摘要
阅读objc-rutime源代码。Objective-C是C的超集,且类和对象和Objective-C中的核心概念，阅读就从类和对象在源码中的实现开始入手在objc.h定义了两个重要的类型Class和id：
// objc.h
typedef struct objc_class *Class;
typedef struct objc_object *id;
本文就主要就讨论objc_class和objc_object，还有对应的两个概念类和元类
代码在哪里？
objc-runtime 代码可以从苹果的开元网站下载objc４。下载的代码不能直接编译，需要添加很多依赖头文件，这种事肯定有很多耐不住寂寞的人去做，Ｓｏ这里objc - 编译Runtime源码objc4-706有完整的步骤，里面也有github地址. 加入不需要编译，下载源码,直接阅读就可以了。
##objc_object 和 objc_class
Class和id是runtime对外暴露的两个类型，
// objc.h
/// An opaque type that represents an Objective-C class.
typedef struct objc_class *Class;
/// Represents an instance of a class.
struct objc_object {
Class isa OBJC_ISA_AVAILABILITY;
};
/// A pointer to an instance of a class.
typedef struct objc_object *id;
通过注释可以了解到Class是一个当做Objective-C类的一个透明类型。而id 是类的一个实例。一个是类一个是对象。虽然objc_object定义很简单，但是这个隐藏很多细节，通过源代码的定义，来一窥究竟。
struct objc_object {
private:
isa_t isa;
public:
// ISA() assumes this is NOT a tagged pointer object
Class ISA();
// getIsa() allows this to be a tagged pointer object
Class getIsa();
...
...
...
};
struct objc_class : objc_object {
// Class ISA;
Class superclass;
cache_t cache; // formerly cache pointer and vtable
class_data_bits_t bits; // class_rw_t * plus custom rr/alloc flags
class_rw_t *data() {
return bits.data();
}
void setData(class_rw_t *newData) {
bits.setData(newData);
}
...
...
...
};
这个是runtim中对objc_object 和 objc_class的定义其中省略了很多方法，关注数据，从内部定义和对外暴露的定义objc_object结构基本一样只是Class isa变成了isa_t isa。为了了解两者的联系和区别，继续看isa_t结构：
union isa_t
{
isa_t() { }
isa_t(uintptr_t value) : bits(value) { }
Class cls;
uintptr_t bits;
# if __arm64__
# define ISA_MASK 0x0000000ffffffff8ULL
# define ISA_MAGIC_MASK 0x000003f000000001ULL
# define ISA_MAGIC_VALUE 0x000001a000000001ULL
struct {
uintptr_t indexed : 1;
uintptr_t has_assoc : 1;
uintptr_t has_cxx_dtor : 1;
uintptr_t shiftcls : 33; // MACH_VM_MAX_ADDRESS 0x1000000000
uintptr_t magic : 6;
uintptr_t weakly_referenced : 1;
uintptr_t deallocating : 1;
uintptr_t has_sidetable_rc : 1;
uintptr_t extra_rc : 19;
# define RC_ONE (1ULL<<45)
# define RC_HALF (1ULL<<18)
};
};
isa_t是一个联合类型从定义可知Class isa和isa_t isa确实等价都是指针类型。
uintptr_t是一个无符号整形，正好可以存一个指针。
去除不关心的内容objc_object 和objc_class定义：
struct objc_object {
Class isa;
};
struct objc_class {
Class isa;
Class superclass;
cache_t cache; // formerly cache pointer and vtable
class_data_bits_t bits; // class_rw_t * plus custom rr/alloc flags
};
typedef struct objc_class *Class;
typedef struct objc_object *id;
类和元类
很多博客中都提到了元类，runtime源码中也通用metaclass这个概念，那元类到底是什么呢？
In object-oriented programming, a metaclass is a class whose instances are classes. Just as an ordinary class defines the behavior of certain objects, a metaclass defines the behavior of certain classes and their instances. Not all object-oriented programming languages support metaclasses. Among those that do, the extent to which metaclasses can override any given aspect of class behavior varies. Metaclasses can be implemented by having classes be first-class citizen, in which case a metaclass is simply an object that constructs classes. Each language has its own metaobject protocol, a set of rules that govern how objects, classes, and metaclasses interact.[1]
每种语言都有自己的metaobject protocol，包含对象，类和元类之间的关系,metaobject protocol 定义了一些词汇或者协议来访问和操作对象的结构和行为主要包含：
- Create or delete a new class(创建和销毁一个类)
- Create a new property or method(创建新的属性或者方法)
- Cause a class to inherit from a different class (“change the class structure”)(修改类的继承结构)
- Generate or change the code defining the methods of a class(生成或者改变类和对象的方法)
##感谢：
2、tarballs
