After yesterday's post I continued to look into Android's new Overscroll functionality introduced in Gingerbread and discovered some more interesting things. The functionality to make a a view scroll beyond its limits and then bounce back (almost exactly like iOS) is sort of built into the framework, but just hidden. I'm not sure exactly why it has been built like it has been, but I will give a few guesses after an explanation of what is actually going on, but first: DEMO!

So What's There and What Isn't?

I'm glad you asked… If we look into the ViewConfiguration class' source we find two variables of interest: OVERSCROLL_DISTANCE and OVERFLING_DISTANCE. These two variables tell the framework how much a view should be able to scroll beyond its limits. They are hard coded in and there are no methods available to set your own custom ones. OVERSCROLL_DISTANCE is set to 0 (!?) and OVERFLING_DISTANCE is set to 4.

For those that don't know, the ViewConfiguration class holds a set of values that Android uses to store the default timeouts / distances etc for certain UI behaviours. It also does some internal scaling and calculations based on screen density etc. If you're interested, have a look at the source

So with OVERSCROLL_DISTANCE set to 0, the view will never move beyond its limits, but you can do something fairly simple to achieve this behaviour.

In complicated terms, just extend the view you wish to use normally, (e.g. ListView) and override the overScrollBy method. Then within the overScrollBy method body, simply call the super.overScrollBy but with your own values for maxOverScrollX and/or maxOverScrollY. If you're gonna do this, make sure you scale your values based on the screen density.

Confused? Have a code sample:

public class BounceListView extends ListView
{
    private static final int MAX\_Y\_OVERSCROLL\_DISTANCE = 200;
    
    private Context mContext;
    private int mMaxYOverscrollDistance;
    
    public BounceListView(Context context) 
    {
        super(context);
        mContext = context;
        initBounceListView();
    }
    
    public BounceListView(Context context, AttributeSet attrs) 
    {
        super(context, attrs);
        mContext = context;
        initBounceListView();
    }
    
    public BounceListView(Context context, AttributeSet attrs, int defStyle) 
    {
        super(context, attrs, defStyle);
        mContext = context;
        initBounceListView();
    }
    
    private void initBounceListView()
    {
        //get the density of the screen and do some maths with it on the max overscroll distance
        //variable so that you get similar behaviors no matter what the screen size
        
        final DisplayMetrics metrics = mContext.getResources().getDisplayMetrics();
            final float density = metrics.density;
        
        mMaxYOverscrollDistance = (int) (density \* MAX\_Y\_OVERSCROLL\_DISTANCE);
    }
    
    @Override
    protected boolean overScrollBy(int deltaX, int deltaY, int scrollX, int scrollY, int scrollRangeX, int scrollRangeY, int maxOverScrollX, int maxOverScrollY, boolean isTouchEvent) 
    { 
        //This is where the magic happens, we have replaced the incoming maxOverScrollY with our own custom variable mMaxYOverscrollDistance; 
        return super.overScrollBy(deltaX, deltaY, scrollX, scrollY, scrollRangeX, scrollRangeY, maxOverScrollX, mMaxYOverscrollDistance, isTouchEvent);  
    }
    
}

And now just use that custom view wherever you would normally have used the standard view!

As promised, some guesses as to why this is happening. My first thought is that the developers had no intention of exposing this functionality and that it is simply meant to be used for the small springback you get after an overfling (remember where OVERFLING_DISTANCE was set to 4). But then is that was the case why set OVERSCROLL_DISTANCE to 0, why not just not include it if that was the case? Maybe they are planning something in the future? But if it was intended to be used, then why not create methods that let you set the overscroll distances for your views? Who knows…