使用@property(copy)与@property(retain)的经验法则是什么? [英] What is the rule of thumb for using @property(copy) vs. @property(retain)?

问题描述

在确定ObjectiveC中的给定属性应为retain还是copy时，我想知道是否遵循经验法则?

I wonder if there is a rule of thumb you follow, when deciding whether or not a given property in ObjectiveC should be a retain or copy?

您如何确定应该是哪个?

How do you decide which it should be?

推荐答案

通常，为了安全起见，将copy用于具有可变变体的类，例如NSString，NSArray，其他集合类等.为什么，考虑这里发生了什么...

Typically you use copy for safety with classes which have mutable variants, like NSString, NSArray, the other collection classes, etc. To see why, consider what happens here...

从前，

@interface MyClass : NSObject
@property (retain) NSString *happyString;
- (void)rejoice;
@end

那一天，

- (void)bigBadMethod {
    MyClass *myObject = [[[MyClass alloc] init] autorelease];
    NSMutableString *theString = [NSMutableString stringWithString:@"I'm happy!"];
    myObject.happyString = theString; // this is allowed because NSMutableString inherits from NSString
    [myObject rejoice]; // prints "I'm happy!"

突然之间...

    [theString setString:@"BRAAAAIIINNNSSSSS"];
    [myObject rejoice]; // prints "BRAAAAIIINNNSSSSS"
}

您不想那样，对吗?因此，如果您不想在不看的时候发生变异，请使用@property (copy)！

And you wouldn't want that, would you? So use @property (copy) if you don't want to get mutated while you're not looking!

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