试图清除 Android SurfaceView/Canvas 时闪烁

我想使用 canvas.drawColor(Color.BLACK) 来清除Canvas，但是如果我只调用一次，显示闪烁，显示旧的图画，这个图画应该被drawColor覆盖。
部分代码:

public void update() { //This method is called by a Thread Canvas canvas = holder.lockCanvas(null); if (canvas != null) { onDraw(canvas); } holder.unlockCanvasAndPost(canvas); } @Override protected void onDraw(Canvas canvas) { if (toClear) { canvas.drawColor(Color.BLACK); //if this is not set to change back to false, it does not flicker toClear = false; } //Draw some objects that are moving around } public void clearScreen() { //This method is called when the user pressed a button toClear = true; }

我查找了关于double buffering的资料，了解到lockCanvas() 和 unlockCanvasAndPost() 可以处理这个问题。现在是哪里出错了呢?