/**
	 * Canvas清屏的操作
	 * 
	 * 参考资料: http://blog.csdn.net/lfdfhl/article/details/9076001
	 * 
	 */
	private void cleanCanvas() {
		mCanvas = mSurfaceHolder.lockCanvas();
		if (mCanvas != null) {
			Paint paint = new Paint();
			// 以下三行为清屏的核心代码
			paint.setXfermode(new PorterDuffXfermode(Mode.CLEAR));
			mCanvas.drawPaint(paint);
			paint.setXfermode(new PorterDuffXfermode(Mode.SRC));
			// 以上三行为清屏的核心代码
			// 在清屏后再执行drawXXX()操作,比如:
			mCanvas.drawBitmap(mBitmap, lastX, 40, mPaint);
		}
	}