@Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        verticalDragRange = h;
        horizontalDragRange = w;

        switch (dragEdge) {
            case TOP:
            case BOTTOM:
                finishAnchor = finishAnchor > 0 ? finishAnchor : verticalDragRange * BACK_FACTOR;
                break;
            case LEFT:
            case RIGHT:
                finishAnchor = finishAnchor > 0 ? finishAnchor : horizontalDragRange * BACK_FACTOR;
                break;
        }
    }
Find out the scrollable child view from a ViewGroup.

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