iphone - cocoa KVO :observeValueForKeyPath:ofObject:change:context: is called twice for one value change

Question

我有一个 NSOperation 子类，它实现 setFinished: 来生成 KVO 通知:

-(void)setFinished:(BOOL)isFinished
{
    LogError(@"%@ will will changing isFinished to %d",self,isFinished);
    [self willChangeValueForKey:@"isFinished"];
    LogError(@"%@ did will changing isFinished to %d",self,isFinished);
    _isFinished = isFinished;
    LogError(@"%@ will did changing isFinished to %d",self,isFinished);
    [self didChangeValueForKey:@"isFinished"];
    LogError(@"%@ did did changing isFinished to %d",self,isFinished);
}

我也确信我只将观察者添加到操作中一次。

奇怪的事情来了，有时这个对象的isFinished键路径的observeValueForKeyPath:ofObject:change:context:被调用了两次，引用日志，我只发现setFinished: 调用一次，而此操作的 addObserver:forKeyPath: 仅调用一次。

另外，我发现它的顺序是这样的:

will will changing isFinished to 1
did will changing isFinished to 1
will did changing isFinished to 1
calling observeValueForKeyPath for object
did did changing isFinished to 1
calling observeValueForKeyPath for object

请问有人有任何线索吗？

Answer 1

如果你想负责调用willChangeValueForKey:和didChangeValueForKey:，你必须告诉系统。您可以通过两种方式做到这一点:

• 您可以在类上重写+automaticallyNotizesObserversForKey:(类方法)，以便在 key 完成时返回NO。这记录在“Manual Change Notification” in the *Key-Value Observing Programming Guide下.

• 您可以实现+automaticallyNotifyingObserversOfFinished来返回NO。这记录在 NSKeyValueObserving.h 头文件的 +automaticallyNotizesObserversForKey: 注释中。

如果您没有正确实现这些类方法之一，系统将在 setFinished: 返回时自动通知观察者。看起来这就是您在日志中看到的内容。