Android Surface was already lock

在画图循环绘制过程中，要在线程里面执行。

 new Thread() {
            @Override
            public void run() {
                Canvas canvas = null;
                try {
                    if (sh != null) {
                        canvas = sh.lockCanvas();
                        canvas.drawColor(0, PorterDuff.Mode.CLEAR);
                        helmetUtils.DrawRectangle(b, canvas);

                    }
                } catch (Exception e) {
                    e.printStackTrace();
                } finally {
                    if (canvas != null) {
                        sh.unlockCanvasAndPost(canvas);
                    }
                }
            }
        }.start();