Track the velocity of a moving view

Question

I'm running into issues when using the VelocityTracker class. Velocity tracking seems to work very well if you are measuring the velocity of touch events on a non-moving view, but as soon as the view starts moving with your finger (using using translateY/X or setY/X), the velocity is completely random (I think the velocity is calculated using the view's position?). Does anyone have any suggestions for getting an accurate velocity of your movement when swiping a view?

Notes: I am using the touch events from my View's onTouchListener for Velocity tracking.

Cheers

Answer 1

You should keep track of how much the view has been moved and call MotionEvent.offsetLocation to "fix" the event coordinates before passing it to a VelocityTracker.

private float mLastX;
private float mTranslationX;
private VelocityTracker mTracker;

@Override
public boolean onTouch(final View view, MotionEvent motionEvent) {
  motionEvent.offsetLocation(mTranslationX, 0);

  switch (motionEvent.getActionMasked()) {
    case MotionEvent.ACTION_DOWN:
      mLastX = motionEvent.getRawX();
      mTracker = VelocityTracker.obtain();
      mTracker.addMovement(motionEvent);
      return true;

    case MotionEvent.ACTION_MOVE:
      mTranslationX += motionEvent.getRawX() - mLastX;
      view.setTranslationX(mTranslationX);
      mTracker.addMovement(motionEvent);
      return true;

    case MotionEvent.ACTION_UP:
      // get your correct velocity from mTracker
      ...
  }
}

Check out the documentation for MotionEvent.getRawX / Y too.

They return the X/Y coordinates of the pointer relative to the entire screen, therefore they aren't affected by the movement of whatever View you're listening on.

Answer 2

The simplest way deals with absolute coordinates write:

motionEvent.setLocation(e.getRawX(), e.getRawY());