@protocol Eat <NSObject>
 
@required  //必须实现
- (void)eat;
 
@optional
- (void)watch;  //可选实现
 
@end

#import “eat.h”
 
@interface Man : NSObject <Eat>
 
@end
 
@implementation Man
 
- (void)eat {
  NSLog(@"Man eat”);
}
 
- (void)watch {
  NSLog(@"Man watch”);
}
 
@end

===============================
 
#import “eat.h”
 
@interface Dog : NSObject <Eat>
 
- (void)eat {
  NSLog(@"Dog eat");
}
 
@end

#import “eat.h”
#import “Man.h”
#import “Dog.h”
 
int main(int argc, const char *argv[]) {
  Man *man = [[Man alloc] init];
  [man eat];  //输出 Man eat
  return 0;
}

@interface Man : NSObject <Eat>
 
// 声明一个指向代理对象的指针变量，遵循Eat,协议一般使用弱引用,若用强引用，会造成双向代理死锁问题
@property (nonatomic,retain) id <Eat> delegate; 
 
@end

#import “eat.h”
#import “Man.h”
#import “Dog.h”
 
int main(int argc, const char *argv[]) {
  Man *man = [[Man alloc] init];
  Dog *dog = [[Dog alloc] init];
 
  man.delegate = dog;
  [man eat];  //输出 Dog eat
  return 0;
}