因为我们是先学习的block 然后多线程的时候又出现block 有点混乱了
参考
http://blog.csdn.net/weiwangchao_/article/details/8107999
http://blog.csdn.net/binyanye1/article/details/8234947
http://mobile.51cto.com/iphone-279757.htm
http://www.cocoachina.com/ios/20120718/4462.html
http://mobile.51cto.com/hot-403914.htm
http://www.cocoachina.com/ios/20120514/4247.html
多线程 你说的是GCD吗