blocks|key|1428960|text|真正的问题是为什么你不知道你是否在观察它。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1428961|如果您是在被观察对象的类中执行此操作，请停止。无论是什么在观察，它都希望继续观察它。如果您在观察者不知情的情况下切断了观察者的通知，那么预计事情会崩溃；更具体地说，期望观察者的状态变得陈旧，因为它不会从以前观察到的对象接收更新。|1428962|如果您是在观察对象的类中执行此操作，则只需记住您正在观察哪些对象(或者，如果您只观察过一个对象，请记住您是否正在观察它)。这是假设观察值是动态的，并且在两个不相关的对象之间；如果观察者拥有观察者，只需在创建或保留观察者之后添加观察者，并在释放观察者之前删除观察者。|1428963|添加和删除作为观察者的对象通常应该发生在观察者的类中，而不应该发生在被观察对象的类中。|1428964|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|L|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|M|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

The real question is why you don't know whether you're observing it or not.

If you're doing this in the class of the object being observed, stop. Whatever's observing it expects to keep observing it. If you cut off the observer's notifications without its knowledge, expect things to break; more specifically, expect the observer's state to go stale as it doesn't receive updates from the formerly-observed object.

If you're doing this in the observing object's class, simply remember which objects you're observing (or, if you only ever observe one object, whether you're observing it). This is assuming that the observation is dynamic and between two otherwise-unrelated objects; if the observer owns the observed, just add the observer after you create or retain the observed, and remove the observer before you release the observed.

Adding and removing an object as an observer should usually happen in the observer's class, and never in the observed object's.

blocks|key|1214181|text|唯一的方法是在添加观察者时设置一个标志。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1214182|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

The only way to do this is to set a flag when you add an observer.

blocks|key|1214262|text|观察者模式的全部要点是允许被观察的类被“密封”--不知道或不关心它是否被观察到。您显然是在试图打破这种模式。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1214263|为什么？|1214264|你遇到的问题是，你假设你正在被观察，而你并没有被观察到。这个对象没有开始观察。如果你想让你的类控制这个过程，那么你应该考虑使用通知中心。这样，您的类就可以完全控制何时可以观察到数据。因此，它并不关心谁在看。|1214265|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

The whole point of the observer pattern is to allow an observed class to be "sealed" -- to not know or care whether it is being observed. You are explicitly trying to break this pattern. 

Why?

The problem you are having is that you are assuming you are being observed when you aren't. This object did not start the observation. If you want your class to have control of this process, then you should consider using the notification center. That way your class has full control on when data can be observed. Hence, it doesn't care who is watching.

blocks|key|1214275|text|在您的removeObserver调用周围放置一个try+catch|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1214276|@try{
+++[someObject+removeObserver:someObserver+forKeyPath:somePath];
}@catch(id+anException){
+++//do+nothing,+obviously+it+wasn't+attached+because+an+exception+was+thrown
}|code-block|syntax|javascript|1214277|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Put a try catch around your removeObserver call

<pre><code>@try{
 [someObject removeObserver:someObserver forKeyPath:somePath];
}@catch(id anException){
 //do nothing, obviously it wasn't attached because an exception was thrown
}
</code></pre>

blocks|key|1214299|text|如果someObject没有任何观察者，那么[someObject+observationInfo]似乎是nil。然而，我不会相信这种行为，因为我还没有看到它的文档。另外，我不知道如何阅读observationInfo来获得特定的观察者。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1214300|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.apple.com/documentation/objectivec/nsobject/1414009-observationinfo?language=objc^0|2|A|M|S|1H|3|2M|F|M|S|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@$9|O|A|P|B|C]|$9|Q|A|R|B|C]|$9|S|A|T|B|C]|$9|U|A|V|B|C]]|D|@$9|W|A|X|1|Y]]|E|$]]|$1|F|3|-4|5|6|7|Z|8|@]|D|@]|E|$]]]|G|$H|$5|I|J|K|E|$L|M]]]]

FWIW, <a href="https://developer.apple.com/documentation/objectivec/nsobject/1414009-observationinfo?language=objc" rel="noreferrer"><code>[someObject observationInfo]</code></a> seems to be <code>nil</code> if <code>someObject</code> doesn't have any observers. I wouldn't trust this behavior, however, as I haven't seen it documented. Also, I don't know how to read <code>observationInfo</code> to get specific observers.

blocks|key|1429171|text|我不喜欢try+catch解决方案，所以我大多数时候做的是为该类中的特定通知创建一个订阅和取消订阅方法。例如，这两种方法订阅或取消订阅对象的全局键盘通知：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1429172|@interface+ObjectA+:+NSObject
-(void)subscribeToKeyboardNotifications;
-(void)unsubscribeToKeyboardNotifications;
@end|code-block|syntax|javascript|1429173|在这些方法中，我使用了一个私有属性，该属性根据订阅状态设置为true或false，如下所示：|1429174|@interface+ObjectA()
@property+(nonatomic,assign)+BOOL+subscribedToKeyboardNotification
@end

