v4/widget/DrawerLayout.java

/*
 * Copyright (C) 2013 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.v4.widget;

import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.PixelFormat;
import android.graphics.drawable.Drawable;
import android.support.v4.view.GravityCompat;
import android.support.v4.view.MotionEventCompat;
import android.support.v4.view.ViewCompat;
import android.util.AttributeSet;
import android.view.Gravity;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;

/**
 * DrawerLayout acts as a top-level container for window content that allows for
 * interactive "drawer" views to be pulled out from the edge of the window.
 *
 * <p>Drawer positioning and layout is controlled using the <code>android:layout_gravity</code>
 * attribute on child views corresponding to </p>
 *
 * <p>As per the Android Design guide, any drawers positioned to the left/start should
 * always contain content for navigating around the application, whereas any drawers
 * positioned to the right/end should always contain actions to take on the current content.
 * This preserves the same navigation left, actions right structure present in the Action Bar
 * and elsewhere.</p>
 */
public class DrawerLayout extends ViewGroup {
    private static final String TAG = "DrawerLayout";

    private static final int INVALID_POINTER = -1;

    /**
     * Indicates that any drawers are in an idle, settled state. No animation is in progress.
     */
    public static final int STATE_IDLE = ViewDragHelper.STATE_IDLE;

    /**
     * Indicates that a drawer is currently being dragged by the user.
     */
    public static final int STATE_DRAGGING = ViewDragHelper.STATE_DRAGGING;

    /**
     * Indicates that a drawer is in the process of settling to a final position.
     */
    public static final int STATE_SETTLING = ViewDragHelper.STATE_SETTLING;

    private static final int MIN_DRAWER_MARGIN = 64; // dp

    private static final int DRAWER_PEEK_DISTANCE = 16; // dp

    private static final int DEFAULT_SCRIM_COLOR = 0x99000000;

    private static final int[] LAYOUT_ATTRS = new int[] {
            android.R.attr.layout_gravity
    };

    private int mMinDrawerMargin;
    private int mDrawerPeekDistance;

    private int mScrimColor = DEFAULT_SCRIM_COLOR;
    private float mScrimOpacity;
    private Paint mScrimPaint = new Paint();

    private final ViewDragHelper mLeftDragger;
    private final ViewDragHelper mRightDragger;
    private int mDrawerState;
    private boolean mInLayout;

    private DrawerListener mListener;

    private float mInitialMotionX;
    private float mInitialMotionY;

    private Drawable mShadowLeft;
    private Drawable mShadowRight;

    /**
     * Listener for monitoring events about drawers.
     */
    public interface DrawerListener {
        /**
         * Called when a drawer's position changes.
         * @param drawerView The child view that was moved
         * @param slideOffset The new offset of this drawer within its range, from 0-1
         */
        public void onDrawerSlide(View drawerView, float slideOffset);

        /**
         * Called when a drawer has settled in a completely open state.
         * The drawer is interactive at this point.
         *
         * @param drawerView Drawer view that is now open
         */
        public void onDrawerOpened(View drawerView);

        /**
         * Called when a drawer has settled in a completely closed state.
         *
         * @param drawerView Drawer view that is now closed
         */
        public void onDrawerClosed(View drawerView);

        /**
         * Called when the drawer motion state changes. The new state will
         * be one of {@link #STATE_IDLE}, {@link #STATE_DRAGGING} or {@link #STATE_SETTLING}.
         *
         * @param newState The new drawer motion state
         */
        public void onDrawerStateChanged(int newState);
    }

    /**
     * Stub/no-op implementations of all methods of {@link DrawerListener}.
     * Override this if you only care about a few of the available callback methods.
     */
    public static abstract class SimpleDrawerListener implements DrawerListener {
        @Override
        public void onDrawerSlide(View drawerView, float slideOffset) {
        }

