v4/view/NestedScrollingChildHelper.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.v4.view;

import android.view.View;
import android.view.ViewParent;

/**
 * Helper class for implementing nested scrolling child views compatible with Android platform
 * versions earlier than Android 5.0 Lollipop (API 21).
 *
 * <p>{@link android.view.View View} subclasses should instantiate a final instance of this
 * class as a field at construction. For each <code>View</code> method that has a matching
 * method signature in this class, delegate the operation to the helper instance in an overriden
 * method implementation. This implements the standard framework policy for nested scrolling.</p>
 *
 * <p>Views invoking nested scrolling functionality should always do so from the relevant
 * {@link android.support.v4.view.ViewCompat}, {@link android.support.v4.view.ViewGroupCompat} or
 * {@link android.support.v4.view.ViewParentCompat} compatibility
 * shim static methods. This ensures interoperability with nested scrolling views on Android
 * 5.0 Lollipop and newer.</p>
 */
public class NestedScrollingChildHelper {
    private final View mView;
    private ViewParent mNestedScrollingParent;
    private boolean mIsNestedScrollingEnabled;
    private int[] mTempNestedScrollConsumed;

    /**
     * Construct a new helper for a given view.
     */
    public NestedScrollingChildHelper(View view) {
        mView = view;
    }

    /**
     * Enable nested scrolling.
     *
     * <p>This is a delegate method. Call it from your {@link android.view.View View} subclass
     * method/{@link android.support.v4.view.NestedScrollingChild} interface method with the same
     * signature to implement the standard policy.</p>
     *
     * @param enabled true to enable nested scrolling dispatch from this view, false otherwise
     */
    public void setNestedScrollingEnabled(boolean enabled) {
        if (mIsNestedScrollingEnabled) {
            ViewCompat.stopNestedScroll(mView);
        }
        mIsNestedScrollingEnabled = enabled;
    }

    /**
     * Check if nested scrolling is enabled for this view.
     *
     * <p>This is a delegate method. Call it from your {@link android.view.View View} subclass
     * method/{@link android.support.v4.view.NestedScrollingChild} interface method with the same
     * signature to implement the standard policy.</p>
     *
     * @return true if nested scrolling is enabled for this view
     */
    public boolean isNestedScrollingEnabled() {
        return mIsNestedScrollingEnabled;
    }

    /**
     * Check if this view has a nested scrolling parent view currently receiving events for
     * a nested scroll in progress.
     *
     * <p>This is a delegate method. Call it from your {@link android.view.View View} subclass
     * method/{@link android.support.v4.view.NestedScrollingChild} interface method with the same
     * signature to implement the standard policy.</p>
     *
     * @return true if this view has a nested scrolling parent, false otherwise
     */
    public boolean hasNestedScrollingParent() {
        return mNestedScrollingParent != null;
    }

    /**
     * Start a new nested scroll for this view.
     *
     * <p>This is a delegate method. Call it from your {@link android.view.View View} subclass
     * method/{@link android.support.v4.view.NestedScrollingChild} interface method with the same
     * signature to implement the standard policy.</p>
     *
     * @param axes Supported nested scroll axes.
     *             See {@link android.support.v4.view.NestedScrollingChild#startNestedScroll(int)}.
     * @return true if a cooperating parent view was found and nested scrolling started successfully
     */
    public boolean startNestedScroll(int axes) {
        if (hasNestedScrollingParent()) {
            // Already in progress
            return true;
        }
        if (isNestedScrollingEnabled()) {
            ViewParent p = mView.getParent();
            View child = mView;
            while (p != null) {
                if (ViewParentCompat.onStartNestedScroll(p, child, mView, axes)) {
                    mNestedScrollingParent = p;
                    ViewParentCompat.onNestedScrollAccepted(p, child, mView, axes);
                    return true;
                }
                if (p instanceof View) {
                    child = (View) p;
                }
                p = p.getParent();
            }
        }
        return false;
    }

    /**
     * Stop a nested scroll in progress.
     *
     * <p>This is a delegate method. Call it from your {@link android.view.View View} subclass
     * method/{@link android.support.v4.view.NestedScrollingChild} interface method with the same
     * signature to implement the standard policy.</p>
     */
    public void stopNestedScroll() {
        if (mNestedScrollingParent != null) {
            ViewParentCompat.onStopNestedScroll(mNestedScrollingParent, mView);
            mNestedScrollingParent = null;
        }
    }

