One canvas, multiple threads

Question

I'm trying to develop a game using a transparent canvas and a surfaceView, I use the canvas to paint and it works fine.

Now I want to add objects that will move on the screen, for example bouncing balls. I want to use a new thread for the balls, (because otherwise if it's the same thread of the surfaceView it doesn't move smoothly ).

So I'm having trouble of doing this. Can you please advise me how and when should I send the same canvas to a new ball object and then return it back to the surfaceview.
I'm not sure what the best way to handle this is, I tried using a ball as a view in the xml layout, and it worked perfect on another thread but it doesn't make sense to create 1000 views when I can just draw them on a canvas.

Any insight will be helpful!

This is my code:

    public class SurfaceMyPaint extends SurfaceView implements Runnable 
   {
        Thread t;
        SurfaceHolder holder;
        Bitmap brush;
        boolean isItOk = false;

    public SurfaceMyPaint(Context context) 
    {
        super(context);
        holder = getHolder();
        initial();
    }

    public void initial() 
    {
        brush= BitmapFactory.decodeResource(getResources(), R.drawable.brush2);       
    }

    public boolean onTouchEvent(MotionEvent event) 
    {
        if(event.getAction()== MotionEvent.ACTION_DOWN)
        {            
           x= event.getX();
           y= event.getY();
        }

       if(event.getAction()== MotionEvent.ACTION_MOVE)
       {
         x = event.getX();
         y= event.getY();
       }
       return true;
    }


    public void run()
    {
        while (isItOk == true)
        {
        if (!holder.getSurface().isValid())
            continue;
        myCanvas_w = getWidth();
        myCanvas_h = getHeight();
        if (result == null)
        result = Bitmap.createBitmap(myCanvas_w, myCanvas_h, Bitmap.Config.ARGB_8888);
        Canvas c = new Canvas(result);
        canvas = holder.lockCanvas(null);
        c.drawBitmap(brush, x - (brush.getWidth() / 2), y - (brush.getWidth() / 2), null);
         canvas.drawBitmap(result, 0, 0, null);

// How can I add this here: Ball ball = new Ball (canvas)????

        holder.unlockCanvasAndPost(canvas);
        }
    }

     public void pause(){
    isItOk = false;
    while (true){
        try {
            t.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        break;
    }
    t=null;
   }
    public void resume()
     {
        isItOk = true;
        t = new Thread(this);
        t.start();
     }

}

Answer 1

Summarizing your rendering code:

1. Create a screen-sized off-screen bitmap, which you hold in a variable called "result" that isn't declared in the code you show. I assume this is a field, and the allocation only happens once?
2. Each time, you create a new Canvas for that Bitmap.
3. You draw on the Bitmap, with drawBitmap().
4. You copy the Bitmap to the Surface, with drawBitmap().

You want to add a step #5, "draw Ball".

The obvious place to do the ball drawing is in step #3, but I'm going to assume you're not doing it there because that has some relatively static content that you update infrequently but want to blend in every frame.

That being the case, you should just draw the balls onto the Canvas you got back from SurfaceHolder. What you must not do is store a reference to the Surface's Canvas in the Ball object itself, which it appears you want to do (based on new Ball(canvas)). Once you call unlockCanvasAndPost() you must not use that Canvas anymore.

I don't really see an advantage to using multiple threads with your current code. If the "background" Bitmap were also changing you could render that with a different thread, and merge in the current version when you draw, but I suspect that will be more trouble than it's worth -- you will have to deal with the synchronization issues, and I don't think it'll solve your non-smoothness.