Objective-C Runtime能做什么？

http://www.cocoachina.com/applenews/devnews/2014/0527/8570.html

https://developer.apple.com/library/ios/documentation/Cocoa/Reference/ObjCRuntimeRef/Reference/reference.html

http://www.cocoachina.com/applenews/devnews/2014/0106/7658.html