lockCanvas()返回不同canvas

每毫秒在一个canvas上画一些东西，是在canvas上添加一些细节，不是每次都重新画一遍。结果我的代码每次都返回了不同的画，这是怎么回事？

public void run() {
    this.run = true;
    Canvas canvas = null;
    while (run) {
        try {
            canvas = this.surfaceHolder.lockCanvas();
            synchronized (this.surfaceHolder) {
                Thread.sleep(delay);
                draw(new Img(canvas, size));
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            if (canvas != null) {
            this.surfaceHolder.unlockCanvasAndPost(canvas);
            }
        }
        synchronized (this) {
            if (wait) {
                try {
                    wait();
                } catch (Exception e) {}
            }
        }
    }
}

可能是由于双缓冲机制，post上去后切换了个缓冲区