weak比assign多了一个功能，当对象消失后自动把指针变成nil
haofanazenmeban[4002:406590] controller:<SecondViewController: 0x7faf7b523340>
2016-05-13 19:57:53.176 haofanazenmeban[4002:406590] view:(null)
weak和assign区别
原文：http://www.cnblogs.com/yintingting/p/5497785.html
