/**

	 * Canvas清屏的操作

	 *

	 * 參考资料: http://blog.csdn.net/lfdfhl/article/details/9076001

	 *

	 */

	private void cleanCanvas() {

		mCanvas = mSurfaceHolder.lockCanvas();

		if (mCanvas != null) {

			Paint paint = new Paint();

			// 下面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

			// 在清屏后再运行drawXXX()操作,比方:

			mCanvas.drawBitmap(mBitmap, lastX, 40, mPaint);

		}

	}