协议与分类--23：代理

@protocol UserDelegate<NSObject>
- (void)doSomething;
@end

@interface User:NSObject
@property (nonatomic，weak) id<UserDelegate> delegate;
@end

#import <Foundation/Foundation.h>
#import "User.h"
@interface APP : NSObject<UserDelegate>
@property (nonatomic,strong) User *user;
@end

 if (user.delegate && [user.delegate respondsToSelector:@selector(doSomething)]) {
        [user.delegate doSomething];
    }