Strong和Copy的区别

Strong和Copy对于NSString、NSDirectory、NSArray的区别

当NSString、NSDirectory、NSArray在声明属性的时候设置为Strong类型时:

(1) 调用set方法赋值

@property (nonatomic, strong) NSString *str;

@property (nonatomic, strong) NSArray *array;

@property (nonatomic, strong) NSDictionary *dic;

 NSMutableString *mustr = [[NSMutableString alloc]initWithFormat:@"123"];
    self.str = mustr;
    NSLog(@"%@",_str);
    [mustr appendString:@"456"];
    NSLog(@"%@",_str);
    
    NSMutableArray *muarray = [NSMutableArray arrayWithObjects:@"a",@"b",@"c",nil];
    self.array = muarray;
    NSLog(@"%@",_array);
    [muarray addObject:@"d"];
    NSLog(@"%@",_array);
    
    NSMutableDictionary *muDic = [NSMutableDictionary dictionaryWithObjectsAndKeys:@"a",@"1",@"b",@"2",nil];
    self.dic = muDic;
    NSLog(@"%@",_dic);
    [muDic setObject:@"3" forKey:@"c"];
    NSLog(@"%@",_dic);

输出结果：

屏幕快照 2017-02-23 17.40.33.png

（2）直接对实例对象赋值

@property (nonatomic, strong) NSString *str;

@property (nonatomic, strong) NSArray *array;

@property (nonatomic, strong) NSDictionary *dic;

NSMutableString *mustr = [[NSMutableString alloc]initWithFormat:@"123"];
    _str = mustr;
    NSLog(@"%@",_str);
    [mustr appendString:@"456"];
    NSLog(@"%@",_str);
    
    NSMutableArray *muarray = [NSMutableArray arrayWithObjects:@"a",@"b",@"c",nil];
    _array = muarray;
    NSLog(@"%@",_array);
    [muarray addObject:@"d"];
    NSLog(@"%@",_array);
    
    NSMutableDictionary *muDic = [NSMutableDictionary dictionaryWithObjectsAndKeys:@"a",@"1",@"b",@"2",nil];
    _dic = muDic;
    NSLog(@"%@",_dic);
    [muDic setObject:@"3" forKey:@"c"];
    NSLog(@"%@",_dic);

输出结果：

屏幕快照 2017-02-23 17.47.13.png

结论：当把NSString、NSDirectory、NSArray声明为strong，那么如果原对象的值发生变化，会影响到当前对象的值,不管当前对象是调用点语法赋值还是直接对实例对象赋值。

当NSString、NSDirectory、NSArray在声明属性的时候设置为Copy类型时:

(1)调用set方法赋值

@property (nonatomic, copy) NSString *str;

@property (nonatomic, copy) NSArray *array;

@property (nonatomic, copy) NSDictionary *dic;

  self.str = mustr;
    NSLog(@"%@",_str);
    [mustr appendString:@"456"];
    NSLog(@"%@",_str);
    
    NSMutableArray *muarray = [NSMutableArray arrayWithObjects:@"a",@"b",@"c",nil];
    self.array = muarray;
    NSLog(@"%@",_array);
    [muarray addObject:@"d"];
    NSLog(@"%@",_array);
    
    NSMutableDictionary *muDic = [NSMutableDictionary dictionaryWithObjectsAndKeys:@"a",@"1",@"b",@"2",nil];
    self.dic = muDic;
    NSLog(@"%@",_dic);
    [muDic setObject:@"3" forKey:@"c"];
    NSLog(@"%@",_dic);

输出结果：

屏幕快照 2017-02-23 18.08.58.png

（2）直接对实例对象赋值

@property (nonatomic, copy) NSString *str;

@property (nonatomic, copy) NSArray *array;

@property (nonatomic, copy) NSDictionary *dic;

NSMutableString *mustr = [[NSMutableString alloc]initWithFormat:@"123"];
  _str = mustr;
  NSLog(@"%@",_str);
  [mustr appendString:@"456"];
  NSLog(@"%@",_str);
  
  NSMutableArray *muarray = [NSMutableArray arrayWithObjects:@"a",@"b",@"c",nil];
  _array = muarray;
  NSLog(@"%@",_array);
  [muarray addObject:@"d"];
  NSLog(@"%@",_array);
  
  NSMutableDictionary *muDic = [NSMutableDictionary dictionaryWithObjectsAndKeys:@"a",@"1",@"b",@"2",nil];
  _dic = muDic;
  NSLog(@"%@",_dic);
  [muDic setObject:@"3" forKey:@"c"];
  NSLog(@"%@",_dic);

当设置为Copy属性时，如果调用点语法赋值，当前对象不会因为原对象的改变而改变，但是如果是直接赋值，那么原对象改变那么当前对象也会改变。

总结：当对NSString这种有可变子类的对象直接赋值时，不管声明属性时声明的是copy还是strong,现对象的值都会随着原对象的值而改变。当调用set方法赋值时，如果对象声明的是copy那么现有对象不会因为原对象的改变而改变，如果声明的是strong，那么现有对象会因为原对象的改变而改变。

个人理解：声明为strong时，其实只是把当前对象的指针指向了原有对象的地址，因此当原有对象改变时，现有对象也会改变。直接赋值也应该是这样。当声明为Copy时，其实是把原有对象复制了一份，然后让现有对象的指针指向复制的这一份。