        @Override
        public void onDrawerOpened(View drawerView) {
        }

        @Override
        public void onDrawerClosed(View drawerView) {
        }

        @Override
        public void onDrawerStateChanged(int newState) {
        }
    }

    public DrawerLayout(Context context) {
        this(context, null);
    }

    public DrawerLayout(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public DrawerLayout(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);

        final float density = getResources().getDisplayMetrics().density;
        mMinDrawerMargin = (int) (MIN_DRAWER_MARGIN * density + 0.5f);
        mDrawerPeekDistance = (int) (DRAWER_PEEK_DISTANCE * density + 0.5f);

        final ViewDragCallback leftCallback = new ViewDragCallback(Gravity.LEFT);
        final ViewDragCallback rightCallback = new ViewDragCallback(Gravity.RIGHT);

        mLeftDragger = ViewDragHelper.create(this, 0.5f, leftCallback);
        mLeftDragger.setEdgeTrackingEnabled(ViewDragHelper.EDGE_LEFT);
        leftCallback.setDragger(mLeftDragger);

        mRightDragger = ViewDragHelper.create(this, 0.5f, rightCallback);
        mRightDragger.setEdgeTrackingEnabled(ViewDragHelper.EDGE_RIGHT);
        rightCallback.setDragger(mRightDragger);
    }

    /**
     * Set a simple drawable used for the left or right shadow.
     * The drawable provided must have a nonzero intrinsic width.
     *
     * @param shadowDrawable Shadow drawable to use at the edge of a drawer
     * @param gravity Which drawer the shadow should apply to
     */
    public void setDrawerShadow(Drawable shadowDrawable, int gravity) {
        /*
         * TODO Someone someday might want to set more complex drawables here.
         * They're probably nuts, but we might want to consider registering callbacks,
         * setting states, etc. properly.
         */

        final int absGravity = GravityCompat.getAbsoluteGravity(gravity,
                ViewCompat.getLayoutDirection(this));
        if ((absGravity & Gravity.LEFT) == Gravity.LEFT) {
            mShadowLeft = shadowDrawable;
            invalidate();
        }
        if ((absGravity & Gravity.RIGHT) == Gravity.RIGHT) {
            mShadowRight = shadowDrawable;
            invalidate();
        }
    }

    /**
     * Set a simple drawable used for the left or right shadow.
     * The drawable provided must have a nonzero intrinsic width.
     *
     * @param resId Resource id of a shadow drawable to use at the edge of a drawer
     * @param gravity Which drawer the shadow should apply to
     */
    public void setDrawerShadow(int resId, int gravity) {
        setDrawerShadow(getResources().getDrawable(resId), gravity);
    }

    /**
     * Set a listener to be notified of drawer events.
     *
     * @param listener Listener to notify when drawer events occur
     * @see DrawerListener
     */
    public void setDrawerListener(DrawerListener listener) {
        mListener = listener;
    }

    /**
     * Resolve the shared state of all drawers from the component ViewDragHelpers.
     * Should be called whenever a ViewDragHelper's state changes.
     */
    void updateDrawerState(int forGravity, int activeState, View activeDrawer) {
        final int leftState = mLeftDragger.getViewDragState();
        final int rightState = mRightDragger.getViewDragState();

        final int state;
        if (leftState == STATE_DRAGGING || rightState == STATE_DRAGGING) {
            state = STATE_DRAGGING;
        } else if (leftState == STATE_SETTLING || rightState == STATE_SETTLING) {
            state = STATE_SETTLING;
        } else {
            state = STATE_IDLE;
        }

        if (activeDrawer != null && activeState == STATE_IDLE) {
            final LayoutParams lp = (LayoutParams) activeDrawer.getLayoutParams();
            if (lp.onScreen == 0) {
                dispatchOnDrawerClosed(activeDrawer);
            } else if (lp.onScreen == 1) {
                dispatchOnDrawerOpened(activeDrawer);
            }
        }

