画布清晰的屏幕

Question

我使用此方法清除屏幕

Canvas canvas = mSurfaceHolder.lockCanvas();
Paint p = new Paint();
p.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
canvas.drawPaint(p);
p.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC));
mSurfaceHolder.unlockCanvasAndPost(canvas);

但它现在有效，屏幕闪烁。

然后我用

for (int i = 0;i < 10;i++) {
    Canvas canvas = mSurfaceHolder.lockCanvas();
    Paint p = new Paint();
    p.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
    canvas.drawPaint(p);
    p.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC));
    mSurfaceHolder.unlockCanvasAndPost(canvas);
}

它有效，为什么？