iOS-copy和strong属性@property标识符

约定不等于承诺〃 2023-10-03 08:34 22阅读 0赞

我们都知道使用@property属性标识符修饰不可变变量如：NSString，NSArray，NSDictionary，NSSet等时都会使用copy修饰，如下：

@property (nonatomic, copy) NSString *cpStr;

那么为什么不使用strong修饰呢，如果使用strong修饰会有什么问题呢？接下来我们一起探讨一下

我声明了四组数据来进行测试

#### 1、**strong** 修饰 **不可变对象** ####

@property (nonatomic, strong) NSString *strongStr;
    @property (nonatomic, strong) NSArray *strongArr;
    @property (nonatomic, strong) NSDictionary *strongDic;
    @property (nonatomic, strong) NSSet *strongSet;
    
    
    /*----strong--不可变--*/
        NSLog(@"--strong--不可变--");
        NSString *str = @"123";
        self.strongStr = str;
        str = @"666";
        NSLog(@"--strong--不可变字符串--");
        NSLog(@"str：%p    self.strongStr:%p",str,self.strongStr);
        NSLog(@"str：%@    self.strongStr:%@",str,self.strongStr);
        
        NSArray *arr = [NSArray arrayWithObjects:@1, nil];
        self.strongArr = arr;
        arr = [arr arrayByAddingObject:@8];//[NSArray arrayWithObjects:@8, nil];
        NSLog(@"--strong--不可变数组--");
        NSLog(@"arr：%p \n self.strongArr：%p",arr,self.strongArr);
        NSLog(@"arr：%@ \n self.strongArr：%@",arr,self.strongArr);
        
        NSDictionary *dic = [NSDictionary dictionaryWithObjects:@[@"123"] forKeys:@[@"key1"]];
        self.strongDic = dic;
        dic = [NSDictionary dictionaryWithObjects:@[@"666"] forKeys:@[@"key3"]];
        NSLog(@"--strong--不可变字典--");
        NSLog(@"dic：%p \n self.strongDic：%p",dic,self.strongDic);
        NSLog(@"dic：%@ \n self.strongDic：%@",dic,self.strongDic);
        
        NSSet *set = [[NSSet alloc] initWithObjects:@"one",nil];
        self.strongSet = set;
        set = [[NSSet alloc] initWithObjects:@"three", nil];
        NSLog(@"--strong--不可变集合--");
        NSLog(@"set：%p \n self.strongSet：%p",set,self.strongSet);
        NSLog(@"set：%@ \n self.strongSet：%@",set,self.strongSet);

输出结果：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L01pbmdnZVFpbmdjaHVu_size_16_color_FFFFFF_t_70][]

内存地址不同，值不同

#### **2、strong 修饰 可变对象** ####

@property (nonatomic, strong) NSString *strongMStr;
    @property (nonatomic, strong) NSArray *strongMArr;
    @property (nonatomic, strong) NSDictionary *strongMDic;
    @property (nonatomic, strong) NSSet *strongMSet;
    
    
    /*----strong--可变--*/
        NSLog(@"--strong--可变--");
        NSMutableString * mString = [NSMutableString stringWithString:@"123"];
        self.strongMStr = mString;
        [mString appendString:@"666"];
        NSLog(@"--strong--可变字符串--");
        NSLog(@"mString：%p    self.strongMStr:%p",mString,self.strongMStr);
        NSLog(@"mString：%@    self.strongMStr:%@",mString,self.strongMStr);
        
        NSMutableArray *mArr = [NSMutableArray arrayWithObjects:@1, nil];
        self.strongMArr = mArr;
        [mArr addObject:@8];
        NSLog(@"--strong--可变数组--");
        NSLog(@"mArr：%p \n self.strongMArr：%p",mArr,self.strongMArr);
        NSLog(@"mArr：%@ \n self.strongMArr：%@",mArr,self.strongMArr);
        
        NSMutableDictionary *mDic = [NSMutableDictionary dictionaryWithObjects:@[@"123"] forKeys:@[@"key1"]];
        self.strongMDic = mDic;
        [mDic setValue:@"666" forKey:@"key3"];
        NSLog(@"--strong--可变字典--");
        NSLog(@"mDic：%p \n self.strongMDic：%p",mDic,self.strongMDic);
        NSLog(@"mDic：%@ \n self.strongMDic：%@",mDic,self.strongMDic);
        
        NSMutableSet *mSet = [[NSMutableSet alloc] initWithObjects:@"one", nil];
        self.strongMSet = mSet;
        [mSet addObject:@"three"];
        NSLog(@"--strong--可变集合--");
        NSLog(@"mSet：%p \n self.strongMSet：%p",mSet,self.strongMSet);
        NSLog(@"mSet：%@ \n self.strongMSet：%@",mSet,self.strongMSet);

输出结果：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L01pbmdnZVFpbmdjaHVu_size_16_color_FFFFFF_t_70 1][]

内存地址相同，值相同

#### **3、copy 修饰 不可变对象** ####

@property (nonatomic, copy) NSString *cpStr;
    @property (nonatomic, copy) NSArray *cpArr;
    @property (nonatomic, copy) NSDictionary *cpDic;
    @property (nonatomic, copy) NSSet *cpSet;
    
    
    /*----copy--不可变--*/
        NSLog(@"");
        NSLog(@"--copy--不可变--");
        NSString *cpStr = @"123";
        self.cpStr = cpStr;
        cpStr = @"666";
        NSLog(@"--copy--不可变字符串--");
        NSLog(@"cpStr：%p    self.cpStr:%p",cpStr,self.cpStr);
        NSLog(@"cpStr：%@    self.cpStr:%@",cpStr,self.cpStr);
        
