Question

I am studying the LunarLander example in the Android sample code: http://developer.android.com/resources/samples/LunarLander/index.html

I am puzzled because the comments say in several places that the code uses ‘invalidate’ to trigger redrawing. But I can’t find it in the code.

More importantly I believe that drawing should always happen in a View’s onDraw and not inline elsewhere in a thread.

Has anyone studied that example and have comments about why invalidate() is not being called?

Answer 1

It isn’t inlined in a Thread but it is called from a Thread.

@Override
public void run() {
    while (mRun) {
        Canvas c = null;
        try {
            c = mSurfaceHolder.lockCanvas(null);
            synchronized (mSurfaceHolder) {
                if (mMode == STATE_RUNNING) updatePhysics();
                doDraw(c);
            }
        } finally {
            // do this in a finally so that if an exception is thrown
            // during the above, we don't leave the Surface in an
            // inconsistent state
            if (c != null) {
                mSurfaceHolder.unlockCanvasAndPost(c);
            }
        }
    }
}

The drawing itself should always be called from a Thread when you do 2D graphics…