#import <Foundation/Foundation.h>@interface Person : NSObject-(void)sendMessage:(NSString *)message;@end

#import "Person.h"
#import <objc/runtime.h>@implementation Person@end

#import "ViewController.h"
#import "Person.h"@interface ViewController ()@end@implementation ViewController- (void)viewDidLoad {[super viewDidLoad];[[[Person alloc]init] sendMessage:@"Hello"];}

+(BOOL)resolveInstanceMethod:(SEL)sel{NSString *methodName = NSStringFromSelector(sel);if ([methodName isEqualToString:@"sendMessage:"]) {//我们可以在这里添加方法的实现return class_addMethod(self, sel, (IMP)sendMessage, "v@:@");}return NO;}void sendMessage (id self, SEL _cmd, NSString *message){NSLog(@"message=%@",message);
}

#import <Foundation/Foundation.h>@interface Animal : NSObject@end

#import "Animal.h"@implementation Animal-(void)sendMessage:(NSString *)message{NSLog(@"message=%@",message);
}@end

-(id)forwardingTargetForSelector:(SEL)aSelector{NSString *methodName = NSStringFromSelector(aSelector);if ([methodName isEqualToString:@"sendMessage:"]) {if ([[Animal new] respondsToSelector:aSelector]) {return [Animal new];}}return [super forwardingTargetForSelector:aSelector];
}

// 若前两种方法都不处理，则走这里
//    1）方法签名
-(NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector{NSString *methodName = NSStringFromSelector(aSelector);if ([methodName isEqualToString:@"sendMessage:"]) {return [NSMethodSignature signatureWithObjCTypes:"v@:@"];}return [super methodSignatureForSelector:aSelector];
}
//  2) 签名后，消息转发,找备用者
-(void)forwardInvocation:(NSInvocation *)anInvocation{SEL selector = [anInvocation selector];Animal *animal = [Animal new];if ([animal respondsToSelector:selector]) {[anInvocation invokeWithTarget:animal];} else{[super forwardInvocation:anInvocation];}
}

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

//  若前3方法都不处理，为了防止崩溃，可调用此方法
-(void)doesNotRecognizeSelector:(SEL)aSelector{NSString *methodName = NSStringFromSelector(aSelector);NSLog(@"找不到 %@ 这个方法的实现",methodName);
}