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介绍

Objective-C 语言将决定尽可能的从编译和链接时推迟到运行时。只要有可能，Objective-C 总是使用动态的方式来解决问题。这意味着 Objective-C 语言不仅需要一个编译器，同时也需要一个运行时系统来执行编译好的代码。这儿的运行时系统扮演的角色类似于 Objective-C 语言的操作系统，Objective-C 基于该系统来工作。

Runtime是由C、C++、汇编实现的底层库，用于支持Objective-C语言的动态特性。我们使用Objective-C创建类或对象都是通过Runtime来实现消息发送、消息转发的。

如下图：

上图中Runtime System Library 就是Runtime的底层，经过编译过，会提供Runtime API、Framework和Service ,最终会提供给相应的Objective-C使用。

Runtime提供的API，请点击查看Objective-C Runtime

平时我们编写代码并运行程序都要先将Objective-C语言转换成C语言，C语言再转换成汇编语言，汇编语言也编译成机器可以识别的二进制，中间的转换过程就是由Runtime来实现的，程序进行起来后，会装载到内存中，通过Runtime提供运行时的功能。

版本

Runtime有两个版本:

Modern (Objective-C 2.0常用版本)
Legacy (早期版本)

版本区别：当更改类的实例变量的布局时，Legacy需要重新编译它的子类，Modern则不用

Runtime Versions and Platforms

There are different versions of the Objective-C runtime on different platforms.

Legacy and Modern Versions

There are two versions of the Objective-C runtime—“modern” and “legacy”. 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; the programming interface for the modern version of the runtime is described in Objective-C Runtime Reference.
The most notable new feature is that instance variables in the modern runtime are “non-fragile”:
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.
In addition, the modern runtime supports instance variable synthesis for declared properties (see Declared Properties in The Objective-C Programming cLanguage).

Objective-C Runtime Programming Guide

iOS进阶之Runtime初探(一)

介绍

版本