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