        if (state != mDrawerState) {
            mDrawerState = state;

            if (mListener != null) {
                mListener.onDrawerStateChanged(state);
            }
        }
    }

    void dispatchOnDrawerClosed(View drawerView) {
        final LayoutParams lp = (LayoutParams) drawerView.getLayoutParams();
        if (lp.knownOpen) {
            lp.knownOpen = false;
            if (mListener != null) {
                mListener.onDrawerClosed(drawerView);
            }
        }
    }

    void dispatchOnDrawerOpened(View drawerView) {
        final LayoutParams lp = (LayoutParams) drawerView.getLayoutParams();
        if (!lp.knownOpen) {
            lp.knownOpen = true;
            if (mListener != null) {
                mListener.onDrawerOpened(drawerView);
            }
        }
    }

    void dispatchOnDrawerSlide(View drawerView, float slideOffset) {
        if (mListener != null) {
            mListener.onDrawerSlide(drawerView, slideOffset);
        }
    }

    void setDrawerViewOffset(View drawerView, float slideOffset) {
        final LayoutParams lp = (LayoutParams) drawerView.getLayoutParams();
        if (slideOffset == lp.onScreen) {
            return;
        }

        lp.onScreen = slideOffset;
        dispatchOnDrawerSlide(drawerView, slideOffset);
    }

    float getDrawerViewOffset(View drawerView) {
        return ((LayoutParams) drawerView.getLayoutParams()).onScreen;
    }

    int getDrawerViewGravity(View drawerView) {
        final int gravity = ((LayoutParams) drawerView.getLayoutParams()).gravity;
        return GravityCompat.getAbsoluteGravity(gravity, ViewCompat.getLayoutDirection(drawerView));
    }

    boolean checkDrawerViewGravity(View drawerView, int checkFor) {
        final int absGrav = getDrawerViewGravity(drawerView);
        return (absGrav & checkFor) == checkFor;
    }

    void moveDrawerToOffset(View drawerView, float slideOffset) {
        final float oldOffset = getDrawerViewOffset(drawerView);
        final int width = drawerView.getWidth();
        final int oldPos = (int) (width * oldOffset);
        final int newPos = (int) (width * slideOffset);
        final int dx = newPos - oldPos;

        drawerView.offsetLeftAndRight(checkDrawerViewGravity(drawerView, Gravity.LEFT) ? dx : -dx);
        setDrawerViewOffset(drawerView, slideOffset);
    }

    View findDrawerWithGravity(int gravity) {
        final int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            final View child = getChildAt(i);
            final int childGravity = getDrawerViewGravity(child);
            if ((childGravity & Gravity.HORIZONTAL_GRAVITY_MASK) ==
                    (gravity & Gravity.HORIZONTAL_GRAVITY_MASK)) {
                return child;
            }
        }
        return null;
    }

    /**
     * Simple gravity to string - only supports LEFT and RIGHT for debugging output.
     *
     * @param gravity Absolute gravity value
     * @return LEFT or RIGHT as appropriate, or a hex string
     */
    static String gravityToString(int gravity) {
        if ((gravity & Gravity.LEFT) == Gravity.LEFT) {
            return "LEFT";
        }
        if ((gravity & Gravity.RIGHT) == Gravity.RIGHT) {
            return "RIGHT";
        }
        return Integer.toHexString(gravity);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        final int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        final int heightSize = MeasureSpec.getSize(heightMeasureSpec);

        if (widthMode != MeasureSpec.EXACTLY || heightMode != MeasureSpec.EXACTLY) {
            throw new IllegalArgumentException(
                    "DrawerLayout must be measured with MeasureSpec.EXACTLY.");
        }

        setMeasuredDimension(widthSize, heightSize);

