http://yanshinian.com/mvvm%EF%BC%8Drac/ (同事博客)
http://www.raywenderlich.com/forums/viewtopic.php?f=45&t=20561
http://www.jianshu.com/p/87ef6720a096
http://www.jianshu.com/p/e10e5ca413b7
http://tech.meituan.com/RACSignalSubscription.html
原文：http://www.cnblogs.com/feng520/p/4977051.html