具体原理参考文章： [http://www.cnblogs.com/skywang12345/p/3310887.html](http://www.cnblogs.com/skywang12345/p/3310887.html) [http://blog.csdn.net/chdjj/article/details/38581035](http://blog.csdn.net/chdjj/article/details/38581035)