViewHolder mSelected = null;

    /**
     * The reference coordinates for the action start. For drag & drop, this is the time long
     * press is completed vs for swipe, this is the initial touch point.
     */
    float mInitialTouchX;

    float mInitialTouchY;

    /**
     * Set when ItemTouchHelper is assigned to a RecyclerView.
     */
    float mSwipeEscapeVelocity;

    /**
     * Set when ItemTouchHelper is assigned to a RecyclerView.
     */
    float mMaxSwipeVelocity;

    /**
     * The diff between the last event and initial touch.
     */
    float mDx;

    float mDy;

    /**
     * The coordinates of the selected view at the time it is selected. We record these values
     * when action starts so that we can consistently position it even if LayoutManager moves the
     * View.
     */
    float mSelectedStartX;

    float mSelectedStartY;

    /**
     * The pointer we are tracking.
     */
    int mActivePointerId = ACTIVE_POINTER_ID_NONE;

    /**
     * Developer callback which controls the behavior of ItemTouchHelper.
     */
    Callback mCallback;

    /**
     * Current mode.
     */
    int mActionState = ACTION_STATE_IDLE;

    /**
     * The direction flags obtained from unmasking
     * {@link Callback#getAbsoluteMovementFlags(RecyclerView, ViewHolder)} for the current
     * action state.
     */
    int mSelectedFlags;

    /**
     * When a View is dragged or swiped and needs to go back to where it was, we create a Recover
     * Animation and animate it to its location using this custom Animator, instead of using
     * framework Animators.
     * Using framework animators has the side effect of clashing with ItemAnimator, creating
     * jumpy UIs.
     */
    List<RecoverAnimation> mRecoverAnimations = new ArrayList<RecoverAnimation>();

    private int mSlop;

    private RecyclerView mRecyclerView;

    /**
     * When user drags a view to the edge, we start scrolling the LayoutManager as long as View
     * is partially out of bounds.
     */
    private final Runnable mScrollRunnable = new Runnable() {
        @Override
        public void run() {
            if (mSelected != null && scrollIfNecessary()) {
                if (mSelected != null) { //it might be lost during scrolling
                    moveIfNecessary(mSelected);
                }
                mRecyclerView.removeCallbacks(mScrollRunnable);
                ViewCompat.postOnAnimation(mRecyclerView, this);
            }
        }
    };

    /**
     * Used for detecting fling swipe
     */
    private VelocityTracker mVelocityTracker;

    //re-used list for selecting a swap target
    private List<ViewHolder> mSwapTargets;

    //re used for for sorting swap targets
    private List<Integer> mDistances;

    /**
     * If drag & drop is supported, we use child drawing order to bring them to front.
     */
    private RecyclerView.ChildDrawingOrderCallback mChildDrawingOrderCallback = null;

    /**
     * This keeps a reference to the child dragged by the user. Even after user stops dragging,
     * until view reaches its final position (end of recover animation), we keep a reference so
     * that it can be drawn above other children.
     */
    private View mOverdrawChild = null;

    /**
     * We cache the position of the overdraw child to avoid recalculating it each time child
     * position callback is called. This value is invalidated whenever a child is attached or
     * detached.
     */
    private int mOverdrawChildPosition = -1;

    /**
     * Used to detect long press.
     */
    private GestureDetectorCompat mGestureDetector;

    private final OnItemTouchListener mOnItemTouchListener
            = new OnItemTouchListener() {
        @Override
        public boolean onInterceptTouchEvent(RecyclerView recyclerView, MotionEvent event) {
            mGestureDetector.onTouchEvent(event);
            if (DEBUG) {
                Log.d(TAG, "intercept: x:" + event.getX() + ",y:" + event.getY() + ", " + event);
            }
            final int action = MotionEventCompat.getActionMasked(event);
            if (action == MotionEvent.ACTION_DOWN) {
                mActivePointerId = MotionEventCompat.getPointerId(event, 0);
                mInitialTouchX = event.getX();
                mInitialTouchY = event.getY();
                obtainVelocityTracker();
                if (mSelected == null) {
                    final RecoverAnimation animation = findAnimation(event);
                    if (animation != null) {
                        mInitialTouchX -= animation.mX;
                        mInitialTouchY -= animation.mY;
                        endRecoverAnimation(animation.mViewHolder, true);
                        if (mPendingCleanup.remove(animation.mViewHolder.itemView)) {
                            mCallback.clearView(mRecyclerView, animation.mViewHolder);
                        }
                        select(animation.mViewHolder, animation.mActionState);
                        updateDxDy(event, mSelectedFlags, 0);
                    }
                }
            } else if (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_UP) {
                mActivePointerId = ACTIVE_POINTER_ID_NONE;
                select(null, ACTION_STATE_IDLE);
            } else if (mActivePointerId != ACTIVE_POINTER_ID_NONE) {
                // in a non scroll orientation, if distance change is above threshold, we
                // can select the item
                final int index = MotionEventCompat.findPointerIndex(event, mActivePointerId);
                if (DEBUG) {
                    Log.d(TAG, "pointer index " + index);
                }
                if (index >= 0) {
                    checkSelectForSwipe(action, event, index);
                }
            }
            if (mVelocityTracker != null) {
                mVelocityTracker.addMovement(event);
            }
            return mSelected != null;
        }

        @Override
        public void onTouchEvent(RecyclerView recyclerView, MotionEvent event) {
            mGestureDetector.onTouchEvent(event);
            if (DEBUG) {
                Log.d(TAG,
                        "on touch: x:" + mInitialTouchX + ",y:" + mInitialTouchY + ", :" + event);
            }
            if (mVelocityTracker != null) {
                mVelocityTracker.addMovement(event);
            }
            if (mActivePointerId == ACTIVE_POINTER_ID_NONE) {
                return;
            }
            final int action = MotionEventCompat.getActionMasked(event);
            final int activePointerIndex = MotionEventCompat
                    .findPointerIndex(event, mActivePointerId);
            if (activePointerIndex >= 0) {
                checkSelectForSwipe(action, event, activePointerIndex);
            }
            ViewHolder viewHolder = mSelected;
            if (viewHolder == null) {
                return;
            }
            switch (action) {
                case MotionEvent.ACTION_MOVE: {
                    // Find the index of the active pointer and fetch its position
                    if (activePointerIndex >= 0) {
                        updateDxDy(event, mSelectedFlags, activePointerIndex);
                        moveIfNecessary(viewHolder);
                        mRecyclerView.removeCallbacks(mScrollRunnable);
                        mScrollRunnable.run();
                        mRecyclerView.invalidate();
                    }
                    break;
                }
                case MotionEvent.ACTION_CANCEL:
                    if (mVelocityTracker != null) {
                        mVelocityTracker.clear();
                    }
                    // fall through
                case MotionEvent.ACTION_UP:
                    select(null, ACTION_STATE_IDLE);
                    mActivePointerId = ACTIVE_POINTER_ID_NONE;
                    break;
                case MotionEvent.ACTION_POINTER_UP: {
                    final int pointerIndex = MotionEventCompat.getActionIndex(event);
                    final int pointerId = MotionEventCompat.getPointerId(event, pointerIndex);
                    if (pointerId == mActivePointerId) {
                        // This was our active pointer going up. Choose a new
                        // active pointer and adjust accordingly.
                        final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
                        mActivePointerId = MotionEventCompat.getPointerId(event, newPointerIndex);
                        updateDxDy(event, mSelectedFlags, pointerIndex);
                    }
                    break;
                }
            }
        }

        @Override
        public void onRequestDisallowInterceptTouchEvent(boolean disallowIntercept) {
            if (!disallowIntercept) {
                return;
            }
            select(null, ACTION_STATE_IDLE);
        }
    };

    /**
     * Temporary rect instance that is used when we need to lookup Item decorations.
     */
    private Rect mTmpRect;

    /**
     * When user started to drag scroll. Reset when we don't scroll
     */
    private long mDragScrollStartTimeInMs;

    /**
     * Creates an ItemTouchHelper that will work with the given Callback.
     * <p>
     * You can attach ItemTouchHelper to a RecyclerView via
     * {@link #attachToRecyclerView(RecyclerView)}. Upon attaching, it will add an item decoration,
     * an onItemTouchListener and a Child attach / detach listener to the RecyclerView.
     *
     * @param callback The Callback which controls the behavior of this touch helper.
     */
    public ItemTouchHelper(Callback callback) {
        mCallback = callback;
    }

    private static boolean hitTest(View child, float x, float y, float left, float top) {
        return x >= left &&
                x <= left + child.getWidth() &&
                y >= top &&
                y <= top + child.getHeight();
    }

    /**
     * Attaches the ItemTouchHelper to the provided RecyclerView. If TouchHelper is already
     * attached
     * to a RecyclerView, it will first detach from the previous one.
     *
     * @param recyclerView The RecyclerView instance to which you want to add this helper.
     */
    public void attachToRecyclerView(RecyclerView recyclerView) {
        if (mRecyclerView == recyclerView) {
            return; // nothing to do
        }
        if (mRecyclerView != null) {
            destroyCallbacks();
        }
        mRecyclerView = recyclerView;
        if (mRecyclerView != null) {
            final Resources resources = recyclerView.getResources();
            mSwipeEscapeVelocity = resources
                    .getDimension(R.dimen.item_touch_helper_swipe_escape_velocity);
            mMaxSwipeVelocity = resources
                    .getDimension(R.dimen.item_touch_helper_swipe_escape_max_velocity);
            setupCallbacks();
        }
    }

    private void setupCallbacks() {
        ViewConfiguration vc = ViewConfiguration.get(mRecyclerView.getContext());
        mSlop = vc.getScaledTouchSlop();
        mRecyclerView.addItemDecoration(this);
        mRecyclerView.addOnItemTouchListener(mOnItemTouchListener);
        mRecyclerView.addOnChildAttachStateChangeListener(this);
        initGestureDetector();
    }

    private void destroyCallbacks() {
        mRecyclerView.removeItemDecoration(this);
        mRecyclerView.removeOnItemTouchListener(mOnItemTouchListener);
        mRecyclerView.removeOnChildAttachStateChangeListener(this);
        // clean all attached
        final int recoverAnimSize = mRecoverAnimations.size();
        for (int i = recoverAnimSize - 1; i >= 0; i--) {
            final RecoverAnimation recoverAnimation = mRecoverAnimations.get(0);
            mCallback.clearView(mRecyclerView, recoverAnimation.mViewHolder);
        }
        mRecoverAnimations.clear();
        mOverdrawChild = null;
        mOverdrawChildPosition = -1;
        releaseVelocityTracker();
    }

    private void initGestureDetector() {
        if (mGestureDetector != null) {
            return;
        }
        mGestureDetector = new GestureDetectorCompat(mRecyclerView.getContext(),
                new ItemTouchHelperGestureListener());
    }

    private void getSelectedDxDy(float[] outPosition) {
        if ((mSelectedFlags & (LEFT | RIGHT)) != 0) {
            outPosition[0] = mSelectedStartX + mDx - mSelected.itemView.getLeft();
        } else {
            outPosition[0] = ViewCompat.getTranslationX(mSelected.itemView);
        }
        if ((mSelectedFlags & (UP | DOWN)) != 0) {
            outPosition[1] = mSelectedStartY + mDy - mSelected.itemView.getTop();
        } else {
            outPosition[1] = ViewCompat.getTranslationY(mSelected.itemView);
        }
    }

