Objective-C is a class-based object system. Each object is an instance of some class; the object's isa pointer points to its class. That class describes the object's data: allocation size and ivar types and layout. The class also describes the object's behavior: the selectors it responds to and instance methods it implements.
一、运行时结构系统
typedef struct objc_class *Class;
typedef struct objc_object *id;
struct objc_class {
Class isa OBJC_ISA_AVAILABILITY;
…
}
struct objc_object {
Class isa OBJC_ISA_AVAILABILITY;
};
1、Class本身是一个类型结构，定义了类类型的组成和存储形式；
2、oc的类信息在编译时被编译为class的实例；即将oc的类信息保存在一个Class实例的结构中。
struct objc_class {
Class isa OBJC_ISA_AVAILABILITY;
#if !__OBJC2__
Class super_class OBJC2_UNAVAILABLE;
const char *name OBJC2_UNAVAILABLE;
long version OBJC2_UNAVAILABLE;
long info OBJC2_UNAVAILABLE;
long instance_size OBJC2_UNAVAILABLE;
struct objc_ivar_list *ivars OBJC2_UNAVAILABLE;
struct objc_method_list **methodLists OBJC2_UNAVAILABLE;
struct objc_cache *cache OBJC2_UNAVAILABLE;
struct objc_protocol_list *protocols OBJC2_UNAVAILABLE;
#endif
} OBJC2_UNAVAILABLE;
3、oc的对象在创建时，会与它的类型信息进行关联。
isa;
二、运行时构建系统
isa
method
三、运行时执行（解释）系统
1、查找执行系统
_objc_msgSend
objc_msgSendSuper
the superclass at which to start searching for the method implementation
2、解释转发系统
_objc_msgForward
ForwardInvocation
3、
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