How can I determine motion direction in an Android Gallery?

Question

I have been creating a Gallery derivative that uses a limited number of Views and, as such, the Adapter needs to be able to populate these Views ahead of time during a scroll or a fling. To do this, I need to get the direction of motion from the onFling(...) and onScroll(...) events.

How can I use the distanceX parameter in onScroll(...) and the velocityX parameter in onFling(...) to determine which way the Gallery is travelling, and therefore which View to prepare next?

Answer 1

The signs of the velocity parameters in onFling(...) and the distance parameters on onScroll(...) are opposite. In order to correctly determine which direction a Gallery is moving, the code should be as follows:

@Override
public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) {
    //if distanceX is POSITIVE, the Views are travelling left
    //therefore the selection position is INCREASING                    
    return super.onScroll(e1, e2, distanceX*mVelocityFactor, distanceY*mVelocityFactor);
}

@Override
public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {
    //if velocityX is NEGATIVE, the Views are travelling left
    //therefore the selection position is DECREASING                    
    return super.onFling(e1, e1, velocityX*mVelocityFactor, velocityY*mVelocityFactor);
}

By the way, mVelocityFactor is just a constant I introduced to make the scroll/fling a little less energetic. I found 0.6 to be quite a nice value—it still feels intuitive to scroll but the flings are less violent.