    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        float dx = 0, dy = 0;
        if (mSelected != null) {
            getSelectedDxDy(mTmpPosition);
            dx = mTmpPosition[0];
            dy = mTmpPosition[1];
        }
        mCallback.onDrawOver(c, parent, mSelected,
                mRecoverAnimations, mActionState, dx, dy);
    }

    @Override
    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
        // we don't know if RV changed something so we should invalidate this index.
        mOverdrawChildPosition = -1;
        float dx = 0, dy = 0;
        if (mSelected != null) {
            getSelectedDxDy(mTmpPosition);
            dx = mTmpPosition[0];
            dy = mTmpPosition[1];
        }
        mCallback.onDraw(c, parent, mSelected,
                mRecoverAnimations, mActionState, dx, dy);
    }

    /**
     * Starts dragging or swiping the given View. Call with null if you want to clear it.
     *
     * @param selected    The ViewHolder to drag or swipe. Can be null if you want to cancel the
     *                    current action
     * @param actionState The type of action
     */
    private void select(ViewHolder selected, int actionState) {
        if (selected == mSelected && actionState == mActionState) {
            return;
        }
        mDragScrollStartTimeInMs = Long.MIN_VALUE;
        final int prevActionState = mActionState;
        // prevent duplicate animations
        endRecoverAnimation(selected, true);
        mActionState = actionState;
        if (actionState == ACTION_STATE_DRAG) {
            // we remove after animation is complete. this means we only elevate the last drag
            // child but that should perform good enough as it is very hard to start dragging a
            // new child before the previous one settles.
            mOverdrawChild = selected.itemView;
            addChildDrawingOrderCallback();
        }
        int actionStateMask = (1 << (DIRECTION_FLAG_COUNT + DIRECTION_FLAG_COUNT * actionState))
                - 1;
        boolean preventLayout = false;

        if (mSelected != null) {
            final ViewHolder prevSelected = mSelected;
            if (prevSelected.itemView.getParent() != null) {
                final int swipeDir = prevActionState == ACTION_STATE_DRAG ? 0
                        : swipeIfNecessary(prevSelected);
                releaseVelocityTracker();
                // find where we should animate to
                final float targetTranslateX, targetTranslateY;
                int animationType;
                switch (swipeDir) {
                    case LEFT:
                    case RIGHT:
                    case START:
                    case END:
                        targetTranslateY = 0;
                        targetTranslateX = Math.signum(mDx) * mRecyclerView.getWidth();
                        break;
                    case UP:
                    case DOWN:
                        targetTranslateX = 0;
                        targetTranslateY = Math.signum(mDy) * mRecyclerView.getHeight();
                        break;
                    default:
                        targetTranslateX = 0;
                        targetTranslateY = 0;
                }
                if (prevActionState == ACTION_STATE_DRAG) {
                    animationType = ANIMATION_TYPE_DRAG;
                } else if (swipeDir > 0) {
                    animationType = ANIMATION_TYPE_SWIPE_SUCCESS;
                } else {
                    animationType = ANIMATION_TYPE_SWIPE_CANCEL;
                }
                getSelectedDxDy(mTmpPosition);
                final float currentTranslateX = mTmpPosition[0];
                final float currentTranslateY = mTmpPosition[1];
                final RecoverAnimation rv = new RecoverAnimation(prevSelected, animationType,
                        prevActionState, currentTranslateX, currentTranslateY,
                        targetTranslateX, targetTranslateY) {
                    @Override
                    public void onAnimationEnd(ValueAnimatorCompat animation) {
                        super.onAnimationEnd(animation);
                        if (this.mOverridden) {
                            return;
                        }
                        if (swipeDir <= 0) {
                            // this is a drag or failed swipe. recover immediately
                            mCallback.clearView(mRecyclerView, prevSelected);
                            // full cleanup will happen on onDrawOver
                        } else {
                            // wait until remove animation is complete.
                            mPendingCleanup.add(prevSelected.itemView);
                            mIsPendingCleanup = true;
                            if (swipeDir > 0) {
                                // Animation might be ended by other animators during a layout.
                                // We defer callback to avoid editing adapter during a layout.
                                postDispatchSwipe(this, swipeDir);
                            }
                        }
                        // removed from the list after it is drawn for the last time
                        if (mOverdrawChild == prevSelected.itemView) {
                            removeChildDrawingOrderCallbackIfNecessary(prevSelected.itemView);
                        }
                    }
                };
                final long duration = mCallback.getAnimationDuration(mRecyclerView, animationType,
                        targetTranslateX - currentTranslateX, targetTranslateY - currentTranslateY);
                rv.setDuration(duration);
                mRecoverAnimations.add(rv);
                rv.start();
                preventLayout = true;
            } else {
                removeChildDrawingOrderCallbackIfNecessary(prevSelected.itemView);
                mCallback.clearView(mRecyclerView, prevSelected);
            }
            mSelected = null;
        }
        if (selected != null) {
            mSelectedFlags =
                    (mCallback.getAbsoluteMovementFlags(mRecyclerView, selected) & actionStateMask)
                            >> (mActionState * DIRECTION_FLAG_COUNT);
            mSelectedStartX = selected.itemView.getLeft();
            mSelectedStartY = selected.itemView.getTop();
            mSelected = selected;

            if (actionState == ACTION_STATE_DRAG) {
                mSelected.itemView.performHapticFeedback(HapticFeedbackConstants.LONG_PRESS);
            }
        }
        final ViewParent rvParent = mRecyclerView.getParent();
        if (rvParent != null) {
            rvParent.requestDisallowInterceptTouchEvent(mSelected != null);
        }
        if (!preventLayout) {
            mRecyclerView.getLayoutManager().requestSimpleAnimationsInNextLayout();
        }
        mCallback.onSelectedChanged(mSelected, mActionState);
        mRecyclerView.invalidate();
    }

    private void postDispatchSwipe(final RecoverAnimation anim, final int swipeDir) {
        // wait until animations are complete.
        mRecyclerView.post(new Runnable() {
            @Override
            public void run() {
                if (mRecyclerView != null && mRecyclerView.isAttachedToWindow() &&
                        !anim.mOverridden &&
                        anim.mViewHolder.getAdapterPosition() != RecyclerView.NO_POSITION) {
                    final RecyclerView.ItemAnimator animator = mRecyclerView.getItemAnimator();
                    // if animator is running or we have other active recover animations, we try
                    // not to call onSwiped because DefaultItemAnimator is not good at merging
                    // animations. Instead, we wait and batch.
                    if ((animator == null || !animator.isRunning(null))
                            && !hasRunningRecoverAnim()) {
                        mCallback.onSwiped(anim.mViewHolder, swipeDir);
                    } else {
                        mRecyclerView.post(this);
                    }
                }
            }
        });
    }

    private boolean hasRunningRecoverAnim() {
        final int size = mRecoverAnimations.size();
        for (int i = 0; i < size; i++) {
            if (!mRecoverAnimations.get(i).mEnded) {
                return true;
            }
        }
        return false;
    }

    /**
     * If user drags the view to the edge, trigger a scroll if necessary.
     */
    private boolean scrollIfNecessary() {
        if (mSelected == null) {
            mDragScrollStartTimeInMs = Long.MIN_VALUE;
            return false;
        }
        final long now = System.currentTimeMillis();
        final long scrollDuration = mDragScrollStartTimeInMs
                == Long.MIN_VALUE ? 0 : now - mDragScrollStartTimeInMs;
        RecyclerView.LayoutManager lm = mRecyclerView.getLayoutManager();
        if (mTmpRect == null) {
            mTmpRect = new Rect();
        }
        int scrollX = 0;
        int scrollY = 0;
        lm.calculateItemDecorationsForChild(mSelected.itemView, mTmpRect);
        if (lm.canScrollHorizontally()) {
            int curX = (int) (mSelectedStartX + mDx);
            final int leftDiff = curX - mTmpRect.left - mRecyclerView.getPaddingLeft();
            if (mDx < 0 && leftDiff < 0) {
                scrollX = leftDiff;
            } else if (mDx > 0) {
                final int rightDiff =
                        curX + mSelected.itemView.getWidth() + mTmpRect.right
                                - (mRecyclerView.getWidth() - mRecyclerView.getPaddingRight());
                if (rightDiff > 0) {
                    scrollX = rightDiff;
                }
            }
        }
        if (lm.canScrollVertically()) {
            int curY = (int) (mSelectedStartY + mDy);
            final int topDiff = curY - mTmpRect.top - mRecyclerView.getPaddingTop();
            if (mDy < 0 && topDiff < 0) {
                scrollY = topDiff;
            } else if (mDy > 0) {
                final int bottomDiff = curY + mSelected.itemView.getHeight() + mTmpRect.bottom -
                        (mRecyclerView.getHeight() - mRecyclerView.getPaddingBottom());
                if (bottomDiff > 0) {
                    scrollY = bottomDiff;
                }
            }
        }
        if (scrollX != 0) {
            scrollX = mCallback.interpolateOutOfBoundsScroll(mRecyclerView,
                    mSelected.itemView.getWidth(), scrollX,
                    mRecyclerView.getWidth(), scrollDuration);
        }
        if (scrollY != 0) {
            scrollY = mCallback.interpolateOutOfBoundsScroll(mRecyclerView,
                    mSelected.itemView.getHeight(), scrollY,
                    mRecyclerView.getHeight(), scrollDuration);
        }
        if (scrollX != 0 || scrollY != 0) {
            if (mDragScrollStartTimeInMs == Long.MIN_VALUE) {
                mDragScrollStartTimeInMs = now;
            }
            mRecyclerView.scrollBy(scrollX, scrollY);
            return true;
        }
        mDragScrollStartTimeInMs = Long.MIN_VALUE;
        return false;
    }

    private List<ViewHolder> findSwapTargets(ViewHolder viewHolder) {
        if (mSwapTargets == null) {
            mSwapTargets = new ArrayList<ViewHolder>();
            mDistances = new ArrayList<Integer>();
        } else {
            mSwapTargets.clear();
            mDistances.clear();
        }
        final int margin = mCallback.getBoundingBoxMargin();
        final int left = Math.round(mSelectedStartX + mDx) - margin;
        final int top = Math.round(mSelectedStartY + mDy) - margin;
        final int right = left + viewHolder.itemView.getWidth() + 2 * margin;
        final int bottom = top + viewHolder.itemView.getHeight() + 2 * margin;
        final int centerX = (left + right) / 2;
        final int centerY = (top + bottom) / 2;
        final RecyclerView.LayoutManager lm = mRecyclerView.getLayoutManager();
        final int childCount = lm.getChildCount();
        for (int i = 0; i < childCount; i++) {
            View other = lm.getChildAt(i);
            if (other == viewHolder.itemView) {
                continue;//myself!
            }
            if (other.getBottom() < top || other.getTop() > bottom
                    || other.getRight() < left || other.getLeft() > right) {
                continue;
            }
            final ViewHolder otherVh = mRecyclerView.getChildViewHolder(other);
            if (mCallback.canDropOver(mRecyclerView, mSelected, otherVh)) {
                // find the index to add
                final int dx = Math.abs(centerX - (other.getLeft() + other.getRight()) / 2);
                final int dy = Math.abs(centerY - (other.getTop() + other.getBottom()) / 2);
                final int dist = dx * dx + dy * dy;

                int pos = 0;
                final int cnt = mSwapTargets.size();
                for (int j = 0; j < cnt; j++) {
                    if (dist > mDistances.get(j)) {
                        pos++;
                    } else {
                        break;
                    }
                }
                mSwapTargets.add(pos, otherVh);
                mDistances.add(pos, dist);
            }
        }
        return mSwapTargets;
    }