        // Gravity value for each drawer we've seen. Only one of each permitted.
        int foundDrawers = 0;
        final int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            final View child = getChildAt(i);

            if (child.getVisibility() == GONE) {
                continue;
            }

            if (isContentView(child)) {
                // Content views get measured at exactly the layout's size.
                child.measure(widthMeasureSpec, heightMeasureSpec);
            } else if (isDrawerView(child)) {
                final int childGravity =
                        getDrawerViewGravity(child) & Gravity.HORIZONTAL_GRAVITY_MASK;
                if ((foundDrawers & childGravity) != 0) {
                    throw new IllegalStateException("Child drawer has absolute gravity " +
                            gravityToString(childGravity) + " but this " + TAG + " already has a " +
                            "drawer view along that edge");
                }
                final int drawerWidthSpec = getChildMeasureSpec(widthMeasureSpec, mMinDrawerMargin,
                        child.getLayoutParams().width);
                child.measure(drawerWidthSpec, heightMeasureSpec);
            } else {
                throw new IllegalStateException("Child " + child + " at index " + i +
                        " does not have a valid layout_gravity - must be Gravity.LEFT, " +
                        "Gravity.RIGHT or Gravity.NO_GRAVITY");
            }
        }
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        mInLayout = true;
        final int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            final View child = getChildAt(i);

            if (child.getVisibility() == GONE) {
                continue;
            }

            if (isContentView(child)) {
                child.layout(0, 0, child.getMeasuredWidth(), child.getMeasuredHeight());
            } else { // Drawer, if it wasn't onMeasure would have thrown an exception.
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();

                final int childWidth = child.getMeasuredWidth();
                int childLeft;

                if (checkDrawerViewGravity(child, Gravity.LEFT)) {
                    childLeft = -childWidth + (int) (childWidth * lp.onScreen);
                } else { // Right; onMeasure checked for us.
                    childLeft = r - l - (int) (childWidth * lp.onScreen);
                }

                child.layout(childLeft, 0, childLeft + childWidth, child.getMeasuredHeight());

                if (lp.onScreen == 0) {
                    child.setVisibility(INVISIBLE);
                }
            }
        }
        mInLayout = false;
    }

    @Override
    public void requestLayout() {
        if (!mInLayout) {
            super.requestLayout();
        }
    }

    @Override
    public void computeScroll() {
        final int childCount = getChildCount();
        float scrimOpacity = 0;
        for (int i = 0; i < childCount; i++) {
            final float onscreen = ((LayoutParams) getChildAt(i).getLayoutParams()).onScreen;
            scrimOpacity = Math.max(scrimOpacity, onscreen);
        }
        mScrimOpacity = scrimOpacity;

        // "|" used on purpose; both need to run.
        if (mLeftDragger.continueSettling(true) | mRightDragger.continueSettling(true)) {
            ViewCompat.postInvalidateOnAnimation(this);
        }
    }

    private static boolean hasOpaqueBackground(View v) {
        final Drawable bg = v.getBackground();
        if (bg != null) {
            return bg.getOpacity() == PixelFormat.OPAQUE;
        }
        return false;
    }

    @Override
    protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
        final boolean drawingContent = isContentView(child);
        int clipLeft = 0, clipRight = getWidth();

        final int restoreCount = canvas.save();
        if (drawingContent) {
            final int childCount = getChildCount();
            for (int i = 0; i < childCount; i++) {
                final View v = getChildAt(i);
                if (v == child || v.getVisibility() != VISIBLE ||
                        !hasOpaqueBackground(v) || !isDrawerView(v)) {
                    continue;
                }

                if (checkDrawerViewGravity(v, Gravity.LEFT)) {
                    final int vright = v.getRight();
                    if (vright > clipLeft) clipLeft = vright;
                } else {
                    final int vleft = v.getLeft();
                    if (vleft < clipRight) clipRight = vleft;
                }
            }
            canvas.clipRect(clipLeft, 0, clipRight, getHeight());
        }
        final boolean result = super.drawChild(canvas, child, drawingTime);
        canvas.restoreToCount(restoreCount);

