你需要调用willChangeValueForKey：和didChangeValueForKey：？ [英] Do you need to call willChangeValueForKey: and didChangeValueForKey:?

问题描述

我认为自煮熟的@property设置应该是这样：

I thought home-cooked @property setters were supposed to look like this:

-(void) setFoo:(Foo *)newFoo {

  // Safeguards 
  // ...

  [self willChangeValueForKey:@"foo"];
  // Switcheroo
  // ...
  [self didChangeValueForKey:@"foo"];
}

但是我看到很多博客文章中的代码做可可太多比我有，这是这样的：

But I see a lot of code in blog posts by people who've been doing Cocoa a lot longer than I have, where it's like this:

-(void) setFoo(Foo *)newFoo {

  // Safeguards 
  // ...

  // Switcheroo
  // ...
}

所以我的问题是，我们需要调用KVO通知方法吗？

So my question is, do we need to call the KVO-notification methods? Or is it being done magically when you update the private iVar, if you're using the modern runtime?

推荐答案

这是神奇的做法除非您选择停用。阅读本节的KVO指南。注意，KVC / KVO存在于从古至今（即在引入 @property 之前），因此设置器 @synthesize d或不。它甚至不涉及旧/新运行时二分法。

It's done magically unless you opt-out. read this section of the KVO guide. Note that KVC/KVO existed from time immemorial (i.e. before the introduction of @property) so it doesn't matter whether the setter is @synthesized or not. It's not even related to the old/new runtime dichotomy.

这个魔法（isa-swizzling）的细节在博文。这是魔法。基本上，当观察到一个键时，运行时会自动替换setter的实现，以便调用KVO通知。

The detail of this magic (isa-swizzling) was detailed in a blog post by Mike Ash. It's magic. Basically, when a key is observed, the runtime automagically replaces the implementation of the setter so that it calls the KVO notification.

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