    /**
     * Checks if we should swap w/ another view holder.
     */
    private void moveIfNecessary(ViewHolder viewHolder) {
        if (mRecyclerView.isLayoutRequested()) {
            return;
        }
        if (mActionState != ACTION_STATE_DRAG) {
            return;
        }

        final float threshold = mCallback.getMoveThreshold(viewHolder);
        final int x = (int) (mSelectedStartX + mDx);
        final int y = (int) (mSelectedStartY + mDy);
        if (Math.abs(y - viewHolder.itemView.getTop()) < viewHolder.itemView.getHeight() * threshold
                && Math.abs(x - viewHolder.itemView.getLeft())
                < viewHolder.itemView.getWidth() * threshold) {
            return;
        }
        List<ViewHolder> swapTargets = findSwapTargets(viewHolder);
        if (swapTargets.size() == 0) {
            return;
        }
        // may swap.
        ViewHolder target = mCallback.chooseDropTarget(viewHolder, swapTargets, x, y);
        if (target == null) {
            mSwapTargets.clear();
            mDistances.clear();
            return;
        }
        final int toPosition = target.getAdapterPosition();
        final int fromPosition = viewHolder.getAdapterPosition();
        if (mCallback.onMove(mRecyclerView, viewHolder, target)) {
            // keep target visible
            mCallback.onMoved(mRecyclerView, viewHolder, fromPosition,
                    target, toPosition, x, y);
        }
    }

    @Override
    public void onChildViewAttachedToWindow(View view) {
    }

    @Override
    public void onChildViewDetachedFromWindow(View view) {
        removeChildDrawingOrderCallbackIfNecessary(view);
        final ViewHolder holder = mRecyclerView.getChildViewHolder(view);
        if (holder == null) {
            return;
        }
        if (mSelected != null && holder == mSelected) {
            select(null, ACTION_STATE_IDLE);
        } else {
            endRecoverAnimation(holder, false); // this may push it into pending cleanup list.
            if (mPendingCleanup.remove(holder.itemView)) {
                mCallback.clearView(mRecyclerView, holder);
            }
        }
    }

    /**
     * Returns the animation type or 0 if cannot be found.
     */
    private int endRecoverAnimation(ViewHolder viewHolder, boolean override) {
        final int recoverAnimSize = mRecoverAnimations.size();
        for (int i = recoverAnimSize - 1; i >= 0; i--) {
            final RecoverAnimation anim = mRecoverAnimations.get(i);
            if (anim.mViewHolder == viewHolder) {
                anim.mOverridden |= override;
                if (!anim.mEnded) {
                    anim.cancel();
                }
                mRecoverAnimations.remove(i);
                return anim.mAnimationType;
            }
        }
        return 0;
    }

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent,
            RecyclerView.State state) {
        outRect.setEmpty();
    }

    private void obtainVelocityTracker() {
        if (mVelocityTracker != null) {
            mVelocityTracker.recycle();
        }
        mVelocityTracker = VelocityTracker.obtain();
    }

    private void releaseVelocityTracker() {
        if (mVelocityTracker != null) {
            mVelocityTracker.recycle();
            mVelocityTracker = null;
        }
    }

    private ViewHolder findSwipedView(MotionEvent motionEvent) {
        final RecyclerView.LayoutManager lm = mRecyclerView.getLayoutManager();
        if (mActivePointerId == ACTIVE_POINTER_ID_NONE) {
            return null;
        }
        final int pointerIndex = MotionEventCompat.findPointerIndex(motionEvent, mActivePointerId);
        final float dx = MotionEventCompat.getX(motionEvent, pointerIndex) - mInitialTouchX;
        final float dy = MotionEventCompat.getY(motionEvent, pointerIndex) - mInitialTouchY;
        final float absDx = Math.abs(dx);
        final float absDy = Math.abs(dy);

        if (absDx < mSlop && absDy < mSlop) {
            return null;
        }
        if (absDx > absDy && lm.canScrollHorizontally()) {
            return null;
        } else if (absDy > absDx && lm.canScrollVertically()) {
            return null;
        }
        View child = findChildView(motionEvent);
        if (child == null) {
            return null;
        }
        return mRecyclerView.getChildViewHolder(child);
    }

    /**
     * Checks whether we should select a View for swiping.
     */
    private boolean checkSelectForSwipe(int action, MotionEvent motionEvent, int pointerIndex) {
        if (mSelected != null || action != MotionEvent.ACTION_MOVE
                || mActionState == ACTION_STATE_DRAG || !mCallback.isItemViewSwipeEnabled()) {
            return false;
        }
        if (mRecyclerView.getScrollState() == RecyclerView.SCROLL_STATE_DRAGGING) {
            return false;
        }
        final ViewHolder vh = findSwipedView(motionEvent);
        if (vh == null) {
            return false;
        }
        final int movementFlags = mCallback.getAbsoluteMovementFlags(mRecyclerView, vh);

        final int swipeFlags = (movementFlags & ACTION_MODE_SWIPE_MASK)
                >> (DIRECTION_FLAG_COUNT * ACTION_STATE_SWIPE);

        if (swipeFlags == 0) {
            return false;
        }

        // mDx and mDy are only set in allowed directions. We use custom x/y here instead of
        // updateDxDy to avoid swiping if user moves more in the other direction
        final float x = MotionEventCompat.getX(motionEvent, pointerIndex);
        final float y = MotionEventCompat.getY(motionEvent, pointerIndex);

        // Calculate the distance moved
        final float dx = x - mInitialTouchX;
        final float dy = y - mInitialTouchY;
        // swipe target is chose w/o applying flags so it does not really check if swiping in that
        // direction is allowed. This why here, we use mDx mDy to check slope value again.
        final float absDx = Math.abs(dx);
        final float absDy = Math.abs(dy);

        if (absDx < mSlop && absDy < mSlop) {
            return false;
        }
        if (absDx > absDy) {
            if (dx < 0 && (swipeFlags & LEFT) == 0) {
                return false;
            }
            if (dx > 0 && (swipeFlags & RIGHT) == 0) {
                return false;
            }
        } else {
            if (dy < 0 && (swipeFlags & UP) == 0) {
                return false;
            }
            if (dy > 0 && (swipeFlags & DOWN) == 0) {
                return false;
            }
        }
        mDx = mDy = 0f;
        mActivePointerId = MotionEventCompat.getPointerId(motionEvent, 0);
        select(vh, ACTION_STATE_SWIPE);
        return true;
    }

    private View findChildView(MotionEvent event) {
        // first check elevated views, if none, then call RV
        final float x = event.getX();
        final float y = event.getY();
        if (mSelected != null) {
            final View selectedView = mSelected.itemView;
            if (hitTest(selectedView, x, y, mSelectedStartX + mDx, mSelectedStartY + mDy)) {
                return selectedView;
            }
        }
        for (int i = mRecoverAnimations.size() - 1; i >= 0; i--) {
            final RecoverAnimation anim = mRecoverAnimations.get(i);
            final View view = anim.mViewHolder.itemView;
            if (hitTest(view, x, y, anim.mX, anim.mY)) {
                return view;
            }
        }
        return mRecyclerView.findChildViewUnder(x, y);
    }

    /**
     * Starts dragging the provided ViewHolder. By default, ItemTouchHelper starts a drag when a
     * View is long pressed. You can disable that behavior via
     * {@link ItemTouchHelper.Callback#isLongPressDragEnabled()}.
     * <p>
     * For this method to work:
     * <ul>
     * <li>The provided ViewHolder must be a child of the RecyclerView to which this
     * ItemTouchHelper
     * is attached.</li>
     * <li>{@link ItemTouchHelper.Callback} must have dragging enabled.</li>
     * <li>There must be a previous touch event that was reported to the ItemTouchHelper
     * through RecyclerView's ItemTouchListener mechanism. As long as no other ItemTouchListener
     * grabs previous events, this should work as expected.</li>
     * </ul>
     *
     * For example, if you would like to let your user to be able to drag an Item by touching one
     * of its descendants, you may implement it as follows:
     * <pre>
     *     viewHolder.dragButton.setOnTouchListener(new View.OnTouchListener() {
     *         public boolean onTouch(View v, MotionEvent event) {
     *             if (MotionEventCompat.getActionMasked(event) == MotionEvent.ACTION_DOWN) {
     *                 mItemTouchHelper.startDrag(viewHolder);
     *             }
     *             return false;
     *         }
     *     });
     * </pre>
     * <p>
     *
     * @param viewHolder The ViewHolder to start dragging. It must be a direct child of
     *                   RecyclerView.
     * @see ItemTouchHelper.Callback#isItemViewSwipeEnabled()
     */
    public void startDrag(ViewHolder viewHolder) {
        if (!mCallback.hasDragFlag(mRecyclerView, viewHolder)) {
            Log.e(TAG, "Start drag has been called but swiping is not enabled");
            return;
        }
        if (viewHolder.itemView.getParent() != mRecyclerView) {
            Log.e(TAG, "Start drag has been called with a view holder which is not a child of "
                    + "the RecyclerView which is controlled by this ItemTouchHelper.");
            return;
        }
        obtainVelocityTracker();
        mDx = mDy = 0f;
        select(viewHolder, ACTION_STATE_DRAG);
    }