        if (mScrimOpacity > 0 && drawingContent) {
            final int baseAlpha = (mScrimColor & 0xff000000) >>> 24;
            final int imag = (int) (baseAlpha * mScrimOpacity);
            final int color = imag << 24 | (mScrimColor & 0xffffff);
            mScrimPaint.setColor(color);

            canvas.drawRect(clipLeft, 0, clipRight, getHeight(), mScrimPaint);
        } else if (mShadowLeft != null && checkDrawerViewGravity(child, Gravity.LEFT)) {
            final int shadowWidth = mShadowLeft.getIntrinsicWidth();
            final int childRight = child.getRight();
            final float alpha =
                    Math.max(0, Math.min((float) childRight / mDrawerPeekDistance, 1.f));
            mShadowLeft.setBounds(childRight, child.getTop(),
                    childRight + shadowWidth, child.getBottom());
            mShadowLeft.setAlpha((int) (0xff * alpha));
            mShadowLeft.draw(canvas);
        } else if (mShadowRight != null && checkDrawerViewGravity(child, Gravity.RIGHT)) {
            final int shadowWidth = mShadowRight.getIntrinsicWidth();
            final int childLeft = child.getLeft();
            final int showing = getWidth() - childLeft;
            final float alpha =
                    Math.max(0, Math.min((float) showing / mDrawerPeekDistance, 1.f));
            mShadowRight.setBounds(childLeft - shadowWidth, child.getTop(),
                    childLeft, child.getBottom());
            mShadowRight.setAlpha((int) (0xff * alpha));
            mShadowRight.draw(canvas);
        }
        return result;
    }

    boolean isContentView(View child) {
        return ((LayoutParams) child.getLayoutParams()).gravity == Gravity.NO_GRAVITY;
    }

    boolean isDrawerView(View child) {
        final int gravity = ((LayoutParams) child.getLayoutParams()).gravity;
        final int absGravity = GravityCompat.getAbsoluteGravity(gravity,
                ViewCompat.getLayoutDirection(child));
        return (absGravity & (Gravity.LEFT | Gravity.RIGHT)) != 0;
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        final int action = MotionEventCompat.getActionMasked(ev);

        // "|" used deliberately here; both methods should be invoked.
        final boolean interceptForDrag = mLeftDragger.shouldInterceptTouchEvent(ev) |
                mRightDragger.shouldInterceptTouchEvent(ev);

        boolean interceptForTap = false;

        switch (action) {
            case MotionEvent.ACTION_DOWN: {
                final float x = ev.getX();
                final float y = ev.getY();
                mInitialMotionX = x;
                mInitialMotionY = y;
                if (mScrimOpacity > 0 &&
                        isContentView(mLeftDragger.findTopChildUnder((int) x, (int) y))) {
                    interceptForTap = true;
                }
                break;
            }

            case MotionEvent.ACTION_CANCEL:
            case MotionEvent.ACTION_UP: {
                closeDrawers(true);
            }
        }
        return interceptForDrag || interceptForTap;
    }

    @Override
    public void requestDisallowInterceptTouchEvent(boolean disallowIntercept) {
        super.requestDisallowInterceptTouchEvent(disallowIntercept);
        if (disallowIntercept) {
            closeDrawers(true);
        }
    }

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        mLeftDragger.processTouchEvent(ev);
        mRightDragger.processTouchEvent(ev);

        final int action = ev.getAction();
        boolean wantTouchEvents = true;

        switch (action & MotionEventCompat.ACTION_MASK) {
            case MotionEvent.ACTION_DOWN: {
                final float x = ev.getX();
                final float y = ev.getY();
                mInitialMotionX = x;
                mInitialMotionY = y;
                break;
            }

