消息转发机制基本分为三个步骤：

1、动态方法解析
新建一个HelloClass的类，定义两个方法：

@interfaceHelloClass:NSObject
- (void)hello;
+ (HelloClass *)hi;@end

对象在接收到未知的消息时，首先会调用所属类的类方法+resolveInstanceMethod:(实例方法)或者+resolveClassMethod:(类方法)。在这个方法中，我们有机会为该未知消息新增一个”处理方法”“。不过使用该方法的前提是我们已经实现了该”处理方法”，只需要在运行时通过class_addMethod函数动态添加到类里面就可以了。

void functionForMethod(id self, SEL _cmd)
{
 NSLog(@"Hello!");
}
Class functionForClassMethod(id self, SEL _cmd)
{
 NSLog(@"Hi!");
 return [HelloClass class];
}

#pragma mark - 1、动态方法解析

+ (BOOL)resolveClassMethod:(SEL)sel
{
NSLog(@"resolveClassMethod");
NSString *selString = NSStringFromSelector(sel);
if ([selString isEqualToString:@"hi"])
{
Class metaClass = objc_getMetaClass("HelloClass");
class_addMethod(metaClass, @selector(hi), (IMP)functionForClassMethod, "v@:");
return YES;
}
return [super resolveClassMethod:sel];
}
+ (BOOL)resolveInstanceMethod:(SEL)sel
{
NSLog(@"resolveInstanceMethod");
NSString *selString = NSStringFromSelector(sel);
if ([selString isEqualToString:@"hello"])
{
class_addMethod(self, @selector(hello), (IMP)functionForMethod, "v@:");
return YES;
}
return [super resolveInstanceMethod:sel];

2、备用接受者
#pragma mark - 2、备用接收者

- (id)forwardingTargetForSelector:(SEL)aSelector
{
NSLog(@"forwardingTargetForSelector");
NSString *selectorString = NSStringFromSelector(aSelector);
// 将消息交给_helper来处理? ? if ([selectorString isEqualToString:@"hello"]) {
 return _helper;
}
return [super forwardingTargetForSelector:aSelector];
}

在本类中需要实现这个新的接受对象

@interfaceHelloClass()
{
RuntimeMethodHelper *_helper;
}
@end
@implementationHelloClass- (instancetype)init
{
self = [super init];
if (self)
{
_helper = [RuntimeMethodHelper new];
}
 return self;
}
RuntimeMethodHelper 类需要实现这个需要转发的方法：

#import"RuntimeMethodHelper.h"
@implementationRuntimeMethodHelper- (void)hello
{
 NSLog(@"%@, %p", self, _cmd);
}@end

3、完整转发
如果动态方法解析和备用接受者都没有处理这个消息，那么就会走完整消息转发：

#pragma mark - 3、完整消息转发

- (void)forwardInvocation:(NSInvocation *)anInvocation
{
NSLog(@"forwardInvocation");
if ([RuntimeMethodHelper instancesRespondToSelector:anInvocation.selector]) {
[anInvocation invokeWithTarget:_helper];
}
}
/*必须重新这个方法，消息转发机制使用从这个方法中获取的信息来创建NSInvocation对象*/
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
{
    NSMethodSignature *signature = [super methodSignatureForSelector:aSelector];
    if (!signature)
    {
        if ([RuntimeMethodHelper instancesRespondToSelector:aSelector]) {
           signature = [RuntimeMethodHelper instanceMethodSignatureForSelector:aSelector];
        }
    }
    return signature;
}

实测示例

2.png