Smooth scrolling in Android

I have no experience with OpenGL nor accelerometer, but swipe (called fling in Android’s API) is not hard to achieve. First thing you need when making such a custom View, is implementing a GestureDetector and call its onTouchEvent() in your view’s onTouchEvent()

GestureDetector mGD = new GestureDetector(getContext(),
                                        new SimpleOnGestureListener() {

    @Override
    public boolean onScroll(MotionEvent e1, MotionEvent e2,
                                float distanceX, float distanceY) {
        // beware, it can scroll to infinity
        scrollBy((int)distanceX, (int)distanceY);
        return true;
    }

    @Override
    public boolean onFling(MotionEvent e1, MotionEvent e2, float vX, float vY) {
        mScroller.fling(getScrollX(), getScrollY(),
                -(int)vX, -(int)vY, 0, (int)mMaxScrollX, 0, (int)mMaxScrollY);
        invalidate(); // don't remember if it's needed
        return true;
    }

    @Override
    public boolean onDown(MotionEvent e) {
        if(!mScroller.isFinished() ) { // is flinging
            mScroller.forceFinished(true); // to stop flinging on touch
        }
        return true; // else won't work
    }
});

@Override
public boolean onTouchEvent(MotionEvent event) {
    return mGD.onTouchEvent(event);
}

While OnGestureListener.onScroll() calls directly View.scrollBy(), for the onFling() method you’ll need a Scroller.

Scroller is a simple object that, as reference says, encapsulates scrolling. It can be used for continuous scrolling or to react to flings. Scroller.fling() begin a “simulation” of fling scroll inside itself, and by watching it you can copy its smoothness with a continuous redrawing animation:

@Override
protected void onDraw(Canvas canvas) {
    // ....your drawings....

    // scrollTo invalidates, so until animation won't finish it will be called
    // (used after a Scroller.fling() )
    if(mScroller.computeScrollOffset()) {
        scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
    }
}

that is, until animation is running, calculate the point we reached and scroll there.

As a last note: remember to return true in your OnGestureListener.onDown(), even if you don’t want to do anything on down, or it won’t work.

And be careful, because Scroller in Android 2.2 has a bug for which the fling animation will not actually end even if it reaches the limits you passed as arguments (yet computed offset respects them, so it won’t actually move).