Wsrtjtyk

I have an Activity below:

<RecyclerView> // with horizontal LinearLayoutManager and PagerSnapHelper

    <RecyclerView> // with vertical LinearLayoutManager

        <RecyclcerView /> // with horizontal LinearLayoutManager

        <HorizontalScrollView />

        // ... and there are other normal list items

    </RecyclerView>

    <RecyclerView> // another RV with vertical LinearLayoutManager

    </RecyclerView>

</RecyclerView>

The problem is when I scroll horizontally, the inner horizontal RecyclerView and HorizontalScrollView doesn't scroll as expected.

Why I have to do this:

I have several ListFragments, each of which takes data from different data sources, but has same type of list items. I don't want a ViewPager, which either hold all pages immediately after ViewPager is created(which may cause OutOfMemoryError), or reload data from network after user scrolling to other pages and scroll back. Further more, with nested RV, pages can share one same RecyclerViewPool, which helps to decrease total count of item views in all pages.

What I have done in the outer horizontal RV:

private static final int INVALID_POINTER = -1;

private int mActivePointerId = INVALID_POINTER;

private float lastX;

private GestureDetector mVerticalScrollGestureDetector = new GestureDetector(getContext(), new GestureDetector.SimpleOnGestureListener() {

    @Override

    public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) {

        return Math.abs(distanceX) < Math.abs(distanceY);

    }

});

private GestureDetector mHorizontalScrollGestureDetector = new GestureDetector(getContext(), new GestureDetector.SimpleOnGestureListener() {

    @Override

    public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) {

        return Math.abs(distanceX) > Math.abs(distanceY);

    }

});



@Override

public boolean onInterceptTouchEvent(MotionEvent e) {

    if (mVerticalScrollGestureDetector.onTouchEvent(e) || !mAllowPageScroll) {

        return false;

    }

    switch (e.getAction()) {

        case MotionEvent.ACTION_DOWN:

            mActivePointerId = e.getPointerId(0);

            break;

        case MotionEvent.ACTION_MOVE:

            int pointerIndex = e.findPointerIndex(mActivePointerId);

            float x = e.getX(pointerIndex);

            float dx = x - lastX;

            lastX = x;

            if (childCanScroll(this, false, (int) dx, (int) x)) {

                Log.d("ScrollX", "not intercept");

                return false;

            }

            Log.d("ScrollX", "maybe intercept");

            break;

        case MotionEvent.ACTION_UP:

        case MotionEvent.ACTION_CANCEL:

            mActivePointerId = INVALID_POINTER;

            break;

    }

    return super.onInterceptTouchEvent(e);

}



private boolean childCanScroll(View v, boolean considerSelf, int dx, int x) {

    if (v instanceof ViewGroup) {

        ViewGroup group = (ViewGroup) v;

        int scrollX = v.getScrollX();

        for (int i = group.getChildCount() - 1; i >= 0; i--) {

            View child = group.getChildAt(i);

            if (x + scrollX >= child.getLeft() && x + scrollX < child.getRight()

                    && childCanScroll(child, true, dx, x + scrollX - child.getLeft())) {

                return true;

            }

        }

    }

    return considerSelf && v.canScrollHorizontally(-dx);

}

Now it works with inner horizontal RV with occasionally not scrolling, and doesn't work with HorizontalScrollView at all.

How can I achieve the goal to make inner horizontally-scrollable view to scroll first?

There are two mistakes I make.

1. In case of ACTION_DOWN I need to record the lastX variable as well as in case of ACTION_MOVE, so that first move of dx is a proper value. This explains why the childCanScroll method doesn't work on things like ScrollView(as RecyclerView's getScrollX() always return 0).




2. In method childCanScroll, the inner if statement should not only consider the x coordinate, but also the y coordinate. If y is not considered, when I scroll outside inner horizontal-scrollable-views, the method also return true, that make things not working properly.

After I fix these two mistakes all things are right.