block的循环引用问题

网址：http://blog.csdn.net/sapphire_aling/article/details/9120433

http://www.360doc.com/content/14/0521/17/11029609_379679415.shtml

http://www.tuicool.com/articles/uM7fIz

http://www.360doc.com/content/15/0331/09/9200790_459475096.shtml

http://www.cnblogs.com/YouXianMing/p/3696031.html