我试图在屏幕上随机画两个矩形使用ondraw方法，我想清除屏幕，使两个新的矩形每2秒。所以我把invalidate（） Package 在一个计时器中。但是，当我运行此代码时，它首先显示两个矩形，并在2秒后停止运行。我猜我的计时器有问题，因为我的代码在添加计时器之前运行良好。为什么这不起作用，我应该如何修复它，使它每2秒绘制2个矩形（删除以前的矩形）？提前谢谢。

protected void onDraw(Canvas canvas){
    super.onDraw(canvas);   
    int width = canvas.getWidth();
    int height = canvas.getHeight();
    int rectangleWidth = width / 5;//Width of a rectangle
    int rectangleHeight = height / 15; //height of a rectangle
    for(int i = 0; i< 2; i++) {
        randomLeft = randomGenerator1.nextInt(width - rectangleWidth);
        colors = new String[]{"red", "black", "blue", "green", "yellow"};
        int randomColorIndex = randomGenerator2.nextInt(colors.length);
        randomColor = new Paint();
        randomColor.setColor(Color.parseColor(colors[randomColorIndex]));
        randomColor.setStyle(Paint.Style.FILL);
        Rectangle newRectangle = new Rectangle(rectangleWidth, rectangleHeight);
        newRectangle.setColor(randomColor);
        rectangles.add(newRectangle);
        redRect.set(randomLeft, y, randomLeft + rectangleWidth, y + rectangleHeight);
        rects.add(redRect);
        canvas.drawRect(redRect, randomColor);
    }
    timer = new Timer();
    timer.schedule(new TimerTask() {
        @Override
        public void run() {
            invalidate();
        }
    }, 2000);
}