OC消息發送機制： 使用運行時，通過selector 去快速查找IMP（函數指針）的過程。

消息轉發：IMP找不到的時候，通過一些方法做轉發處理。

#import "Person.h"
#import "Dog.h"
#import "Animal.h"
#import <objc/runtime.h>

@implementation Person

void runNew(id self, SEL sel)
{
    NSLog(@"runNew------執行了");
}

+(BOOL)resolveInstanceMethod:(SEL)sel
{
    
    
 
//在這個方法裏可以動態添加一個方法的實現
    if (sel == @selector(run)) {
        class_addMethod(self, sel, (IMP)runNew, "v@:");
    }
    return [super resolveInstanceMethod:sel];
}

- (id)forwardingTargetForSelector:(SEL)aSelector
{
    //在這個方法裏做消息分發處理
//    if (aSelector == @selector(run)) {
//        return [[Dog alloc]init];
 
//去執行dog類裏的run方法
//    }
//    else if (aSelector == @selector(eat))
//    {
//        return [[Animal alloc]init];//去執行Animal類裏的eat方法
//    }
    return [super forwardingTargetForSelector:aSelector];
}

- (void)forwardInvocation:(NSInvocation *)anInvocation
{
    return [super forwardInvocation:anInvocation];
}

 
-(NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
{
    if (aSelector == @selector(run)) {
        
        return [NSMethodSignature signatureWithObjCTypes:"v@:"];
    }
    if (aSelector == @selector(eat)) {
        return [NSMethodSignature signatureWithObjCTypes:"v@:"];
    }
    return [NSMethodSignature methodSignatureForSelector:aSelector];
}

-(void)doesNotRecognizeSelector:(SEL)aSelector
{
    //最後， 都沒有找到方法實現， 則拋出異常，但程序不會崩潰
    NSLog(@"%@---消息不存在",NSStringFromSelector(aSelector));
}


@end

OC消息轉發機制