Question

I am making a board game. The board doesn’t ever move, but pieces on top of it sometimes do depending on user interaction. There are also UI elements which may update periodically.

Right now the way I set it up is by overwriting the onDraw() method of a SurfaceView subclass. I have a drawing thread that constantly calls postInvalidate() in a while loop:

class PanelThread extends Thread
{
        //...
    long sleepTime = 0;
    long nextGameTick = System.currentTimeMillis();

    @Override
    public void run()
    {
        Canvas c;
        while (_run)
        { // When setRunning(false) occurs, _run is
            c = null; // set to false and loop ends, stopping thread
            try
            {
                c = _surfaceHolder.lockCanvas(null);
                synchronized (_surfaceHolder)
                {
                    // Insert methods to modify positions of items in onDraw()
                    _panel.postInvalidate();
                }
            } finally
            {
                if (c != null)
                {
                    _surfaceHolder.unlockCanvasAndPost(c);
                }
            }
        }
            nextGameTick += MILLISECONDS_PER_FRAME;
        sleepTime = nextGameTick - System.currentTimeMillis();
        if(sleepTime >= 0)
        {
            try
            {
                sleep(sleepTime, 0);
            } catch (InterruptedException e)
            {
                continue;
            }
        }
        else
        {
            //we're behind, oh well.
            System.out.println("behind!");
            nextGameTick = System.currentTimeMillis();
        }
    }
}

This is not efficient and is taking a lot of CPU. Is there a easy way to get android to only update when something changes?

Answer 1

You have the right idea, but it needs a bit of refinement.

You definitely do not want to loop as fast as the CPU can handle it though.
You should be sleeping your Thread in every loop for a little while. You most certainly do not need to do everything in your loop every millisecond.

I found this guide to FPS control to be incredible helpful in designing a game loop.

This Android-specific game loop guide also provides a lot of great sample code and an in-depth explanation.