@Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        boolean handled = false;
        ensureTarget();
        if (isEnabled()) {
            handled = viewDragHelper.shouldInterceptTouchEvent(ev);
        } else {
            viewDragHelper.cancel();
        }
        return !handled ? super.onInterceptTouchEvent(ev) : handled;
    }
Find out the scrollable child view from a ViewGroup.

@param viewGroup
library/src/main/java/com/liuguangqiang/swipeback/SwipeBackLayout.java