@implementation

-(void)subscribeToKeyboardNotifications+{
++++if+(!self.subscribedToKeyboardNotification)+{
++++++++[[NSNotificationCenter+defaultCenter]addObserver:self+selector:@selector(onKeyboardShow:)+name:UIKeyboardWillShowNotification+object:nil];
++++++++[[NSNotificationCenter+defaultCenter]addObserver:self+selector:@selector(onKeyboardHide:)+name:UIKeyboardWillHideNotification+object:nil];
++++++++self.subscribedToKeyboardNotification+=+YES;
++++}
}

-(void)unsubscribeToKeyboardNotifications+{
++++if+(self.subscribedToKeyboardNotification)+{
++++++++[[NSNotificationCenter+defaultCenter]removeObserver:self+name:UIKeyboardWillShowNotification+object:nil];
++++++++[[NSNotificationCenter+defaultCenter]removeObserver:self+name:UIKeyboardWillHideNotification+object:nil];
++++++++self.subscribedToKeyboardNotification+=+NO;
++++}
}
@end|1429175|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

I am not a fan of that try catch solution so
what i do most of the time is that i create a subscribe and unsubscribe method for a specific notification inside that class. For example these two methods subcribe or unsubscribe the object to the global keyboard notification:

<pre><code>@interface ObjectA : NSObject
-(void)subscribeToKeyboardNotifications;
-(void)unsubscribeToKeyboardNotifications;
@end
</code></pre>

Inside those methods i use a private property which is set to true or false depending on the subscription state like so:

<pre><code>@interface ObjectA()
@property (nonatomic,assign) BOOL subscribedToKeyboardNotification
@end

@implementation

-(void)subscribeToKeyboardNotifications {
 if (!self.subscribedToKeyboardNotification) {
 [[NSNotificationCenter defaultCenter]addObserver:self selector:@selector(onKeyboardShow:) name:UIKeyboardWillShowNotification object:nil];
 [[NSNotificationCenter defaultCenter]addObserver:self selector:@selector(onKeyboardHide:) name:UIKeyboardWillHideNotification object:nil];
 self.subscribedToKeyboardNotification = YES;
 }
}

-(void)unsubscribeToKeyboardNotifications {
 if (self.subscribedToKeyboardNotification) {
 [[NSNotificationCenter defaultCenter]removeObserver:self name:UIKeyboardWillShowNotification object:nil];
 [[NSNotificationCenter defaultCenter]removeObserver:self name:UIKeyboardWillHideNotification object:nil];
 self.subscribedToKeyboardNotification = NO;
 }
}
@end
</code></pre>

blocks|key|1429188|text|如果没有观察者，则[someObject+observationInfo]返回nil。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1429189|if+([tableMessage+observationInfo]+==+nil)
{
+++NSLog(@"add+your+observer");
}
else
{
++NSLog(@"remove+your+observer");

}|code-block|syntax|javascript|1429190|entityMap^0|9|S|13|3|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|L|$]]

<code>[someObject observationInfo]</code> return <code>nil</code> if there is no observer.

<pre><code>if ([tableMessage observationInfo] == nil)
{
 NSLog(@"add your observer");
}
else
{
 NSLog(@"remove your observer");

}
</code></pre>

blocks|key|1214450|text|除了Adam的回答之外，我还建议使用这样的宏|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1214451|#define+SafeRemoveObserver(sender,+observer,+keyPath)+\
@try{\
+++[sender+removeObserver:observer+forKeyPath:keyPath];\
}@catch(id+anException){\
}|code-block|syntax|javascript|1214452|用法示例|1214453|-+(void)dealloc+{
++++SafeRemoveObserver(someObject,+self,+somePath);
}|1214454|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

In addition to Adam's answer I would like to suggest to use macro like this

<pre><code>#define SafeRemoveObserver(sender, observer, keyPath) \
@try{\
 [sender removeObserver:observer forKeyPath:keyPath];\
}@catch(id anException){\
}
</code></pre>

example of usage

<pre><code>- (void)dealloc {
 SafeRemoveObserver(someObject, self, somePath);
}
</code></pre>

if you tell an objective c object to removeObservers: for a key path and that key path has not been registered, it cracks the sads. like -

'Cannot remove an observer for the key path "theKeyPath" from because it is not registered as an observer.'

is there a way to determine if an object has a registered observer, so i can do this

<pre><code>if (object has observer){
 remove observer
}
else{
 go on my merry way
}
</code></pre>

How can i tell if an object has a key value observer attached

如果您将一个对象C对象告诉removeObservers: for一个键路径，而该键路径还没有注册，它就会破解sads。就像-‘无法从删除密钥路径’theKeyPath‘的观察者，因为它未注册为观察者。’有没有一种方法可以确定一个对象是否有注册的观察者，这样我就可以这样做了？if (object has observe...

问如何判断对象是否附加了键值观察器
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