22: 理解NSCopying协议

1.NSCopying协议

若想令自定义对象具有拷贝功能，则需要实现NSCopying协议

• 实现copyWithZone方法
  方法中应该用全能初始化方法，来初始化待拷贝的对象

//.h
 @interface Person : NSObject 
  @property (nonatomic,copy) NSString *name;
  @property (nonatomic,readonly) NSArray *friends;
  @property (nonatomic,assign) int age;
  - (instancetype)initWithName:(NSString *)name age:(int)age;
 @end
 //.m
 @interface Person ()
  @property (nonatomic,readwrite,strong) NSMutableArray *friends;
 @end
 @implementation Person
  - (instancetype)initWithName:(NSString *)name age:(int)age
  {
  self = [super init];
  if (self) {
  self.name = name;
  self.age = age;
  _friends = [NSMutableArray array];
  }
  return self;
 }
 ... 
 - (id)copyWithZone:(NSZone *)zone{
  Person *p = [[[self class] allocWithZone:zone] initWithName:_name age:_age];
  return p;
 }
 @end

• 如果全能初始化不能满足要求，还应该手动的加上一些操作

 //.h
  @interface Person : NSObject 
  @property (nonatomic,copy) NSString *name;
  @property (nonatomic,readonly) NSArray *friends
  @property (nonatomic,assign) int age;
  - (instancetype)initWithName:(NSString *)name age:(int)age;
  @end
  //.m
  @interface Person ()
  @property (nonatomic,readwrite,strong) NSMutableArray *friends;
  @end
  @implementation Person
  - (instancetype)initWithName:(NSString *)name age:(int)age
  {
  self = [super init];
  if (self) {
  self.name = name;
  self.age = age;
  _friends = [NSMutableArray array];
  }
  return self;
  }
  ... 
  - (id)copyWithZone:(NSZone *)zone{
  Person *p = [[[self class] allocWithZone:zone] initWithName:_name age:_age];
  p->_friends = [_friends mutableCopy]; //额外的代码
 
 
 return p;
  }
 @end

• 如果自定义对象分为可变版本和不可变版本,那么就要同时实现NSCopying与NSMutableCopying协议

• 深拷贝浅拷贝的对比图

Paste_Image.png

• 复制对象时应该决定是深拷贝还是浅拷贝，一般情况下是浅拷贝,如果你所写的对象需要深拷贝，那么需要新增一个专门执行深拷贝的方法