sample-BiDirectionViewPager#onTouchEvent().kt

    override fun onTouchEvent(event: MotionEvent): Boolean {
        // swapping the motionEvent's x and y, so that when finger moves right, it becomes moving down
        // for VerticalViewPager effect
        event.swapXY()

        // this portion is used for injection ACTION_DOWN
        if (firstTime && event.actionMasked == MotionEvent.ACTION_MOVE) {
            injectActionDown(event)
            firstTime = false
        }
        if (event.actionMasked == MotionEvent.ACTION_UP) {
            firstTime = true
        }
        super.onTouchEvent(event)
        return true
    }