    /**
     * Starts swiping the provided ViewHolder. By default, ItemTouchHelper starts swiping a View
     * when user swipes their finger (or mouse pointer) over the View. You can disable this
     * behavior
     * by overriding {@link ItemTouchHelper.Callback}
     * <p>
     * For this method to work:
     * <ul>
     * <li>The provided ViewHolder must be a child of the RecyclerView to which this
     * ItemTouchHelper is attached.</li>
     * <li>{@link ItemTouchHelper.Callback} must have swiping enabled.</li>
     * <li>There must be a previous touch event that was reported to the ItemTouchHelper
     * through RecyclerView's ItemTouchListener mechanism. As long as no other ItemTouchListener
     * grabs previous events, this should work as expected.</li>
     * </ul>
     *
     * For example, if you would like to let your user to be able to swipe an Item by touching one
     * of its descendants, you may implement it as follows:
     * <pre>
     *     viewHolder.dragButton.setOnTouchListener(new View.OnTouchListener() {
     *         public boolean onTouch(View v, MotionEvent event) {
     *             if (MotionEventCompat.getActionMasked(event) == MotionEvent.ACTION_DOWN) {
     *                 mItemTouchHelper.startSwipe(viewHolder);
     *             }
     *             return false;
     *         }
     *     });
     * </pre>
     *
     * @param viewHolder The ViewHolder to start swiping. It must be a direct child of
     *                   RecyclerView.
     */
    public void startSwipe(ViewHolder viewHolder) {
        if (!mCallback.hasSwipeFlag(mRecyclerView, viewHolder)) {
            Log.e(TAG, "Start swipe has been called but dragging is not enabled");
            return;
        }
        if (viewHolder.itemView.getParent() != mRecyclerView) {
            Log.e(TAG, "Start swipe has been called with a view holder which is not a child of "
                    + "the RecyclerView controlled by this ItemTouchHelper.");
            return;
        }
        obtainVelocityTracker();
        mDx = mDy = 0f;
        select(viewHolder, ACTION_STATE_SWIPE);
    }

    private RecoverAnimation findAnimation(MotionEvent event) {
        if (mRecoverAnimations.isEmpty()) {
            return null;
        }
        View target = findChildView(event);
        for (int i = mRecoverAnimations.size() - 1; i >= 0; i--) {
            final RecoverAnimation anim = mRecoverAnimations.get(i);
            if (anim.mViewHolder.itemView == target) {
                return anim;
            }
        }
        return null;
    }

    private void updateDxDy(MotionEvent ev, int directionFlags, int pointerIndex) {
        final float x = MotionEventCompat.getX(ev, pointerIndex);
        final float y = MotionEventCompat.getY(ev, pointerIndex);

        // Calculate the distance moved
        mDx = x - mInitialTouchX;
        mDy = y - mInitialTouchY;
        if ((directionFlags & LEFT) == 0) {
            mDx = Math.max(0, mDx);
        }
        if ((directionFlags & RIGHT) == 0) {
            mDx = Math.min(0, mDx);
        }
        if ((directionFlags & UP) == 0) {
            mDy = Math.max(0, mDy);
        }
        if ((directionFlags & DOWN) == 0) {
            mDy = Math.min(0, mDy);
        }
    }

    private int swipeIfNecessary(ViewHolder viewHolder) {
        if (mActionState == ACTION_STATE_DRAG) {
            return 0;
        }
        final int originalMovementFlags = mCallback.getMovementFlags(mRecyclerView, viewHolder);
        final int absoluteMovementFlags = mCallback.convertToAbsoluteDirection(
                originalMovementFlags,
                ViewCompat.getLayoutDirection(mRecyclerView));
        final int flags = (absoluteMovementFlags
                & ACTION_MODE_SWIPE_MASK) >> (ACTION_STATE_SWIPE * DIRECTION_FLAG_COUNT);
        if (flags == 0) {
            return 0;
        }
        final int originalFlags = (originalMovementFlags
                & ACTION_MODE_SWIPE_MASK) >> (ACTION_STATE_SWIPE * DIRECTION_FLAG_COUNT);
        int swipeDir;
        if (Math.abs(mDx) > Math.abs(mDy)) {
            if ((swipeDir = checkHorizontalSwipe(viewHolder, flags)) > 0) {
                // if swipe dir is not in original flags, it should be the relative direction
                if ((originalFlags & swipeDir) == 0) {
                    // convert to relative
                    return Callback.convertToRelativeDirection(swipeDir,
                            ViewCompat.getLayoutDirection(mRecyclerView));
                }
                return swipeDir;
            }
            if ((swipeDir = checkVerticalSwipe(viewHolder, flags)) > 0) {
                return swipeDir;
            }
        } else {
            if ((swipeDir = checkVerticalSwipe(viewHolder, flags)) > 0) {
                return swipeDir;
            }
            if ((swipeDir = checkHorizontalSwipe(viewHolder, flags)) > 0) {
                // if swipe dir is not in original flags, it should be the relative direction
                if ((originalFlags & swipeDir) == 0) {
                    // convert to relative
                    return Callback.convertToRelativeDirection(swipeDir,
                            ViewCompat.getLayoutDirection(mRecyclerView));
                }
                return swipeDir;
            }
        }
        return 0;
    }

    private int checkHorizontalSwipe(ViewHolder viewHolder, int flags) {
        if ((flags & (LEFT | RIGHT)) != 0) {
            final int dirFlag = mDx > 0 ? RIGHT : LEFT;
            if (mVelocityTracker != null && mActivePointerId > -1) {
                mVelocityTracker.computeCurrentVelocity(PIXELS_PER_SECOND,
                        mCallback.getSwipeVelocityThreshold(mMaxSwipeVelocity));
                final float xVelocity = VelocityTrackerCompat
                        .getXVelocity(mVelocityTracker, mActivePointerId);
                final float yVelocity = VelocityTrackerCompat
                        .getYVelocity(mVelocityTracker, mActivePointerId);
                final int velDirFlag = xVelocity > 0f ? RIGHT : LEFT;
                final float absXVelocity = Math.abs(xVelocity);
                if ((velDirFlag & flags) != 0 && dirFlag == velDirFlag &&
                        absXVelocity >= mCallback.getSwipeEscapeVelocity(mSwipeEscapeVelocity) &&
                        absXVelocity > Math.abs(yVelocity)) {
                    return velDirFlag;
                }
            }

            final float threshold = mRecyclerView.getWidth() * mCallback
                    .getSwipeThreshold(viewHolder);

            if ((flags & dirFlag) != 0 && Math.abs(mDx) > threshold) {
                return dirFlag;
            }
        }
        return 0;
    }

    private int checkVerticalSwipe(ViewHolder viewHolder, int flags) {
        if ((flags & (UP | DOWN)) != 0) {
            final int dirFlag = mDy > 0 ? DOWN : UP;
            if (mVelocityTracker != null && mActivePointerId > -1) {
                mVelocityTracker.computeCurrentVelocity(PIXELS_PER_SECOND,
                        mCallback.getSwipeVelocityThreshold(mMaxSwipeVelocity));
                final float xVelocity = VelocityTrackerCompat
                        .getXVelocity(mVelocityTracker, mActivePointerId);
                final float yVelocity = VelocityTrackerCompat
                        .getYVelocity(mVelocityTracker, mActivePointerId);
                final int velDirFlag = yVelocity > 0f ? DOWN : UP;
                final float absYVelocity = Math.abs(yVelocity);
                if ((velDirFlag & flags) != 0 && velDirFlag == dirFlag &&
                        absYVelocity >= mCallback.getSwipeEscapeVelocity(mSwipeEscapeVelocity) &&
                        absYVelocity > Math.abs(xVelocity)) {
                    return velDirFlag;
                }
            }

            final float threshold = mRecyclerView.getHeight() * mCallback
                    .getSwipeThreshold(viewHolder);
            if ((flags & dirFlag) != 0 && Math.abs(mDy) > threshold) {
                return dirFlag;
            }
        }
        return 0;
    }

    private void addChildDrawingOrderCallback() {
        if (Build.VERSION.SDK_INT >= 21) {
            return;// we use elevation on Lollipop
        }
        if (mChildDrawingOrderCallback == null) {
            mChildDrawingOrderCallback = new RecyclerView.ChildDrawingOrderCallback() {
                @Override
                public int onGetChildDrawingOrder(int childCount, int i) {
                    if (mOverdrawChild == null) {
                        return i;
                    }
                    int childPosition = mOverdrawChildPosition;
                    if (childPosition == -1) {
                        childPosition = mRecyclerView.indexOfChild(mOverdrawChild);
                        mOverdrawChildPosition = childPosition;
                    }
                    if (i == childCount - 1) {
                        return childPosition;
                    }
                    return i < childPosition ? i : i + 1;
                }
            };
        }
        mRecyclerView.setChildDrawingOrderCallback(mChildDrawingOrderCallback);
    }

    private void removeChildDrawingOrderCallbackIfNecessary(View view) {
        if (view == mOverdrawChild) {
            mOverdrawChild = null;
            // only remove if we've added
            if (mChildDrawingOrderCallback != null) {
                mRecyclerView.setChildDrawingOrderCallback(null);
            }
        }
    }

    /**
     * An interface which can be implemented by LayoutManager for better integration with
     * {@link ItemTouchHelper}.
     */
    public static interface ViewDropHandler {

        /**
         * Called by the {@link ItemTouchHelper} after a View is dropped over another View.
         * <p>
         * A LayoutManager should implement this interface to get ready for the upcoming move
         * operation.
         * <p>
         * For example, LinearLayoutManager sets up a "scrollToPositionWithOffset" calls so that
         * the View under drag will be used as an anchor View while calculating the next layout,
         * making layout stay consistent.
         *
         * @param view   The View which is being dragged. It is very likely that user is still
         *               dragging this View so there might be other
         *               {@link #prepareForDrop(View, View, int, int)} after this one.
         * @param target The target view which is being dropped on.
         * @param x      The <code>left</code> offset of the View that is being dragged. This value
         *               includes the movement caused by the user.
         * @param y      The <code>top</code> offset of the View that is being dragged. This value
         *               includes the movement caused by the user.
         */
        public void prepareForDrop(View view, View target, int x, int y);
    }

