private int getDragRange() {
        switch (dragEdge) {
            case TOP:
            case BOTTOM:
                return verticalDragRange;
            case LEFT:
            case RIGHT:
                return horizontalDragRange;
            default:
                return verticalDragRange;
        }
    }
Find out the scrollable child view from a ViewGroup.

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