            case MotionEvent.ACTION_UP: {
                final float x = ev.getX();
                final float y = ev.getY();
                boolean peekingOnly = true;
                if (isContentView(mLeftDragger.findTopChildUnder((int) x, (int) y))) {
                    final float dx = x - mInitialMotionX;
                    final float dy = y - mInitialMotionY;
                    final int slop = mLeftDragger.getTouchSlop();
                    if (dx * dx + dy * dy < slop * slop) {
                        // Taps close a dimmed open pane.
                        peekingOnly = false;
                    }
                }
                closeDrawers(peekingOnly);
                break;
            }

            case MotionEvent.ACTION_CANCEL: {
                closeDrawers(true);
                break;
            }
        }

        return wantTouchEvents;
    }

    /**
     * Close all currently open drawer views by animating them out of view.
     */
    public void closeDrawers() {
        closeDrawers(false);
    }

    void closeDrawers(boolean peekingOnly) {
        boolean needsInvalidate = false;
        final int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            final View child = getChildAt(i);
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();

            if (!isDrawerView(child) || (peekingOnly && !lp.isPeeking)) {
                continue;
            }

            final int childWidth = child.getWidth();

            if (checkDrawerViewGravity(child, Gravity.LEFT)) {
                needsInvalidate |= mLeftDragger.smoothSlideViewTo(child,
                        -childWidth, child.getTop());
            } else {
                needsInvalidate |= mRightDragger.smoothSlideViewTo(child,
                        getWidth(), child.getTop());
            }

            lp.isPeeking = false;
        }

        if (needsInvalidate) {
            invalidate();
        }
    }

    /**
     * Open the specified drawer view by animating it into view.
     *
     * @param drawerView Drawer view to open
     */
    public void openDrawer(View drawerView) {
        if (!isDrawerView(drawerView)) {
            throw new IllegalArgumentException("View " + drawerView + " is not a sliding drawer");
        }

        if (checkDrawerViewGravity(drawerView, Gravity.LEFT)) {
            mLeftDragger.smoothSlideViewTo(drawerView, 0, drawerView.getTop());
        } else {
            mRightDragger.smoothSlideViewTo(drawerView, getWidth() - drawerView.getWidth(),
                    drawerView.getTop());
        }
        invalidate();
    }

    /**
     * Open the specified drawer by animating it out of view.
     *
     * @param gravity Gravity.LEFT to move the left drawer or Gravity.RIGHT for the right.
     *                GravityCompat.START or GravityCompat.END may also be used.
     */
    public void openDrawer(int gravity) {
        final int absGravity = GravityCompat.getAbsoluteGravity(gravity,
                ViewCompat.getLayoutDirection(this));
        final View drawerView = findDrawerWithGravity(absGravity);

        if (drawerView == null) {
            throw new IllegalArgumentException("No drawer view found with absolute gravity " +
                    gravityToString(absGravity));
        }
        openDrawer(drawerView);
    }

    /**
     * Close the specified drawer view by animating it into view.
     *
     * @param drawerView Drawer view to close
     */
    public void closeDrawer(View drawerView) {
        if (!isDrawerView(drawerView)) {
            throw new IllegalArgumentException("View " + drawerView + " is not a sliding drawer");
        }

        if (checkDrawerViewGravity(drawerView, Gravity.LEFT)) {
            mLeftDragger.smoothSlideViewTo(drawerView, -drawerView.getWidth(), drawerView.getTop());
        } else {
            mRightDragger.smoothSlideViewTo(drawerView, getWidth(), drawerView.getTop());
        }
        invalidate();
    }

    /**
     * Close the specified drawer by animating it out of view.
     *
     * @param gravity Gravity.LEFT to move the left drawer or Gravity.RIGHT for the right.
     *                GravityCompat.START or GravityCompat.END may also be used.
     */
    public void closeDrawer(int gravity) {
        final int absGravity = GravityCompat.getAbsoluteGravity(gravity,
                ViewCompat.getLayoutDirection(this));
        final View drawerView = findDrawerWithGravity(absGravity);

        if (drawerView == null) {
            throw new IllegalArgumentException("No drawer view found with absolute gravity " +
                    gravityToString(absGravity));
        }
        closeDrawer(drawerView);
    }