    /**
     * This class is the contract between ItemTouchHelper and your application. It lets you control
     * which touch behaviors are enabled per each ViewHolder and also receive callbacks when user
     * performs these actions.
     * <p>
     * To control which actions user can take on each view, you should override
     * {@link #getMovementFlags(RecyclerView, ViewHolder)} and return appropriate set
     * of direction flags. ({@link #LEFT}, {@link #RIGHT}, {@link #START}, {@link #END},
     * {@link #UP}, {@link #DOWN}). You can use
     * {@link #makeMovementFlags(int, int)} to easily construct it. Alternatively, you can use
     * {@link SimpleCallback}.
     * <p>
     * If user drags an item, ItemTouchHelper will call
     * {@link Callback#onMove(RecyclerView, ViewHolder, ViewHolder)
     * onMove(recyclerView, dragged, target)}.
     * Upon receiving this callback, you should move the item from the old position
     * ({@code dragged.getAdapterPosition()}) to new position ({@code target.getAdapterPosition()})
     * in your adapter and also call {@link RecyclerView.Adapter#notifyItemMoved(int, int)}.
     * To control where a View can be dropped, you can override
     * {@link #canDropOver(RecyclerView, ViewHolder, ViewHolder)}. When a
     * dragging View overlaps multiple other views, Callback chooses the closest View with which
     * dragged View might have changed positions. Although this approach works for many use cases,
     * if you have a custom LayoutManager, you can override
     * {@link #chooseDropTarget(ViewHolder, java.util.List, int, int)} to select a
     * custom drop target.
     * <p>
     * When a View is swiped, ItemTouchHelper animates it until it goes out of bounds, then calls
     * {@link #onSwiped(ViewHolder, int)}. At this point, you should update your
     * adapter (e.g. remove the item) and call related Adapter#notify event.
     */
    @SuppressWarnings("UnusedParameters")
    public abstract static class Callback {

        public static final int DEFAULT_DRAG_ANIMATION_DURATION = 200;

        public static final int DEFAULT_SWIPE_ANIMATION_DURATION = 250;

        static final int RELATIVE_DIR_FLAGS = START | END |
                ((START | END) << DIRECTION_FLAG_COUNT) |
                ((START | END) << (2 * DIRECTION_FLAG_COUNT));

        private static final ItemTouchUIUtil sUICallback;

        private static final int ABS_HORIZONTAL_DIR_FLAGS = LEFT | RIGHT |
                ((LEFT | RIGHT) << DIRECTION_FLAG_COUNT) |
                ((LEFT | RIGHT) << (2 * DIRECTION_FLAG_COUNT));

        private static final Interpolator sDragScrollInterpolator = new Interpolator() {
            public float getInterpolation(float t) {
                return t * t * t * t * t;
            }
        };

        private static final Interpolator sDragViewScrollCapInterpolator = new Interpolator() {
            public float getInterpolation(float t) {
                t -= 1.0f;
                return t * t * t * t * t + 1.0f;
            }
        };

        /**
         * Drag scroll speed keeps accelerating until this many milliseconds before being capped.
         */
        private static final long DRAG_SCROLL_ACCELERATION_LIMIT_TIME_MS = 2000;

        private int mCachedMaxScrollSpeed = -1;

        static {
            if (Build.VERSION.SDK_INT >= 21) {
                sUICallback = new ItemTouchUIUtilImpl.Lollipop();
            } else if (Build.VERSION.SDK_INT >= 11) {
                sUICallback = new ItemTouchUIUtilImpl.Honeycomb();
            } else {
                sUICallback = new ItemTouchUIUtilImpl.Gingerbread();
            }
        }

        /**
         * Returns the {@link ItemTouchUIUtil} that is used by the {@link Callback} class for visual
         * changes on Views in response to user interactions. {@link ItemTouchUIUtil} has different
         * implementations for different platform versions.
         * <p>
         * By default, {@link Callback} applies these changes on
         * {@link RecyclerView.ViewHolder#itemView}.
         * <p>
         * For example, if you have a use case where you only want the text to move when user
         * swipes over the view, you can do the following:
         * <pre>
         *     public void clearView(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder){
         *         getDefaultUIUtil().clearView(((ItemTouchViewHolder) viewHolder).textView);
         *     }
         *     public void onSelectedChanged(RecyclerView.ViewHolder viewHolder, int actionState) {
         *         if (viewHolder != null){
         *             getDefaultUIUtil().onSelected(((ItemTouchViewHolder) viewHolder).textView);
         *         }
         *     }
         *     public void onChildDraw(Canvas c, RecyclerView recyclerView,
         *             RecyclerView.ViewHolder viewHolder, float dX, float dY, int actionState,
         *             boolean isCurrentlyActive) {
         *         getDefaultUIUtil().onDraw(c, recyclerView,
         *                 ((ItemTouchViewHolder) viewHolder).textView, dX, dY,
         *                 actionState, isCurrentlyActive);
         *         return true;
         *     }
         *     public void onChildDrawOver(Canvas c, RecyclerView recyclerView,
         *             RecyclerView.ViewHolder viewHolder, float dX, float dY, int actionState,
         *             boolean isCurrentlyActive) {
         *         getDefaultUIUtil().onDrawOver(c, recyclerView,
         *                 ((ItemTouchViewHolder) viewHolder).textView, dX, dY,
         *                 actionState, isCurrentlyActive);
         *         return true;
         *     }
         * </pre>
         *
         * @return The {@link ItemTouchUIUtil} instance that is used by the {@link Callback}
         */
        public static ItemTouchUIUtil getDefaultUIUtil() {
            return sUICallback;
        }

        /**
         * Replaces a movement direction with its relative version by taking layout direction into
         * account.
         *
         * @param flags           The flag value that include any number of movement flags.
         * @param layoutDirection The layout direction of the View. Can be obtained from
         *                        {@link ViewCompat#getLayoutDirection(android.view.View)}.
         * @return Updated flags which uses relative flags ({@link #START}, {@link #END}) instead
         * of {@link #LEFT}, {@link #RIGHT}.
         * @see #convertToAbsoluteDirection(int, int)
         */
        public static int convertToRelativeDirection(int flags, int layoutDirection) {
            int masked = flags & ABS_HORIZONTAL_DIR_FLAGS;
            if (masked == 0) {
                return flags;// does not have any abs flags, good.
            }
            flags &= ~masked; //remove left / right.
            if (layoutDirection == ViewCompat.LAYOUT_DIRECTION_LTR) {
                // no change. just OR with 2 bits shifted mask and return
                flags |= masked << 2; // START is 2 bits after LEFT, END is 2 bits after RIGHT.
                return flags;
            } else {
                // add RIGHT flag as START
                flags |= ((masked << 1) & ~ABS_HORIZONTAL_DIR_FLAGS);
                // first clean RIGHT bit then add LEFT flag as END
                flags |= ((masked << 1) & ABS_HORIZONTAL_DIR_FLAGS) << 2;
            }
            return flags;
        }

        /**
         * Convenience method to create movement flags.
         * <p>
         * For instance, if you want to let your items be drag & dropped vertically and swiped
         * left to be dismissed, you can call this method with:
         * <code>makeMovementFlags(UP | DOWN, LEFT);</code>
         *
         * @param dragFlags  The directions in which the item can be dragged.
         * @param swipeFlags The directions in which the item can be swiped.
         * @return Returns an integer composed of the given drag and swipe flags.
         */
        public static int makeMovementFlags(int dragFlags, int swipeFlags) {
            return makeFlag(ACTION_STATE_IDLE, swipeFlags | dragFlags) |
                    makeFlag(ACTION_STATE_SWIPE, swipeFlags) | makeFlag(ACTION_STATE_DRAG,
                    dragFlags);
        }

        /**
         * Shifts the given direction flags to the offset of the given action state.
         *
         * @param actionState The action state you want to get flags in. Should be one of
         *                    {@link #ACTION_STATE_IDLE}, {@link #ACTION_STATE_SWIPE} or
         *                    {@link #ACTION_STATE_DRAG}.
         * @param directions  The direction flags. Can be composed from {@link #UP}, {@link #DOWN},
         *                    {@link #RIGHT}, {@link #LEFT} {@link #START} and {@link #END}.
         * @return And integer that represents the given directions in the provided actionState.
         */
        public static int makeFlag(int actionState, int directions) {
            return directions << (actionState * DIRECTION_FLAG_COUNT);
        }

        /**
         * Should return a composite flag which defines the enabled move directions in each state
         * (idle, swiping, dragging).
         * <p>
         * Instead of composing this flag manually, you can use {@link #makeMovementFlags(int,
         * int)}
         * or {@link #makeFlag(int, int)}.
         * <p>
         * This flag is composed of 3 sets of 8 bits, where first 8 bits are for IDLE state, next
         * 8 bits are for SWIPE state and third 8 bits are for DRAG state.
         * Each 8 bit sections can be constructed by simply OR'ing direction flags defined in
         * {@link ItemTouchHelper}.
         * <p>
         * For example, if you want it to allow swiping LEFT and RIGHT but only allow starting to
         * swipe by swiping RIGHT, you can return:
         * <pre>
         *      makeFlag(ACTION_STATE_IDLE, RIGHT) | makeFlag(ACTION_STATE_SWIPE, LEFT | RIGHT);
         * </pre>
         * This means, allow right movement while IDLE and allow right and left movement while
         * swiping.
         *
         * @param recyclerView The RecyclerView to which ItemTouchHelper is attached.
         * @param viewHolder   The ViewHolder for which the movement information is necessary.
         * @return flags specifying which movements are allowed on this ViewHolder.
         * @see #makeMovementFlags(int, int)
         * @see #makeFlag(int, int)
         */
        public abstract int getMovementFlags(RecyclerView recyclerView,
                ViewHolder viewHolder);

        /**
         * Converts a given set of flags to absolution direction which means {@link #START} and
         * {@link #END} are replaced with {@link #LEFT} and {@link #RIGHT} depending on the layout
         * direction.
         *
         * @param flags           The flag value that include any number of movement flags.
         * @param layoutDirection The layout direction of the RecyclerView.
         * @return Updated flags which includes only absolute direction values.
         */
        public int convertToAbsoluteDirection(int flags, int layoutDirection) {
            int masked = flags & RELATIVE_DIR_FLAGS;
            if (masked == 0) {
                return flags;// does not have any relative flags, good.
            }
            flags &= ~masked; //remove start / end
            if (layoutDirection == ViewCompat.LAYOUT_DIRECTION_LTR) {
                // no change. just OR with 2 bits shifted mask and return
                flags |= masked >> 2; // START is 2 bits after LEFT, END is 2 bits after RIGHT.
                return flags;
            } else {
                // add START flag as RIGHT
                flags |= ((masked >> 1) & ~RELATIVE_DIR_FLAGS);
                // first clean start bit then add END flag as LEFT
                flags |= ((masked >> 1) & RELATIVE_DIR_FLAGS) >> 2;
            }
            return flags;
        }

        final int getAbsoluteMovementFlags(RecyclerView recyclerView,
                ViewHolder viewHolder) {
            final int flags = getMovementFlags(recyclerView, viewHolder);
            return convertToAbsoluteDirection(flags, ViewCompat.getLayoutDirection(recyclerView));
        }

        private boolean hasDragFlag(RecyclerView recyclerView, ViewHolder viewHolder) {
            final int flags = getAbsoluteMovementFlags(recyclerView, viewHolder);
            return (flags & ACTION_MODE_DRAG_MASK) != 0;
        }

        private boolean hasSwipeFlag(RecyclerView recyclerView,
                ViewHolder viewHolder) {
            final int flags = getAbsoluteMovementFlags(recyclerView, viewHolder);
            return (flags & ACTION_MODE_SWIPE_MASK) != 0;
        }

        /**
         * Return true if the current ViewHolder can be dropped over the the target ViewHolder.
         * <p>
         * This method is used when selecting drop target for the dragged View. After Views are
         * eliminated either via bounds check or via this method, resulting set of views will be
         * passed to {@link #chooseDropTarget(ViewHolder, java.util.List, int, int)}.
         * <p>
         * Default implementation returns true.
         *
         * @param recyclerView The RecyclerView to which ItemTouchHelper is attached to.
         * @param current      The ViewHolder that user is dragging.
         * @param target       The ViewHolder which is below the dragged ViewHolder.
         * @return True if the dragged ViewHolder can be replaced with the target ViewHolder, false
         * otherwise.
         */
        public boolean canDropOver(RecyclerView recyclerView, ViewHolder current,
                ViewHolder target) {
            return true;
        }

