BiDirectionViewPager#onInterceptTouchEvent().kt

    override fun onInterceptTouchEvent(event: MotionEvent): Boolean {
        val action = event.actionMasked
        val currentPoint = Point(event.x.toInt(), event.y.toInt())

        if (action == MotionEvent.ACTION_DOWN) {
            // mark the beginning, when finger touched down
            initialTouchPoint = Point(currentPoint)
        } else if (action == MotionEvent.ACTION_UP) {
            // reset the marking, when finger is lifted up
            initialTouchPoint = Point(0, 0)
        } else {
            val moveDistance = currentPoint.distanceFrom(initialTouchPoint)
            if (moveDistance > FINGER_MOVE_THRESHOLD) {
                val direction = MotionUtil.getDirection(initialTouchPoint, currentPoint)
                // check if the scrolling is vertical
                if (direction == MotionUtil.Direction.up || direction == MotionUtil.Direction.down) {
                    return true
                }
            }
        }
        return false
    }