    /**
     * Check if the given drawer view is currently in an open state.
     * To be considered "open" the drawer must have settled into its fully
     * visible state. To check for partial visibility use
     * {@link #isDrawerVisible(android.view.View)}.
     *
     * @param drawer Drawer view to check
     * @return true if the given drawer view is in an open state
     * @see #isDrawerVisible(android.view.View)
     */
    public boolean isDrawerOpen(View drawer) {
        if (!isDrawerView(drawer)) {
            throw new IllegalArgumentException("View " + drawer + " is not a drawer");
        }
        return ((LayoutParams) drawer.getLayoutParams()).knownOpen;
    }

    /**
     * Check if a given drawer view is currently visible on-screen. The drawer
     * may be only peeking onto the screen, fully extended, or anywhere inbetween.
     *
     * @param drawer Drawer view to check
     * @return true if the given drawer is visible on-screen
     * @see #isDrawerOpen(android.view.View)
     */
    public boolean isDrawerVisible(View drawer) {
        if (!isDrawerView(drawer)) {
            throw new IllegalArgumentException("View " + drawer + " is not a drawer");
        }
        return ((LayoutParams) drawer.getLayoutParams()).onScreen > 0;
    }

    @Override
    protected ViewGroup.LayoutParams generateDefaultLayoutParams() {
        return new LayoutParams(LayoutParams.FILL_PARENT, LayoutParams.FILL_PARENT);
    }

    @Override
    protected ViewGroup.LayoutParams generateLayoutParams(ViewGroup.LayoutParams p) {
        return p instanceof LayoutParams
                ? new LayoutParams((LayoutParams) p)
                : new LayoutParams(p);
    }

    @Override
    protected boolean checkLayoutParams(ViewGroup.LayoutParams p) {
        return p instanceof LayoutParams && super.checkLayoutParams(p);
    }

    @Override
    public ViewGroup.LayoutParams generateLayoutParams(AttributeSet attrs) {
        return new LayoutParams(getContext(), attrs);
    }

    private class ViewDragCallback extends ViewDragHelper.Callback {

        private final int mGravity;
        private ViewDragHelper mDragger;

        public ViewDragCallback(int gravity) {
            mGravity = gravity;
        }

        public void setDragger(ViewDragHelper dragger) {
            mDragger = dragger;
        }

        @Override
        public boolean tryCaptureView(View child, int pointerId) {
            // Only capture views where the gravity matches what we're looking for.
            // This lets us use two ViewDragHelpers, one for each side drawer.
            return isDrawerView(child) && checkDrawerViewGravity(child, mGravity);
        }

        @Override
        public void onViewDragStateChanged(int state) {
            updateDrawerState(mGravity, state, mDragger.getCapturedView());
        }

        @Override
        public void onViewPositionChanged(View changedView, int left, int top, int dx, int dy) {
            float offset;
            final int childWidth = changedView.getWidth();

            // This reverses the positioning shown in onLayout.
            if (checkDrawerViewGravity(changedView, Gravity.LEFT)) {
                offset = (float) (childWidth + left) / childWidth;
            } else {
                final int width = getWidth();
                offset = (float) (width - left) / childWidth;
            }
            setDrawerViewOffset(changedView, offset);
            changedView.setVisibility(offset == 0 ? INVISIBLE : VISIBLE);
            invalidate();
        }

        @Override
        public void onViewCaptured(View capturedChild, int activePointerId) {
            final LayoutParams lp = (LayoutParams) capturedChild.getLayoutParams();
            lp.isPeeking = false;

            closeOtherDrawer();
        }

        private void closeOtherDrawer() {
            final int otherGrav = mGravity == Gravity.LEFT ? Gravity.RIGHT : Gravity.LEFT;
            final View toClose = findDrawerWithGravity(otherGrav);
            if (toClose != null) {
                closeDrawer(toClose);
            }
        }

