Android SurfaceHolder.unlockCanvasAndPost() does not cause redraw

Question

I'm implementing a fairly standard app with the Android sdk that involves drawing using the SurfaceView, SurfaceHolder, Callback setup.

In my main thread (UI thread) I have no drawing or handling of the SurfaceHolder (or the canvas you retrieve with it).

In a separate thread I have the following:

Log.i("GAME.DrawThread", "run()");
        Log.i("GAME.DrawThread", Thread.currentThread().getName());
        Canvas canvas = null;
        try {
            canvas = holder.lockCanvas();
            synchronized(holder) {
                Log.i("GAME", "draw():synchronized");
                Paint paint = new Paint();
                paint.setColor(R.color.draw_color);
                canvas.drawColor(R.color.draw_color);
                canvas.drawLine(0, 0, 500, 500, paint);
            }
        } catch (SurfaceHolder.BadSurfaceTypeException e) {
            Log.e("GAME", "onDraw():  BadSurfaceTypeException");
        } finally {
            if (canvas != null) {
                holder.unlockCanvasAndPost(canvas);
            }
        } 

This code is being executed, throws no exceptions, and has no negative side effects that I can find; however, the unlockCanvasAndPost() call never causes onDraw() to be called.

In other words, unlockCanvasAndPost() does not cause a redraw of the SurfaceView.

Any ideas what could cause this symptom? I have plenty of java experience, a fair amount of android experience, and a lot of debugging experience and cannot track this one down.

Thanks in advance.

Answer 1

That's not how SurfaceView works. Calling unlockCanvasAndPost() does not invoke onDraw(), that's the whole point of using a SurfaceView. A SurfaceView's surface lives in a different window.

Answer 2

So it turns out that when using SurfaceView you draw to a Surface that is underneath a Window. I was setting the background color of the View in xml; it turns out that sets the background color of the Window, not the Surface. In effect, I made the Window opaque so that you couldn't see the Surface underneath.

Lesson Learned.