说明：  苹果公司推出的objietive-c语言的官方说明文档，是目前为止最具权威性的说明文档。纯英文原版，更有助于理解，英文的难度不大，有四级水平足矣。
(Apple official documentation objietive-c language, is by far the most authoritative documentation. Plain English original, but also help to understand the difficulty of the English, there are four levels of enough.)