1 这个作者深入浅出，一下就看懂了闭包的本质
http://wanghuanming.com/2015/01/closure-implementations/
2 iOS闭包
http://blog.devtang.com/2013/07/28/a-look-inside-blocks/
https://onevcat.com/2011/11/objc-block/
http://tech.meituan.com/block-in-Meituan-iOS.html
http://tanqisen.github.io/blog/2013/04/19/gcd-block-cycle-retain/
下面这个写的有点好
http://www.starfelix.com/blog/2014/10/19/ru-he-geng-an-quan-de-shi-yong-block/
这个写的更好
https://www.zybuluo.com/MicroCai/note/58470
