runtime in Objective-C

/** 
 * Sends a message with a simple return value to an instance of a class.
 * 
 * @param self A pointer to the instance of the class that is to receive the message. // self 是一个指向接受消息的实例的指针
 * @param op The selector of the method that handles the message. // op 是处理消息的方法选择器
 * @param ... 
 *   A variable argument list containing the arguments to the method. // 方法的参数们
 * 
 * @return The return value of the method. // objc_msgSend的返回值和方法的返回值是一样的
 * 
 * @note When it encounters a method call, the compiler generates a call to one of the
 *  functions \c objc_msgSend, \c objc_msgSend_stret, \c objc_msgSendSuper, or \c objc_msgSendSuper_stret.
 *  Messages sent to an object’s superclass (using the \c super keyword) are sent using \c objc_msgSendSuper; 
 *  other messages are sent using \c objc_msgSend. Methods that have data structures as return values
 *  are sent using \c objc_msgSendSuper_stret and \c objc_msgSend_stret.
 */

id returnValue = objc_msgSend(someObject,
							  @selector(messageName:),
							  parameter);

/// 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;

struct objc_class {
    Class isa  OBJC_ISA_AVAILABILITY;

#if !__OBJC2__
    Class super_class                                        OBJC2_UNAVAILABLE; // 父类
    const char *name                                         OBJC2_UNAVAILABLE; // 类名
    long version                                             OBJC2_UNAVAILABLE; // 类的版本信息，默认为0
    long info                                                OBJC2_UNAVAILABLE; // 类信息，供运行时期使用的一些位标识
    long instance_size                                       OBJC2_UNAVAILABLE; // 该类的实例变量大小
    struct objc_ivar_list *ivars                             OBJC2_UNAVAILABLE; // 该类的成员变量链表
    struct objc_method_list **methodLists                    OBJC2_UNAVAILABLE; // 方法定义链表
    struct objc_cache *cache                                 OBJC2_UNAVAILABLE; // 方法缓存
    struct objc_protocol_list *protocols                     OBJC2_UNAVAILABLE; // 协议链表
#endif

} OBJC2_UNAVAILABLE;
/* Use `Class` instead of `struct objc_class *` */


struct objc_method {
    SEL method_name                                          OBJC2_UNAVAILABLE; // 方法名
    char *method_types                                       OBJC2_UNAVAILABLE; // 函数类型的字符串
    IMP method_imp                                           OBJC2_UNAVAILABLE; // 函数的实现IMP
}                                                            OBJC2_UNAVAILABLE;

struct objc_method_list {
    struct objc_method_list *obsolete                        OBJC2_UNAVAILABLE;

    int method_count                                         OBJC2_UNAVAILABLE;
#ifdef __LP64__
    int space                                                OBJC2_UNAVAILABLE;
#endif
    /* variable length structure */
    struct objc_method method_list[1]                        OBJC2_UNAVAILABLE;
}                                                            OBJC2_UNAVAILABLE;

void foo(id obj, SEL _cmd)  
{
    NSLog(@"Call a method");
}

+ (BOOL)resolveInstanceMethod:(SEL)aSEL
{
    if(aSEL == @selector(messageName:)){
        class_addMethod([self class], aSEL, (IMP)foo, "v@:");
        return YES;
    }
    return [super resolveInstanceMethod];
}

- (id)forwardingTargetForSelector:(SEL)aSelector
{
    if(aSelector == @selector(messageName:)){
        return alternateObject;
    }
    return [super forwardingTargetForSelector:aSelector];
}

- (void)forwardInvocation:(NSInvocation *)invocation
{
    SEL sel = invocation.selector;

    if([alternateObject respondsToSelector:sel]) {
        [invocation invokeWithTarget:alternateObject];
    } 
    else {
        [self doesNotRecognizeSelector:sel];
    }
}