为什么我更喜欢unsafe_unretained限定符而不是赋值给弱引用属性？ [英] Why should I prefer the unsafe_unretained qualifier over assign for weak referencing properties?

问题描述

可能重复：

使用ARC，终身限定符分配和unsafe_unretained

两者有什么区别？

@property(unsafe_unretained) MyClass *delegate;
@property(assign) MyClass *delegate;

两者都是非归零弱引用，对吧？那么有什么理由我应该写更长更难阅读 unsafe_unretained 而不是 assign ？

Both are non-zeroing weak references, right? So is there any reason why I should write the longer and harder to read unsafe_unretained instead of assign?

注意：我知道弱这是一个归零参考。但它只有iOS> = 5.

Note: I know there is weak which is a zeroing reference. But it's only iOS >= 5.

推荐答案

在属性访问器中，是的，它们是相同的。对于这种情况， assign 属性将映射到 unsafe_unretained 。但是考虑手动编写ivar而不是使用ivar合成。

In a property accessor, yes those are the same. assign properties will map to unsafe_unretained for this case. But consider writing the ivar manually rather than using ivar synthesis.

@interface Foo
{
   Bar *test;
}
@property(assign) Bar *test;
@end

这个代码现在在ARC下是不正确的，而在ARC之前它是'吨。所有Obj-C对象的默认属性是 __ strong 向前移动。推进这项工作的正确方法如下：

This code is now incorrect under ARC, whereas prior to ARC it wasn't. The default attribute for all Obj-C objects is __strong moving forward. The proper way to do this moving forward is as follows.

@interface Foo
{
   __unsafe_unretained Bar *test;
}
@property(unsafe_unretained) Bar *test;
@end

或使用ivar合成只需 @property（unsafe_unretained ）Bar * test

or with ivar synthesis just @property(unsafe_unretained) Bar *test

所以它真的只是一种不同的写作方式，但它表现出不同的意图。

So really its just a different way of writing the same thing, but it shows a different intent.

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