Links:
----- official apple website
http://www.opensource.apple.com/tarballs/objc4/
http://www.opensource.apple.com/source/objc4/
----- open source project on github
https://github.com/RetVal/objc-runtime
----- how to read runtime source code
http://www.zhihu.com/question/33634266
----- api document
https://developer.apple.com/library/ios/documentation/Cocoa/Reference/ObjCRuntimeRef/#//apple_ref/doc/uid/TP40001418-CH1g-126286