Question

touch 事件会先被 onInterceptTouchEvent() 捕获，进行判断是否拦截。

@Override
public boolean onInterceptTouchEvent(CoordinatorLayout parent, V child, MotionEvent event) {
    if (!child.isShown()) {
        return false;
    }
    int action = MotionEventCompat.getActionMasked(event);
    // Record the velocity
    if (action == MotionEvent.ACTION_DOWN) {
        reset();
    }
    if (mVelocityTracker == null) {
        mVelocityTracker = VelocityTracker.obtain();
    }
    mVelocityTracker.addMovement(event);
    switch (action) {
        case MotionEvent.ACTION_UP:
        case MotionEvent.ACTION_CANCEL:
            mTouchingScrollingChild = false;
            mActivePointerId = MotionEvent.INVALID_POINTER_ID;
            // Reset the ignore flag
            if (mIgnoreEvents) {
                mIgnoreEvents = false;
                return false;
            }
            break;
        case MotionEvent.ACTION_DOWN:
            int initialX = (int) event.getX();
            mInitialY = (int) event.getY();
            View scroll = mNestedScrollingChildRef.get();
            if (scroll != null && parent.isPointInChildBounds(scroll, initialX, mInitialY)) {
                mActivePointerId = event.getPointerId(event.getActionIndex());
                mTouchingScrollingChild = true;
            }
            mIgnoreEvents = mActivePointerId == MotionEvent.INVALID_POINTER_ID &&
                    !parent.isPointInChildBounds(child, initialX, mInitialY);
            break;
    }
    if (!mIgnoreEvents && mViewDragHelper.shouldInterceptTouchEvent(event)) {
        return true;
    }
    // We have to handle cases that the ViewDragHelper does not capture the bottom sheet because
    // it is not the top most view of its parent. This is not necessary when the touch event is
    // happening over the scrolling content as nested scrolling logic handles that case.
    View scroll = mNestedScrollingChildRef.get();
    return action == MotionEvent.ACTION_MOVE && scroll != null &&
            !mIgnoreEvents && mState != STATE_DRAGGING &&
            !parent.isPointInChildBounds(scroll, (int) event.getX(), (int) event.getY()) &&
            Math.abs(mInitialY - event.getY()) > mViewDragHelper.getTouchSlop();
}

onInterceptTouchEvent 做了几件事情：

1. 判断是否拦截事件.先使用 mViewDragHelper.shouldInterceptTouchEvent(event) 拦截。
2. 使用 mVelocityTracker 记录手指动作，用于后期计算 Y 轴速率。
3. 判断点击事件是否在 NestedChildView 上，将 boolean 存到 mTouchingScrollingChild 标记位中，这个主要是用于 ViewDragHelper.Callback 中的判断。
4. ACTION_UP 和 ACTION_CANCEL 对标记位进行复位，好在下一轮 Touch 事件中使用。

onTouchEvent 处理

 @Override
    public boolean onTouchEvent(CoordinatorLayout parent, V child, MotionEvent event) {
        if (!child.isShown()) {
            return false;
        }
        int action = MotionEventCompat.getActionMasked(event);
        if (mState == STATE_DRAGGING && action == MotionEvent.ACTION_DOWN) {
            return true;
        }
        mViewDragHelper.processTouchEvent(event);
        // Record the velocity
        if (action == MotionEvent.ACTION_DOWN) {
            reset();
        }
        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        }
        mVelocityTracker.addMovement(event);
        // The ViewDragHelper tries to capture only the top-most View. We have to explicitly tell it
        // to capture the bottom sheet in case it is not captured and the touch slop is passed.
        if (action == MotionEvent.ACTION_MOVE) {
            if (Math.abs(mInitialY - event.getY()) > mViewDragHelper.getTouchSlop()) {
                mViewDragHelper.captureChildView(child, event.getPointerId(event.getActionIndex()));
            }
        }
        return true;
    }

onTouchEvent 主要做了几件事情：

1. 使用 mVelocityTracker 记录手指动作.用于后期计算 Y 轴速率。
2. 使用 mViewDragHelper 处理 Touch 事件.可能会产生拖动效果。
3. mViewDragHelper 在滑动的时候对 BehaviorView 的再一次捕获.再一次明确告诉 ViewDragHelper 我要移动的是 BehaviorView 组件.什么情况需要主动告诉 ViewDragHelper ?比如:当你点击在 BehaviorView 的区域，但是 BehaviorView 的视图的层级不是最高的，或者你点击的区域不在 BehaviorView 上,ViewDragHelper 在做处理滑动的时候找不到 BehaviorView, 这个时候你要手动告知它现在要移动的是 BehaviorView,情景类似 ViewDragHelper 处理 EdgeDrag 的样子。

注意：即使你的 onInterceptTouchEvent 返回 false,也可能因为下面的 View 没有处理这个 Touch 事件，而导致 Touch 事件上发被 Behavior 的 onTouchEvent 被截取。