        /**
         * Called when ItemTouchHelper wants to move the dragged item from its old position to
         * the new position.
         * <p>
         * If this method returns true, ItemTouchHelper assumes {@code viewHolder} has been moved
         * to the adapter position of {@code target} ViewHolder
         * ({@link ViewHolder#getAdapterPosition()
         * ViewHolder#getAdapterPosition()}).
         * <p>
         * If you don't support drag & drop, this method will never be called.
         *
         * @param recyclerView The RecyclerView to which ItemTouchHelper is attached to.
         * @param viewHolder   The ViewHolder which is being dragged by the user.
         * @param target       The ViewHolder over which the currently active item is being
         *                     dragged.
         * @return True if the {@code viewHolder} has been moved to the adapter position of
         * {@code target}.
         * @see #onMoved(RecyclerView, ViewHolder, int, ViewHolder, int, int, int)
         */
        public abstract boolean onMove(RecyclerView recyclerView,
                ViewHolder viewHolder, ViewHolder target);

        /**
         * Returns whether ItemTouchHelper should start a drag and drop operation if an item is
         * long pressed.
         * <p>
         * Default value returns true but you may want to disable this if you want to start
         * dragging on a custom view touch using {@link #startDrag(ViewHolder)}.
         *
         * @return True if ItemTouchHelper should start dragging an item when it is long pressed,
         * false otherwise. Default value is <code>true</code>.
         * @see #startDrag(ViewHolder)
         */
        public boolean isLongPressDragEnabled() {
            return true;
        }

        /**
         * Returns whether ItemTouchHelper should start a swipe operation if a pointer is swiped
         * over the View.
         * <p>
         * Default value returns true but you may want to disable this if you want to start
         * swiping on a custom view touch using {@link #startSwipe(ViewHolder)}.
         *
         * @return True if ItemTouchHelper should start swiping an item when user swipes a pointer
         * over the View, false otherwise. Default value is <code>true</code>.
         * @see #startSwipe(ViewHolder)
         */
        public boolean isItemViewSwipeEnabled() {
            return true;
        }

        /**
         * When finding views under a dragged view, by default, ItemTouchHelper searches for views
         * that overlap with the dragged View. By overriding this method, you can extend or shrink
         * the search box.
         *
         * @return The extra margin to be added to the hit box of the dragged View.
         */
        public int getBoundingBoxMargin() {
            return 0;
        }

        /**
         * Returns the fraction that the user should move the View to be considered as swiped.
         * The fraction is calculated with respect to RecyclerView's bounds.
         * <p>
         * Default value is .5f, which means, to swipe a View, user must move the View at least
         * half of RecyclerView's width or height, depending on the swipe direction.
         *
         * @param viewHolder The ViewHolder that is being dragged.
         * @return A float value that denotes the fraction of the View size. Default value
         * is .5f .
         */
        public float getSwipeThreshold(ViewHolder viewHolder) {
            return .5f;
        }

        /**
         * Returns the fraction that the user should move the View to be considered as it is
         * dragged. After a view is moved this amount, ItemTouchHelper starts checking for Views
         * below it for a possible drop.
         *
         * @param viewHolder The ViewHolder that is being dragged.
         * @return A float value that denotes the fraction of the View size. Default value is
         * .5f .
         */
        public float getMoveThreshold(ViewHolder viewHolder) {
            return .5f;
        }

        /**
         * Defines the minimum velocity which will be considered as a swipe action by the user.
         * <p>
         * You can increase this value to make it harder to swipe or decrease it to make it easier.
         * Keep in mind that ItemTouchHelper also checks the perpendicular velocity and makes sure
         * current direction velocity is larger then the perpendicular one. Otherwise, user's
         * movement is ambiguous. You can change the threshold via
         * {@link #getSwipeVelocityThreshold(float)}.
         * <p>
         * The velocity is calculated in pixels per second.
         * <p>
         * The default framework value is passed as a parameter so that you can modify it with a
         * multiplier.
         *
         * @param defaultValue The default value (in pixels per second) used by the ItemTouchHelper.
         *
         * @return The minimum swipe velocity. The default implementation returns the
         * <code>defaultValue</code> parameter.
         *
         * @see #getSwipeVelocityThreshold(float)
         * @see #getSwipeThreshold(ViewHolder)
         */
        public float getSwipeEscapeVelocity(float defaultValue) {
            return defaultValue;
        }

        /**
         * Defines the maximum velocity ItemTouchHelper will ever calculate for pointer movements.
         * <p>
         * To consider a movement as swipe, ItemTouchHelper requires it to be larger than the
         * perpendicular movement. If both directions reach to the max threshold, none of them will
         * be considered as a swipe because it is usually an indication that user rather tried to
         * scroll then swipe.
         * <p>
         * The velocity is calculated in pixels per second.
         * <p>
         * You can customize this behavior by changing this method. If you increase the value, it
         * will be easier for the user to swipe diagonally and if you decrease the value, user will
         * need to make a rather straight finger movement to trigger a swipe.
         *
         * @param defaultValue The default value(in pixels per second) used by the ItemTouchHelper.
         *
         * @return The velocity cap for pointer movements. The default implementation returns the
         * <code>defaultValue</code> parameter.
         *
         * @see #getSwipeEscapeVelocity(float)
         */
        public float getSwipeVelocityThreshold(float defaultValue) {
            return defaultValue;
        }

        /**
         * Called by ItemTouchHelper to select a drop target from the list of ViewHolders that
         * are under the dragged View.
         * <p>
         * Default implementation filters the View with which dragged item have changed position
         * in the drag direction. For instance, if the view is dragged UP, it compares the
         * <code>view.getTop()</code> of the two views before and after drag started. If that value
         * is different, the target view passes the filter.
         * <p>
         * Among these Views which pass the test, the one closest to the dragged view is chosen.
         * <p>
         * This method is called on the main thread every time user moves the View. If you want to
         * override it, make sure it does not do any expensive operations.
         *
         * @param selected    The ViewHolder being dragged by the user.
         * @param dropTargets The list of ViewHolder that are under the dragged View and
         *                    candidate as a drop.
         * @param curX        The updated left value of the dragged View after drag translations
         *                    are applied. This value does not include margins added by
         *                    {@link RecyclerView.ItemDecoration}s.
         * @param curY        The updated top value of the dragged View after drag translations
         *                    are applied. This value does not include margins added by
         *                    {@link RecyclerView.ItemDecoration}s.
         * @return A ViewHolder to whose position the dragged ViewHolder should be
         * moved to.
         */
        public ViewHolder chooseDropTarget(ViewHolder selected,
                List<ViewHolder> dropTargets, int curX, int curY) {
            int right = curX + selected.itemView.getWidth();
            int bottom = curY + selected.itemView.getHeight();
            ViewHolder winner = null;
            int winnerScore = -1;
            final int dx = curX - selected.itemView.getLeft();
            final int dy = curY - selected.itemView.getTop();
            final int targetsSize = dropTargets.size();
            for (int i = 0; i < targetsSize; i++) {
                final ViewHolder target = dropTargets.get(i);
                if (dx > 0) {
                    int diff = target.itemView.getRight() - right;
                    if (diff < 0 && target.itemView.getRight() > selected.itemView.getRight()) {
                        final int score = Math.abs(diff);
                        if (score > winnerScore) {
                            winnerScore = score;
                            winner = target;
                        }
                    }
                }
                if (dx < 0) {
                    int diff = target.itemView.getLeft() - curX;
                    if (diff > 0 && target.itemView.getLeft() < selected.itemView.getLeft()) {
                        final int score = Math.abs(diff);
                        if (score > winnerScore) {
                            winnerScore = score;
                            winner = target;
                        }
                    }
                }
                if (dy < 0) {
                    int diff = target.itemView.getTop() - curY;
                    if (diff > 0 && target.itemView.getTop() < selected.itemView.getTop()) {
                        final int score = Math.abs(diff);
                        if (score > winnerScore) {
                            winnerScore = score;
                            winner = target;
                        }
                    }
                }

                if (dy > 0) {
                    int diff = target.itemView.getBottom() - bottom;
                    if (diff < 0 && target.itemView.getBottom() > selected.itemView.getBottom()) {
                        final int score = Math.abs(diff);
                        if (score > winnerScore) {
                            winnerScore = score;
                            winner = target;
                        }
                    }
                }
            }
            return winner;
        }

        /**
         * Called when a ViewHolder is swiped by the user.
         * <p>
         * If you are returning relative directions ({@link #START} , {@link #END}) from the
         * {@link #getMovementFlags(RecyclerView, ViewHolder)} method, this method
         * will also use relative directions. Otherwise, it will use absolute directions.
         * <p>
         * If you don't support swiping, this method will never be called.
         * <p>
         * ItemTouchHelper will keep a reference to the View until it is detached from
         * RecyclerView.
         * As soon as it is detached, ItemTouchHelper will call
         * {@link #clearView(RecyclerView, ViewHolder)}.
         *
         * @param viewHolder The ViewHolder which has been swiped by the user.
         * @param direction  The direction to which the ViewHolder is swiped. It is one of
         *                   {@link #UP}, {@link #DOWN},
         *                   {@link #LEFT} or {@link #RIGHT}. If your
         *                   {@link #getMovementFlags(RecyclerView, ViewHolder)}
         *                   method
         *                   returned relative flags instead of {@link #LEFT} / {@link #RIGHT};
         *                   `direction` will be relative as well. ({@link #START} or {@link
         *                   #END}).
         */
        public abstract void onSwiped(ViewHolder viewHolder, int direction);

        /**
         * Called when the ViewHolder swiped or dragged by the ItemTouchHelper is changed.
         * <p/>
         * If you override this method, you should call super.
         *
         * @param viewHolder  The new ViewHolder that is being swiped or dragged. Might be null if
         *                    it is cleared.
         * @param actionState One of {@link ItemTouchHelper#ACTION_STATE_IDLE},
         *                    {@link ItemTouchHelper#ACTION_STATE_SWIPE} or
         *                    {@link ItemTouchHelper#ACTION_STATE_DRAG}.
         *
         * @see #clearView(RecyclerView, RecyclerView.ViewHolder)
         */
        public void onSelectedChanged(ViewHolder viewHolder, int actionState) {
            if (viewHolder != null) {
                sUICallback.onSelected(viewHolder.itemView);
            }
        }

        private int getMaxDragScroll(RecyclerView recyclerView) {
            if (mCachedMaxScrollSpeed == -1) {
                mCachedMaxScrollSpeed = recyclerView.getResources().getDimensionPixelSize(
                        R.dimen.item_touch_helper_max_drag_scroll_per_frame);
            }
            return mCachedMaxScrollSpeed;
        }

