读 RunTime

Legacy and Modern Versions

 The modern version was introduced with Objective-C 2.0 and includes a number of new features.
 The programming interface for the legacy version of the runtime is described in Objective-C 1 Runtime Reference;

• In the legacy runtime, if you change the layout of instance variables in a class, you must recompile classes that inherit from it.
• In the modern runtime, if you change the layout of instance variables in a class, you do not have to recompile classes that inherit from it.

1. Objective-C Source Code

 When you compile code containing Objective-C classes and methods, 
 the compiler creates the data structures and function calls that implement the dynamic characteristics of the language ，
 The data structures capture information found in class and category definitions and in protocol declarations;
 they include the class and protocol objects discussed in Defining a Class and Protocols in *The Objective-C Programming Language*, as well as method selectors, instance variable templates, and other information distilled from source code.
 The principal runtime function is the one that sends messages,

参考链接

Objective-C Runtime Programming Guide
Objective-C Runtime 运行时之一：类与对象
iOS安全