        @Override
        public void onViewReleased(View releasedChild, float xvel, float yvel) {
            // Offset is how open the drawer is, therefore left/right values
            // are reversed from one another.
            final float offset = getDrawerViewOffset(releasedChild);
            final int childWidth = releasedChild.getWidth();

            int left;
            if (checkDrawerViewGravity(releasedChild, Gravity.LEFT)) {
                left = xvel > 0 || xvel == 0 && offset > 0.5f ? 0 : -childWidth;
            } else {
                final int width = getWidth();
                left = xvel < 0 || xvel == 0 && offset < 0.5f ? width - childWidth : width;
            }

            mDragger.settleCapturedViewAt(left, releasedChild.getTop());
            invalidate();
        }

        @Override
        public void onEdgeTouched(int edgeFlags, int pointerId) {
            final View toCapture;
            final int childLeft;
            final boolean leftEdge =
                    (edgeFlags & ViewDragHelper.EDGE_LEFT) == ViewDragHelper.EDGE_LEFT;
            if (leftEdge) {
                toCapture = findDrawerWithGravity(Gravity.LEFT);
                childLeft = -toCapture.getWidth() + mDrawerPeekDistance;
            } else {
                toCapture = findDrawerWithGravity(Gravity.RIGHT);
                childLeft = getWidth() - mDrawerPeekDistance;
            }

            // Only peek if it would mean making the drawer more visible
            if (toCapture != null && ((leftEdge && toCapture.getLeft() < childLeft) ||
                    (!leftEdge && toCapture.getLeft() > childLeft))) {
                final LayoutParams lp = (LayoutParams) toCapture.getLayoutParams();
                mDragger.smoothSlideViewTo(toCapture, childLeft, toCapture.getTop());
                lp.isPeeking = true;
                invalidate();

                closeOtherDrawer();
            }
        }

        @Override
        public void onEdgeDragStarted(int edgeFlags, int pointerId) {
            final View toCapture;
            if ((edgeFlags & ViewDragHelper.EDGE_LEFT) == ViewDragHelper.EDGE_LEFT) {
                toCapture = findDrawerWithGravity(Gravity.LEFT);
            } else {
                toCapture = findDrawerWithGravity(Gravity.RIGHT);
            }

            if (toCapture != null) {
                mDragger.captureChildView(toCapture, pointerId);
            }
        }

        @Override
        public int getViewHorizontalDragRange(View child) {
            return child.getWidth();
        }

        @Override
        public int clampViewPositionHorizontal(View child, int left, int dx) {
            if (checkDrawerViewGravity(child, Gravity.LEFT)) {
                return Math.max(-child.getWidth(), Math.min(left, 0));
            } else {
                final int width = getWidth();
                return Math.max(width - child.getWidth(), Math.min(left, width));
            }
        }
    }

    public static class LayoutParams extends ViewGroup.LayoutParams {

        public int gravity = Gravity.NO_GRAVITY;
        float onScreen;
        boolean isPeeking;
        boolean knownOpen;

        public LayoutParams(Context c, AttributeSet attrs) {
            super(c, attrs);

            final TypedArray a = c.obtainStyledAttributes(attrs, LAYOUT_ATTRS);
            this.gravity = a.getInt(0, Gravity.NO_GRAVITY);
            a.recycle();
        }

        public LayoutParams(int width, int height) {
            super(width, height);
        }

        public LayoutParams(int width, int height, int gravity) {
            this(width, height);
            this.gravity = gravity;
        }

        public LayoutParams(LayoutParams source) {
            super(source);
            this.gravity = source.gravity;
        }

        public LayoutParams(ViewGroup.LayoutParams source) {
            super(source);
        }
    }
}