        /**
         * Called when {@link #onMove(RecyclerView, ViewHolder, ViewHolder)} returns true.
         * <p>
         * ItemTouchHelper does not create an extra Bitmap or View while dragging, instead, it
         * modifies the existing View. Because of this reason, it is important that the View is
         * still part of the layout after it is moved. This may not work as intended when swapped
         * Views are close to RecyclerView bounds or there are gaps between them (e.g. other Views
         * which were not eligible for dropping over).
         * <p>
         * This method is responsible to give necessary hint to the LayoutManager so that it will
         * keep the View in visible area. For example, for LinearLayoutManager, this is as simple
         * as calling {@link LinearLayoutManager#scrollToPositionWithOffset(int, int)}.
         *
         * Default implementation calls {@link RecyclerView#scrollToPosition(int)} if the View's
         * new position is likely to be out of bounds.
         * <p>
         * It is important to ensure the ViewHolder will stay visible as otherwise, it might be
         * removed by the LayoutManager if the move causes the View to go out of bounds. In that
         * case, drag will end prematurely.
         *
         * @param recyclerView The RecyclerView controlled by the ItemTouchHelper.
         * @param viewHolder   The ViewHolder under user's control.
         * @param fromPos      The previous adapter position of the dragged item (before it was
         *                     moved).
         * @param target       The ViewHolder on which the currently active item has been dropped.
         * @param toPos        The new adapter position of the dragged item.
         * @param x            The updated left value of the dragged View after drag translations
         *                     are applied. This value does not include margins added by
         *                     {@link RecyclerView.ItemDecoration}s.
         * @param y            The updated top value of the dragged View after drag translations
         *                     are applied. This value does not include margins added by
         *                     {@link RecyclerView.ItemDecoration}s.
         */
        public void onMoved(final RecyclerView recyclerView,
                final ViewHolder viewHolder, int fromPos, final ViewHolder target, int toPos, int x,
                int y) {
            final RecyclerView.LayoutManager layoutManager = recyclerView.getLayoutManager();
            if (layoutManager instanceof ViewDropHandler) {
                ((ViewDropHandler) layoutManager).prepareForDrop(viewHolder.itemView,
                        target.itemView, x, y);
                return;
            }

            // if layout manager cannot handle it, do some guesswork
            if (layoutManager.canScrollHorizontally()) {
                final int minLeft = layoutManager.getDecoratedLeft(target.itemView);
                if (minLeft <= recyclerView.getPaddingLeft()) {
                    recyclerView.scrollToPosition(toPos);
                }
                final int maxRight = layoutManager.getDecoratedRight(target.itemView);
                if (maxRight >= recyclerView.getWidth() - recyclerView.getPaddingRight()) {
                    recyclerView.scrollToPosition(toPos);
                }
            }

            if (layoutManager.canScrollVertically()) {
                final int minTop = layoutManager.getDecoratedTop(target.itemView);
                if (minTop <= recyclerView.getPaddingTop()) {
                    recyclerView.scrollToPosition(toPos);
                }
                final int maxBottom = layoutManager.getDecoratedBottom(target.itemView);
                if (maxBottom >= recyclerView.getHeight() - recyclerView.getPaddingBottom()) {
                    recyclerView.scrollToPosition(toPos);
                }
            }
        }

        private void onDraw(Canvas c, RecyclerView parent, ViewHolder selected,
                List<ItemTouchHelper.RecoverAnimation> recoverAnimationList,
                int actionState, float dX, float dY) {
            final int recoverAnimSize = recoverAnimationList.size();
            for (int i = 0; i < recoverAnimSize; i++) {
                final ItemTouchHelper.RecoverAnimation anim = recoverAnimationList.get(i);
                anim.update();
                final int count = c.save();
                onChildDraw(c, parent, anim.mViewHolder, anim.mX, anim.mY, anim.mActionState,
                        false);
                c.restoreToCount(count);
            }
            if (selected != null) {
                final int count = c.save();
                onChildDraw(c, parent, selected, dX, dY, actionState, true);
                c.restoreToCount(count);
            }
        }

        private void onDrawOver(Canvas c, RecyclerView parent, ViewHolder selected,
                List<ItemTouchHelper.RecoverAnimation> recoverAnimationList,
                int actionState, float dX, float dY) {
            final int recoverAnimSize = recoverAnimationList.size();
            for (int i = 0; i < recoverAnimSize; i++) {
                final ItemTouchHelper.RecoverAnimation anim = recoverAnimationList.get(i);
                final int count = c.save();
                onChildDrawOver(c, parent, anim.mViewHolder, anim.mX, anim.mY, anim.mActionState,
                        false);
                c.restoreToCount(count);
            }
            if (selected != null) {
                final int count = c.save();
                onChildDrawOver(c, parent, selected, dX, dY, actionState, true);
                c.restoreToCount(count);
            }
            boolean hasRunningAnimation = false;
            for (int i = recoverAnimSize - 1; i >= 0; i--) {
                final RecoverAnimation anim = recoverAnimationList.get(i);
                if (anim.mEnded && !anim.mIsPendingCleanup) {
                    recoverAnimationList.remove(i);
                } else if (!anim.mEnded) {
                    hasRunningAnimation = true;
                }
            }
            if (hasRunningAnimation) {
                parent.invalidate();
            }
        }

        /**
         * Called by the ItemTouchHelper when the user interaction with an element is over and it
         * also completed its animation.
         * <p>
         * This is a good place to clear all changes on the View that was done in
         * {@link #onSelectedChanged(RecyclerView.ViewHolder, int)},
         * {@link #onChildDraw(Canvas, RecyclerView, ViewHolder, float, float, int,
         * boolean)} or
         * {@link #onChildDrawOver(Canvas, RecyclerView, ViewHolder, float, float, int, boolean)}.
         *
         * @param recyclerView The RecyclerView which is controlled by the ItemTouchHelper.
         * @param viewHolder   The View that was interacted by the user.
         */
        public void clearView(RecyclerView recyclerView, ViewHolder viewHolder) {
            sUICallback.clearView(viewHolder.itemView);
        }

        /**
         * Called by ItemTouchHelper on RecyclerView's onDraw callback.
         * <p>
         * If you would like to customize how your View's respond to user interactions, this is
         * a good place to override.
         * <p>
         * Default implementation translates the child by the given <code>dX</code>,
         * <code>dY</code>.
         * ItemTouchHelper also takes care of drawing the child after other children if it is being
         * dragged. This is done using child re-ordering mechanism. On platforms prior to L, this
         * is
         * achieved via {@link android.view.ViewGroup#getChildDrawingOrder(int, int)} and on L
         * and after, it changes View's elevation value to be greater than all other children.)
         *
         * @param c                 The canvas which RecyclerView is drawing its children
         * @param recyclerView      The RecyclerView to which ItemTouchHelper is attached to
         * @param viewHolder        The ViewHolder which is being interacted by the User or it was
         *                          interacted and simply animating to its original position
         * @param dX                The amount of horizontal displacement caused by user's action
         * @param dY                The amount of vertical displacement caused by user's action
         * @param actionState       The type of interaction on the View. Is either {@link
         *                          #ACTION_STATE_DRAG} or {@link #ACTION_STATE_SWIPE}.
         * @param isCurrentlyActive True if this view is currently being controlled by the user or
         *                          false it is simply animating back to its original state.
         * @see #onChildDrawOver(Canvas, RecyclerView, ViewHolder, float, float, int,
         * boolean)
         */
        public void onChildDraw(Canvas c, RecyclerView recyclerView,
                ViewHolder viewHolder,
                float dX, float dY, int actionState, boolean isCurrentlyActive) {
            sUICallback.onDraw(c, recyclerView, viewHolder.itemView, dX, dY, actionState,
                    isCurrentlyActive);
        }

        /**
         * Called by ItemTouchHelper on RecyclerView's onDraw callback.
         * <p>
         * If you would like to customize how your View's respond to user interactions, this is
         * a good place to override.
         * <p>
         * Default implementation translates the child by the given <code>dX</code>,
         * <code>dY</code>.
         * ItemTouchHelper also takes care of drawing the child after other children if it is being
         * dragged. This is done using child re-ordering mechanism. On platforms prior to L, this
         * is
         * achieved via {@link android.view.ViewGroup#getChildDrawingOrder(int, int)} and on L
         * and after, it changes View's elevation value to be greater than all other children.)
         *
         * @param c                 The canvas which RecyclerView is drawing its children
         * @param recyclerView      The RecyclerView to which ItemTouchHelper is attached to
         * @param viewHolder        The ViewHolder which is being interacted by the User or it was
         *                          interacted and simply animating to its original position
         * @param dX                The amount of horizontal displacement caused by user's action
         * @param dY                The amount of vertical displacement caused by user's action
         * @param actionState       The type of interaction on the View. Is either {@link
         *                          #ACTION_STATE_DRAG} or {@link #ACTION_STATE_SWIPE}.
         * @param isCurrentlyActive True if this view is currently being controlled by the user or
         *                          false it is simply animating back to its original state.
         * @see #onChildDrawOver(Canvas, RecyclerView, ViewHolder, float, float, int,
         * boolean)
         */
        public void onChildDrawOver(Canvas c, RecyclerView recyclerView,
                ViewHolder viewHolder,
                float dX, float dY, int actionState, boolean isCurrentlyActive) {
            sUICallback.onDrawOver(c, recyclerView, viewHolder.itemView, dX, dY, actionState,
                    isCurrentlyActive);
        }

        /**
         * Called by the ItemTouchHelper when user action finished on a ViewHolder and now the View
         * will be animated to its final position.
         * <p>
         * Default implementation uses ItemAnimator's duration values. If
         * <code>animationType</code> is {@link #ANIMATION_TYPE_DRAG}, it returns
         * {@link RecyclerView.ItemAnimator#getMoveDuration()}, otherwise, it returns
         * {@link RecyclerView.ItemAnimator#getRemoveDuration()}. If RecyclerView does not have
         * any {@link RecyclerView.ItemAnimator} attached, this method returns
         * {@code DEFAULT_DRAG_ANIMATION_DURATION} or {@code DEFAULT_SWIPE_ANIMATION_DURATION}
         * depending on the animation type.
         *
         * @param recyclerView  The RecyclerView to which the ItemTouchHelper is attached to.
         * @param animationType The type of animation. Is one of {@link #ANIMATION_TYPE_DRAG},
         *                      {@link #ANIMATION_TYPE_SWIPE_CANCEL} or
         *                      {@link #ANIMATION_TYPE_SWIPE_SUCCESS}.
         * @param animateDx     The horizontal distance that the animation will offset
         * @param animateDy     The vertical distance that the animation will offset
         * @return The duration for the animation
         */
        public long getAnimationDuration(RecyclerView recyclerView, int animationType,
                float animateDx, float animateDy) {
            final RecyclerView.ItemAnimator itemAnimator = recyclerView.getItemAnimator();
            if (itemAnimator == null) {
                return animationType == ANIMATION_TYPE_DRAG ? DEFAULT_DRAG_ANIMATION_DURATION
                        : DEFAULT_SWIPE_ANIMATION_DURATION;
            } else {
                return animationType == ANIMATION_TYPE_DRAG ? itemAnimator.getMoveDuration()
                        : itemAnimator.getRemoveDuration();
            }
        }

