Objective-C中的协议

先给出一个demo案例：
model类.h文件：

#import 

@interface LQUserModel : NSObject

@property (nonatomic, copy, readonly) NSString *userName;
@property (nonatomic, assign, readonly) NSUInteger age;

- (instancetype)initWithUserName:(NSString *)userName age:(NSUInteger)age;
+ (instancetype)userWithUserName:(NSString *)userName age:(NSUInteger)age;

@end

model类.m文件：

#import "LQUserModel.h"

@implementation LQUserModel

- (instancetype)initWithUserName:(NSString *)userName age:(NSUInteger)age{
    if (self = [super init]) {
        _userName = [userName copy];
        _age = age;
    }
    return self;
}
+ (instancetype)userWithUserName:(NSString *)userName age:(NSUInteger)age{
    return [[LQUserModel alloc]initWithUserName:userName age:age];
}

@end

定义了一个有LQUserModel对象的控制器CopyingTestViewController：

#import 
@class LQUserModel;
@interface CopyingTestViewController : UIViewController

@property (nonatomic, copy) LQUserModel *userModel;

@end

现在我们来进行一个页面切换，我将代码写在了touch方法里：

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{
    CopyingTestViewController *copyingVC = [[CopyingTestViewController alloc]init];
    LQUserModel *model = [[LQUserModel alloc]initWithUserName:@"John" age:18];
//    LQUserModel *model = [LQUserModel userWithUserName:@"John" age:18];
    copyingVC.userModel = model;
    [self presentViewController:copyingVC animated:YES completion:nil];
}

运行程序，当进行切换操作时，crash了...

2018-01-17 14:22:02.697557+0800 RLAudioRecord[10091:2180904] -[LQUserModel copyWithZone:]: unrecognized selector sent to instance 0x10089297

LQUserModel对象没有实现方法copyWithZone:,而copyWithZone:是协议NSCopying的方法，所以我们需要遵守该协议

@interface LQUserModel : NSObject

- (id)copyWithZone:(nullable NSZone *)zone{
    LQUserModel *model = [[LQUserModel allocWithZone:zone]init];
    [LQUserModel userWithUserName:_userName age:_age];
    return model;
}

ok，问题得到修复，我们来讲下NSCopying这个协议方法的什么时候使用的，我们在定义LQUserModel对象时，指定其属性为copy：

@property (nonatomic, copy) LQUserModel *userModel;

如果自定义类需要有copy属性，该类需要遵守协议，并实现其协议方法copyWithZone:。
系统中和协议方法：

@protocol NSCopying

- (id)copyWithZone:(nullable NSZone *)zone;

@end

@protocol NSMutableCopying

- (id)mutableCopyWithZone:(nullable NSZone *)zone;

@end

使用对象实例可变时，使用协议NSMutableCopying ，不可变时使用NSCopying。