ios消息

Class

 typedef struct objc_class *Class;

 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;

Method

 typedef struct objc_method *Method;

 typedef struct objc_ method {

     SEL method_name;

     char *method_types;

     IMP method_imp;

 };

SEl

 const char *sel_getName(SEL sel) {
 #ifndef NO_GC
     if ((uintptr_t)sel == kIgnore) return "<ignored selector>";
 #endif
     return sel ? (const char *)sel : "<null selector>";
 }

http://opensource.apple.com/source/objc4/objc4-437/runtime/objc-sel.mm

从这里我们可以看除SEL，实际上就是字符串

IMP

 typedef id (*IMP)(id, SEL, ...)

可以看出IMP就是一个函数指针，它指向的函数返回值为id，包含id类型（消息接收对象），SEL类型（方法名）和可变参数（方法参数）