What handler is triggered when a user performs a vertical swipe in an app?

In Android, the GestureDetector class with the GestureDetector.OnGestureListener interface implementation is usually used to handle vertical swipes. The onFling method is called during a quick finger movement (swipe). To determine a vertical swipe, the onFling method checks the direction of movement along the Y coordinate.

Example of handling a vertical swipe:

GestureDetector gestureDetector = new GestureDetector(context, new GestureDetector.SimpleOnGestureListener() {
    private static final int SWIPE_THRESHOLD = 100;
    private static final int SWIPE_VELOCITY_THRESHOLD = 100;

    @Override
    public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {
        float diffY = e2.getY() - e1.getY();
        float diffX = e2.getX() - e1.getX();
        if (Math.abs(diffY) > Math.abs(diffX)) { // vertical swipe
            if (Math.abs(diffY) > SWIPE_THRESHOLD && Math.abs(velocityY) > SWIPE_VELOCITY_THRESHOLD) {
                if (diffY > 0) {
                    // swipe down
                } else {
                    // swipe up
                }
                return true;
            }
        }
        return false;
    }
});

Thus, the onFling handler in GestureDetector with direction check is responsible for vertical swipes.