CorrectiveAppBarLayoutFlingBehavior.java

/**
 * This Behavior corrects the bug in RecyclerView that doesn't report scroll consumption
 * properly by plugging in a scroll listener to RecyclerView that properly reports the
 * same(on reaching the top)
 *
 * This is courtesy of @mak-sing on SO:(top rated answer is FLAWED!)
 * https://stackoverflow.com/questions/30923889/flinging-with-recyclerview-appbarlayout
 */
public final class CorrectiveAppBarLayoutFlingBehavior extends AppBarLayout.Behavior {
    private Map<RecyclerView, RecyclerViewScrollListener> scrollListenerMap = new HashMap<>(); //keep scroll listener map, the custom scroll listener also keep the current scroll Y position.

    public CorrectiveAppBarLayoutFlingBehavior() {
    }

    public CorrectiveAppBarLayoutFlingBehavior(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    /**
     *
     * @param coordinatorLayout
     * @param child The child that attached the behavior (AppBarLayout)
     * @param target The scrolling target e.g. a recyclerView or NestedScrollView
     * @param velocityX
     * @param velocityY
     * @param consumed The fling should be consumed by the scrolling target or not
     * @return
     */
    @Override
    public boolean onNestedFling(CoordinatorLayout coordinatorLayout, AppBarLayout child, View target, float velocityX, float velocityY, boolean consumed) {
        if (target instanceof RecyclerView) {
            final RecyclerView recyclerView = (RecyclerView) target;
            if (scrollListenerMap.get(recyclerView) == null) {
                RecyclerViewScrollListener recyclerViewScrollListener = new RecyclerViewScrollListener(coordinatorLayout, child, this);
                scrollListenerMap.put(recyclerView, recyclerViewScrollListener);
                recyclerView.addOnScrollListener(recyclerViewScrollListener);
            }
            scrollListenerMap.get(recyclerView).setVelocity(velocityY);
            consumed = scrollListenerMap.get(recyclerView).getScrolledY() > 0; //recyclerView only consume the fling when it's not scrolled to the top
        }
        return super.onNestedFling(coordinatorLayout, child, target, velocityX, velocityY, consumed);
    }

    private static class RecyclerViewScrollListener extends RecyclerView.OnScrollListener {
        private int scrolledY;
        private int scrollState;
        private float velocity;
        private WeakReference<CoordinatorLayout> coordinatorLayoutRef;
        private WeakReference<AppBarLayout> childRef;
        private WeakReference<CorrectiveAppBarLayoutFlingBehavior> behaviorWeakReference;

        public RecyclerViewScrollListener(CoordinatorLayout coordinatorLayout, AppBarLayout child, CorrectiveAppBarLayoutFlingBehavior barBehavior) {
            coordinatorLayoutRef = new WeakReference<CoordinatorLayout>(coordinatorLayout);
            childRef = new WeakReference<AppBarLayout>(child);
            behaviorWeakReference = new WeakReference<CorrectiveAppBarLayoutFlingBehavior>(barBehavior);
        }

        @Override
        public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
            scrollState = newState;
        }

        public void setVelocity(float velocity) {
            this.velocity = velocity;
        }

        public int getScrolledY() {
            return scrolledY;
        }

        @Override
        public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
            scrolledY += dy;

            if (scrolledY <= 0 && (scrollState != RecyclerView.SCROLL_STATE_IDLE)
                    && childRef.get() != null &&
                    coordinatorLayoutRef.get() != null &&
                    behaviorWeakReference.get() != null) {

                if(scrollState == RecyclerView.SCROLL_STATE_SETTLING){
                    // If the RecyclerView is settling on reaching top then we don't need
                    // a fling with full velocity, just a scroll. We do a fling faking a scroll
                    velocity = velocity * 0.35f;
                }
                //manually trigger the fling when it's scrolled at the top
                behaviorWeakReference.get().onNestedFling(coordinatorLayoutRef.get(), childRef.get(), recyclerView, 0, velocity, false);
            }
        }
    }
}