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消息转发机制
http://www.jianshu.com/p/801f4e86259f
脑图：http://naotu.baidu.com/viewshare.html?shareId=axevyea7pc4o
http://www.cocoachina.com/ios/20150715/12540.html
runtime 消息转发
原文地址：http://www.cnblogs.com/pencilCool/p/4693574.html
