NSString 属性为啥用copy 不用strong

copy不能修改，strong可以修改，防止字符串被意外修改.demo:

——————————————————code 你要的 demo——————————————————

@property (nonatomic, copy) NSString * copystring;

@property (nonatomic, strong) NSString *strongString;

NSMutableString *string = [NSMutableString stringWithString:@"a"];
   
    self.copystring = string;
    self.strongString = string;
   
    [string appendString: @"b"];

NSLog(@"copyString:%@--StrongString:%@",self.copystring, self.strongString); // opyString:a--StrongString:ab

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