OC Protocol----协议

类似Java的泛型与接口的结合体，用于类型的<>中，可以多继承（按照OC的说法叫遵从某些协议）

1、定义协议

@protocol Client <NSObject>
-(void)defaultMethod;//默认必须实现  

@required//必须
-(void)required；  

@optional//可选
-(void)optional;
@end  

2、符合（实现）协议

#import "Client.h"
@interface JackClient : NSObject<Client>  

@end   

@implementation JackClient
-(void)defaultMethod{
    NSLog(@"this is defaultMethod");
}  

-(void)required{
    NSLog(@"this is required method");
}
@end  

3、使用并调用回调

#import "Client.h"
#protocol JackClient  

@interface Test : NSObject<Client>{
    id<Client> client;
}
-(id)initWithClient:(id<Client>) client;
-(void) turnTheCrank;
@property(retain) id<Client> client;  

@end  

@implementation Test  

-(id)initWithClient:(id<Client>)client{
    self = [super init];
    if (self) {
        _client = client;
    }
    return self;
}  

-(void) turnTheCrank{
    [self.client defaultMethod];
    [self.client required];
    if ([self.client respondsToSelector:@selector(optional)]) {//可选回调可先判断是否实现，再调用
        [self.client optional];
    }
}  

@end  

最后

[[[Test alloc]initWithClient:[[JackClient alloc] init]] turnTheCrank];