How to get the scroll speed on a ListView?

Question

I have a ListView with onScrollStateChanged and onScroll event listeners. I want to be able to get the scroll speed of the ListView or some way to get the finalX location of

Answer 1

So this is just some theoretical pseudo-code, but wouldn't something like this work?

I think the accepted answer doesn't really work the resolution is super low (i.e. you only get a velocity after an item has been scrolled off the entire screen, but what if you have large item views?).

   int mTrackingPosition = -1;
   int mLastTop;
   long mLastEventTime;

   @Override
   public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {

       // Get a new tracking position if our old one is no longer on screen
       // (this also includes the first time)
       if (first > mTrackingPosition || last < mTrackingPosition) {

           // We get the middle position here since that's likely to stay 
           // on screen for a bit when scrolling up or down.
           mTrackingPosition = firstVisibleItem + visibleItemCount / 2;

           // Reset our times since we can't really get velocity from this
           // one measurement
           mLastTop = mLastEventTime = -1;

           // Handle the case that this happens more than once in a
           // row if that's even reasonably possible (i.e. they 
           // scrolled rediculously fast)
       }

       // Get the measurements of the tracking view
       View v = view.getViewForPosition(mTrackingPosition);
       int top = v.getTop();
       long time = System.currentTimeMillis();

       // We can only get speed if we have a last recorded time
       if (mLastTop != -1 && mLastEventTime != -1) {
           // Velocity = distance / time
           float velocity = (mLastTop - top) / (time - mLastEventTime);

           // What do you want to do with the velocity?
           ...
       }

       // Update the last top and time so that we can track the difference
       mLastTop = top;
       mLastEventTime = time;
   }

Again this is just pseudo-code that I haven't tested, but I think it should work. You'll also have to reset the last time and top position values when the scroll state is STATE_IDLE.