        /**
         * Called by the ItemTouchHelper when user is dragging a view out of bounds.
         * <p>
         * You can override this method to decide how much RecyclerView should scroll in response
         * to this action. Default implementation calculates a value based on the amount of View
         * out of bounds and the time it spent there. The longer user keeps the View out of bounds,
         * the faster the list will scroll. Similarly, the larger portion of the View is out of
         * bounds, the faster the RecyclerView will scroll.
         *
         * @param recyclerView        The RecyclerView instance to which ItemTouchHelper is attached
         *                            to.
         * @param viewSize            The total size of the View in scroll direction, excluding
         *                            item decorations.
         * @param viewSizeOutOfBounds The total size of the View that is out of bounds. This value
         *                            is negative if the View is dragged towards left or top edge.
         * @param totalSize           The total size of RecyclerView in the scroll direction.
         * @param msSinceStartScroll  The time passed since View is kept out of bounds.
         *
         * @return The amount that RecyclerView should scroll. Keep in mind that this value will
         * be passed to {@link RecyclerView#scrollBy(int, int)} method.
         */
        public int interpolateOutOfBoundsScroll(RecyclerView recyclerView,
                int viewSize, int viewSizeOutOfBounds,
                int totalSize, long msSinceStartScroll) {
            final int maxScroll = getMaxDragScroll(recyclerView);
            final int absOutOfBounds = Math.abs(viewSizeOutOfBounds);
            final int direction = (int) Math.signum(viewSizeOutOfBounds);
            // might be negative if other direction
            float outOfBoundsRatio = Math.min(1f, 1f * absOutOfBounds / viewSize);
            final int cappedScroll = (int) (direction * maxScroll *
                    sDragViewScrollCapInterpolator.getInterpolation(outOfBoundsRatio));
            final float timeRatio;
            if (msSinceStartScroll > DRAG_SCROLL_ACCELERATION_LIMIT_TIME_MS) {
                timeRatio = 1f;
            } else {
                timeRatio = (float) msSinceStartScroll / DRAG_SCROLL_ACCELERATION_LIMIT_TIME_MS;
            }
            final int value = (int) (cappedScroll * sDragScrollInterpolator
                    .getInterpolation(timeRatio));
            if (value == 0) {
                return viewSizeOutOfBounds > 0 ? 1 : -1;
            }
            return value;
        }
    }

    /**
     * A simple wrapper to the default Callback which you can construct with drag and swipe
     * directions and this class will handle the flag callbacks. You should still override onMove
     * or
     * onSwiped depending on your use case.
     *
     * <pre>
     * ItemTouchHelper mIth = new ItemTouchHelper(
     *     new ItemTouchHelper.SimpleCallback(ItemTouchHelper.UP | ItemTouchHelper.DOWN,
     *         ItemTouchHelper.LEFT) {
     *         public abstract boolean onMove(RecyclerView recyclerView,
     *             ViewHolder viewHolder, ViewHolder target) {
     *             final int fromPos = viewHolder.getAdapterPosition();
     *             final int toPos = viewHolder.getAdapterPosition();
     *             // move item in `fromPos` to `toPos` in adapter.
     *             return true;// true if moved, false otherwise
     *         }
     *         public void onSwiped(ViewHolder viewHolder, int direction) {
     *             // remove from adapter
     *         }
     * });
     * </pre>
     */
    public abstract static class SimpleCallback extends Callback {

        private int mDefaultSwipeDirs;

        private int mDefaultDragDirs;

        /**
         * Creates a Callback for the given drag and swipe allowance. These values serve as
         * defaults
         * and if you want to customize behavior per ViewHolder, you can override
         * {@link #getSwipeDirs(RecyclerView, ViewHolder)}
         * and / or {@link #getDragDirs(RecyclerView, ViewHolder)}.
         *
         * @param dragDirs  Binary OR of direction flags in which the Views can be dragged. Must be
         *                  composed of {@link #LEFT}, {@link #RIGHT}, {@link #START}, {@link
         *                  #END},
         *                  {@link #UP} and {@link #DOWN}.
         * @param swipeDirs Binary OR of direction flags in which the Views can be swiped. Must be
         *                  composed of {@link #LEFT}, {@link #RIGHT}, {@link #START}, {@link
         *                  #END},
         *                  {@link #UP} and {@link #DOWN}.
         */
        public SimpleCallback(int dragDirs, int swipeDirs) {
            mDefaultSwipeDirs = swipeDirs;
            mDefaultDragDirs = dragDirs;
        }

        /**
         * Updates the default swipe directions. For example, you can use this method to toggle
         * certain directions depending on your use case.
         *
         * @param defaultSwipeDirs Binary OR of directions in which the ViewHolders can be swiped.
         */
        public void setDefaultSwipeDirs(int defaultSwipeDirs) {
            mDefaultSwipeDirs = defaultSwipeDirs;
        }

        /**
         * Updates the default drag directions. For example, you can use this method to toggle
         * certain directions depending on your use case.
         *
         * @param defaultDragDirs Binary OR of directions in which the ViewHolders can be dragged.
         */
        public void setDefaultDragDirs(int defaultDragDirs) {
            mDefaultDragDirs = defaultDragDirs;
        }

        /**
         * Returns the swipe directions for the provided ViewHolder.
         * Default implementation returns the swipe directions that was set via constructor or
         * {@link #setDefaultSwipeDirs(int)}.
         *
         * @param recyclerView The RecyclerView to which the ItemTouchHelper is attached to.
         * @param viewHolder   The RecyclerView for which the swipe drection is queried.
         * @return A binary OR of direction flags.
         */
        public int getSwipeDirs(RecyclerView recyclerView, ViewHolder viewHolder) {
            return mDefaultSwipeDirs;
        }

        /**
         * Returns the drag directions for the provided ViewHolder.
         * Default implementation returns the drag directions that was set via constructor or
         * {@link #setDefaultDragDirs(int)}.
         *
         * @param recyclerView The RecyclerView to which the ItemTouchHelper is attached to.
         * @param viewHolder   The RecyclerView for which the swipe drection is queried.
         * @return A binary OR of direction flags.
         */
        public int getDragDirs(RecyclerView recyclerView, ViewHolder viewHolder) {
            return mDefaultDragDirs;
        }

        @Override
        public int getMovementFlags(RecyclerView recyclerView, ViewHolder viewHolder) {
            return makeMovementFlags(getDragDirs(recyclerView, viewHolder),
                    getSwipeDirs(recyclerView, viewHolder));
        }
    }

    private class ItemTouchHelperGestureListener extends GestureDetector.SimpleOnGestureListener {

        @Override
        public boolean onDown(MotionEvent e) {
            return true;
        }

        @Override
        public void onLongPress(MotionEvent e) {
            View child = findChildView(e);
            if (child != null) {
                ViewHolder vh = mRecyclerView.getChildViewHolder(child);
                if (vh != null) {
                    if (!mCallback.hasDragFlag(mRecyclerView, vh)) {
                        return;
                    }
                    int pointerId = MotionEventCompat.getPointerId(e, 0);
                    // Long press is deferred.
                    // Check w/ active pointer id to avoid selecting after motion
                    // event is canceled.
                    if (pointerId == mActivePointerId) {
                        final int index = MotionEventCompat
                                .findPointerIndex(e, mActivePointerId);
                        final float x = MotionEventCompat.getX(e, index);
                        final float y = MotionEventCompat.getY(e, index);
                        mInitialTouchX = x;
                        mInitialTouchY = y;
                        mDx = mDy = 0f;
                        if (DEBUG) {
                            Log.d(TAG,
                                    "onlong press: x:" + mInitialTouchX + ",y:" + mInitialTouchY);
                        }
                        if (mCallback.isLongPressDragEnabled()) {
                            select(vh, ACTION_STATE_DRAG);
                        }
                    }
                }
            }
        }
    }

    private class RecoverAnimation implements AnimatorListenerCompat {

        final float mStartDx;

        final float mStartDy;

        final float mTargetX;

        final float mTargetY;

        final ViewHolder mViewHolder;

        final int mActionState;

        private final ValueAnimatorCompat mValueAnimator;

        private final int mAnimationType;

        public boolean mIsPendingCleanup;

        float mX;

        float mY;

        // if user starts touching a recovering view, we put it into interaction mode again,
        // instantly.
        boolean mOverridden = false;

        private boolean mEnded = false;

        private float mFraction;

        public RecoverAnimation(ViewHolder viewHolder, int animationType,
                int actionState, float startDx, float startDy, float targetX, float targetY) {
            mActionState = actionState;
            mAnimationType = animationType;
            mViewHolder = viewHolder;
            mStartDx = startDx;
            mStartDy = startDy;
            mTargetX = targetX;
            mTargetY = targetY;
            mValueAnimator = AnimatorCompatHelper.emptyValueAnimator();
            mValueAnimator.addUpdateListener(
                    new AnimatorUpdateListenerCompat() {
                        @Override
                        public void onAnimationUpdate(ValueAnimatorCompat animation) {
                            setFraction(animation.getAnimatedFraction());
                        }
                    });
            mValueAnimator.setTarget(viewHolder.itemView);
            mValueAnimator.addListener(this);
            setFraction(0f);
        }

        public void setDuration(long duration) {
            mValueAnimator.setDuration(duration);
        }

        public void start() {
            mViewHolder.setIsRecyclable(false);
            mValueAnimator.start();
        }

        public void cancel() {
            mValueAnimator.cancel();
        }

        public void setFraction(float fraction) {
            mFraction = fraction;
        }

        /**
         * We run updates on onDraw method but use the fraction from animator callback.
         * This way, we can sync translate x/y values w/ the animators to avoid one-off frames.
         */
        public void update() {
            if (mStartDx == mTargetX) {
                mX = ViewCompat.getTranslationX(mViewHolder.itemView);
            } else {
                mX = mStartDx + mFraction * (mTargetX - mStartDx);
            }
            if (mStartDy == mTargetY) {
                mY = ViewCompat.getTranslationY(mViewHolder.itemView);
            } else {
                mY = mStartDy + mFraction * (mTargetY - mStartDy);
            }
        }

        @Override
        public void onAnimationStart(ValueAnimatorCompat animation) {

        }

        @Override
        public void onAnimationEnd(ValueAnimatorCompat animation) {
            if (!mEnded) {
                mViewHolder.setIsRecyclable(true);
            }
            mEnded = true;
        }

        @Override
        public void onAnimationCancel(ValueAnimatorCompat animation) {
            setFraction(1f); //make sure we recover the view's state.
        }

        @Override
        public void onAnimationRepeat(ValueAnimatorCompat animation) {

        }
    }
}