        NSArray *cpArr = [NSArray arrayWithObjects:@1,nil];
        self.cpArr = cpArr;
        cpArr = [NSArray arrayWithObjects:@8, nil];
        NSLog(@"--copy--不可变字数组--");
        NSLog(@"cpArr：%p \n self.cpArr：%p",cpArr,self.cpArr);
        NSLog(@"cpArr：%@ \n self.cpArr：%@",cpArr,self.cpArr);
        
        NSDictionary *cpDic = [NSDictionary dictionaryWithObjects:@[@"123"] forKeys:@[@"key1"]];
        self.cpDic = cpDic;
        cpDic = [NSDictionary dictionaryWithObjects:@[@"666"] forKeys:@[@"key3"]];
        NSLog(@"--copy--不可变字典--");
        NSLog(@"cpDic：%p \n self.cpDic：%p",cpDic,self.cpDic);
        NSLog(@"cpDic：%@ \n self.cpDic：%@",cpDic,self.cpDic);
        
        NSSet *cpSet = [[NSSet alloc] initWithObjects:@"one", nil];
        self.cpSet = cpSet;
        cpSet = [[NSSet alloc] initWithObjects:@"three", nil];
        NSLog(@"--copy--不可变集合--");
        NSLog(@"cpSet：%p \n self.cpSet：%p",set,self.cpSet);
        NSLog(@"cpSet：%@ \n self.cpSet：%@",set,self.cpSet);

输出结果

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L01pbmdnZVFpbmdjaHVu_size_16_color_FFFFFF_t_70 2][]

内存地址不同，值不同

#### 4、copy 修饰 可变对象 ####

@property (nonatomic, copy) NSString *cpMStr;
    @property (nonatomic, copy) NSArray *cpMArr;
    @property (nonatomic, copy) NSDictionary *cpMDic;
    @property (nonatomic, copy) NSSet *cpMSet;
    
    
    /*----copy--可变--*/
        NSLog(@"--copy--可变--");
        NSMutableString * cpMStr = [NSMutableString stringWithString:@"123"];
        self.cpMStr = cpMStr;
        [cpMStr appendString:@"666"];
        NSLog(@"--copy--可变字符串--");
        NSLog(@"cpMStr：%p    self.cpMStr:%p",cpMStr,self.cpMStr);
        NSLog(@"cpMStr：%@    self.cpMStr:%@",cpMStr,self.cpMStr);
        
        NSMutableArray *cpMArr = [NSMutableArray arrayWithObjects:@1, nil];
        self.cpMArr = cpMArr;
        [cpMArr addObject:@6];
        NSLog(@"--copy--可变数组--");
        NSLog(@"cpMArr：%p \n self.cpMArr:%p",cpMArr,self.cpMArr);
        NSLog(@"cpMArr：%@ \n self.cpMArr:%@",cpMArr,self.cpMArr);
        
        NSMutableDictionary *cpMDic = [NSMutableDictionary dictionaryWithObjects:@[@"123"] forKeys:@[@"key1"]];
        self.cpMDic = cpMDic;
        [cpMDic setValue:@"666" forKey:@"key3"];
        NSLog(@"--copy--可变字典--");
        NSLog(@"cpMDic：%p \n self.cpMDic：%p",cpMDic,self.cpMDic);
        NSLog(@"cpMDic：%@ \n self.cpMDic：%@",cpMDic,self.cpMDic);
        
        NSMutableSet *cpMSet = [[NSMutableSet alloc] initWithObjects:@"one", nil];
        self.cpMSet = cpMSet;
        [cpMSet addObject:@"three"];
        NSLog(@"--copy--可变集合--");
        NSLog(@"cpMSet：%p \n self.cpMSet：%p",mSet,self.cpMSet);
        NSLog(@"cpMSet：%@ \n self.cpMSet：%@",mSet,self.cpMSet);

输出结果

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L01pbmdnZVFpbmdjaHVu_size_16_color_FFFFFF_t_70 3][]

内存地址不同，值不同

#### **总结：** ####

在这里我们会发现，只有在第二种情况：strong修饰 可变对象时，如果可变对象的值发生了改变，那么strong修饰的对象会跟着改变。

当使用 strong 修饰属性的时候，属性的setter方法会直接强引用该对象，这样当原对象的值发生改变时，新对象的属性也跟随改变

对于NSString，NSArray，NSDictionary，NSSet等这些不可变对象，如果对这个对象进行可变对象的操作则会产生异常crash。

因此，我们使用copy修饰NSString，NSArray，NSDictionary，NSSet等这些不可变对象，copy可以让本对象不受外界（子对象）影响，无论给我传入的是一个可变对象还是一个不可变对象，都能保证自身持有的是一个不可变副本；防止可变对象更改自身的值的时候不会影响到property对象属性。

针对不可变对象使用copy修饰，针对可变对象使用strong修饰。

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[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L01pbmdnZVFpbmdjaHVu_size_16_color_FFFFFF_t_70 1]: https://img-blog.csdnimg.cn/20210619143405460.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L01pbmdnZVFpbmdjaHVu,size_16,color_FFFFFF,t_70
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L01pbmdnZVFpbmdjaHVu_size_16_color_FFFFFF_t_70 2]: https://img-blog.csdnimg.cn/20210619143504560.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L01pbmdnZVFpbmdjaHVu,size_16,color_FFFFFF,t_70
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L01pbmdnZVFpbmdjaHVu_size_16_color_FFFFFF_t_70 3]: https://img-blog.csdnimg.cn/20210619143618334.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L01pbmdnZVFpbmdjaHVu,size_16,color_FFFFFF,t_70