private void updateListViewScrollState(AbsListView listView) {
        if (listView.getChildCount() == 0) {
            setDraggable(true);
        } else {
            if (listView.getFirstVisiblePosition() == 0) {
                View firstChild = listView.getChildAt(0);
                if (firstChild.getTop() == listView.getPaddingTop()) {
                    setDraggable(true);
                    return;
                }
            }
            setDraggable(false);
        }
    }
Set associated list view, then this layout will only be able to drag down when the list
view is scrolled to top.

@param recyclerView
scrolllayout/src/main/java/com/yinglan/scrolllayout/ScrollLayout.java