    /**
     * Dispatch one step of a nested scrolling operation to the current nested scrolling parent.
     *
     * <p>This is a delegate method. Call it from your {@link android.view.View View} subclass
     * method/{@link android.support.v4.view.NestedScrollingChild} interface method with the same
     * signature to implement the standard policy.</p>
     *
     * @return true if the parent consumed any of the nested scroll
     */
    public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed,
            int dxUnconsumed, int dyUnconsumed, int[] offsetInWindow) {
        if (isNestedScrollingEnabled() && mNestedScrollingParent != null) {
            if (dxConsumed != 0 || dyConsumed != 0 || dxUnconsumed != 0 || dyUnconsumed != 0) {
                int startX = 0;
                int startY = 0;
                if (offsetInWindow != null) {
                    mView.getLocationInWindow(offsetInWindow);
                    startX = offsetInWindow[0];
                    startY = offsetInWindow[1];
                }

                ViewParentCompat.onNestedScroll(mNestedScrollingParent, mView, dxConsumed,
                        dyConsumed, dxUnconsumed, dyUnconsumed);

                if (offsetInWindow != null) {
                    mView.getLocationInWindow(offsetInWindow);
                    offsetInWindow[0] -= startX;
                    offsetInWindow[1] -= startY;
                }
                return true;
            } else if (offsetInWindow != null) {
                // No motion, no dispatch. Keep offsetInWindow up to date.
                offsetInWindow[0] = 0;
                offsetInWindow[1] = 0;
            }
        }
        return false;
    }

    /**
     * Dispatch one step of a nested pre-scrolling operation to the current nested scrolling parent.
     *
     * <p>This is a delegate method. Call it from your {@link android.view.View View} subclass
     * method/{@link android.support.v4.view.NestedScrollingChild} interface method with the same
     * signature to implement the standard policy.</p>
     *
     * @return true if the parent consumed any of the nested scroll
     */
    public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow) {
        if (isNestedScrollingEnabled() && mNestedScrollingParent != null) {
            if (dx != 0 || dy != 0) {
                int startX = 0;
                int startY = 0;
                if (offsetInWindow != null) {
                    mView.getLocationInWindow(offsetInWindow);
                    startX = offsetInWindow[0];
                    startY = offsetInWindow[1];
                }

                if (consumed == null) {
                    if (mTempNestedScrollConsumed == null) {
                        mTempNestedScrollConsumed = new int[2];
                    }
                    consumed = mTempNestedScrollConsumed;
                }
                consumed[0] = 0;
                consumed[1] = 0;
                ViewParentCompat.onNestedPreScroll(mNestedScrollingParent, mView, dx, dy, consumed);

                if (offsetInWindow != null) {
                    mView.getLocationInWindow(offsetInWindow);
                    offsetInWindow[0] -= startX;
                    offsetInWindow[1] -= startY;
                }
                return consumed[0] != 0 || consumed[1] != 0;
            } else if (offsetInWindow != null) {
                offsetInWindow[0] = 0;
                offsetInWindow[1] = 0;
            }
        }
        return false;
    }

    /**
     * Dispatch a nested fling operation to the current nested scrolling parent.
     *
     * <p>This is a delegate method. Call it from your {@link android.view.View View} subclass
     * method/{@link android.support.v4.view.NestedScrollingChild} interface method with the same
     * signature to implement the standard policy.</p>
     *
     * @return true if the parent consumed the nested fling
     */
    public boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed) {
        if (isNestedScrollingEnabled() && mNestedScrollingParent != null) {
            return ViewParentCompat.onNestedFling(mNestedScrollingParent, mView, velocityX,
                    velocityY, consumed);
        }
        return false;
    }

    /**
     * Dispatch a nested pre-fling operation to the current nested scrolling parent.
     *
     * <p>This is a delegate method. Call it from your {@link android.view.View View} subclass
     * method/{@link android.support.v4.view.NestedScrollingChild} interface method with the same
     * signature to implement the standard policy.</p>
     *
     * @return true if the parent consumed the nested fling
     */
    public boolean dispatchNestedPreFling(float velocityX, float velocityY) {
        if (isNestedScrollingEnabled() && mNestedScrollingParent != null) {
            return ViewParentCompat.onNestedPreFling(mNestedScrollingParent, mView, velocityX,
                    velocityY);
        }
        return false;
    }

    /**
     * View subclasses should always call this method on their
     * <code>NestedScrollingChildHelper</code> when detached from a window.
     *
     * <p>This is a delegate method. Call it from your {@link android.view.View View} subclass
     * method/{@link android.support.v4.view.NestedScrollingChild} interface method with the same
     * signature to implement the standard policy.</p>
     */
    public void onDetachedFromWindow() {
        ViewCompat.stopNestedScroll(mView);
    }

    /**
     * Called when a nested scrolling child stops its current nested scroll operation.
     *
     * <p>This is a delegate method. Call it from your {@link android.view.View View} subclass
     * method/{@link android.support.v4.view.NestedScrollingChild} interface method with the same
     * signature to implement the standard policy.</p>
     *
     * @param child Child view stopping its nested scroll. This may not be a direct child view.
     */
    public void onStopNestedScroll(View child) {
        ViewCompat.stopNestedScroll(mView);
    }
}