NS_ROOT_CLASS
@interface NSObject <NSObject> {Class    isa;
}+ (void)load;+ (void)initialize;
- (id)init;+ (id)new;
+ (id)allocWithZone:(NSZone *)zone;
+ (id)alloc;
- (void)dealloc;- (void)finalize;- (id)copy;
- (id)mutableCopy;+ (id)copyWithZone:(NSZone *)zone NS_AUTOMATED_REFCOUNT_UNAVAILABLE;
+ (id)mutableCopyWithZone:(NSZone *)zone NS_AUTOMATED_REFCOUNT_UNAVAILABLE;+ (Class)superclass;
+ (Class)class;
+ (BOOL)instancesRespondToSelector:(SEL)aSelector;
+ (BOOL)conformsToProtocol:(Protocol *)protocol;
- (IMP)methodForSelector:(SEL)aSelector;
+ (IMP)instanceMethodForSelector:(SEL)aSelector;
- (void)doesNotRecognizeSelector:(SEL)aSelector;- (id)forwardingTargetForSelector:(SEL)aSelector NS_AVAILABLE(10_5, 2_0);
- (void)forwardInvocation:(NSInvocation *)anInvocation;
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector;+ (NSMethodSignature *)instanceMethodSignatureForSelector:(SEL)aSelector;- (BOOL)allowsWeakReference NS_UNAVAILABLE;
- (BOOL)retainWeakReference NS_UNAVAILABLE;+ (NSString *)description;+ (BOOL)isSubclassOfClass:(Class)aClass;+ (BOOL)resolveClassMethod:(SEL)sel NS_AVAILABLE(10_5, 2_0);
+ (BOOL)resolveInstanceMethod:(SEL)sel NS_AVAILABLE(10_5, 2_0);@end

#include <sys/types.h>      // for __DARWIN_NULL
#include <Availability.h>
#include <objc/objc-api.h>typedef struct objc_class *Class;
typedef struct objc_object {Class isa;
} *id;

typedef struct objc_method *Method;
typedef struct objc_ivar *Ivar;
typedef struct objc_category *Category;
typedef struct objc_property *objc_property_t;struct objc_class {Class isa;#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;

现在明了了，super_class是其父类的指针，name是类名，version,info,instance_size是表明这个类的信息， ivars是这个类的变量列表，methodLists是函数列表,cache，是一个方法的list的cache，protocols当然是一个protocol的list。

[aObjec performSelector:aSEL]; 当这行代码执行的时候，aObject 首先会去找cache，里头有没有对应函数，如果找不到，会去找methodLists 列表中是否含有对应的函数，如果再没有就会去找 super_class 中的cache，然后..... 继续这样的步骤.......

所以我们现在了解了NSObject 对于消息的发送的基本的机制流程，后续我会介绍NSOject 我们在这个机制上能做的一些事情。