How can i tell if an object has a key value observ

问题:

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

if (object has observer){
  remove observer
}
else{
  go on my merry way
}

回答1:

Put a try catch around your removeObserver call

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

回答2:

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.

回答3:

FWIW, [someObject observationInfo] seems to be nil if someObject 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 observationInfo to get specific observers.

回答4:

When you add an observer to an object you could add it to a NSMutableArray like this:

- (void)addObservedObject:(id)object {
    if (![_observedObjects containsObject:object]) {
        [_observedObjects addObject:object];
    }
}

If you want to unobserve the objects you can do something like:

for (id object in _observedObjects) {
    if ([object isKindOfClass:[MyClass class]]) {
        MyClass *myObject = (MyClass *)object;
        [self unobserveMethod:myObject];
    }
}
[_observedObjects removeAllObjects];

Remember, if you unobserve a single object remove it from the _observedObjects array:

- (void)removeObservedObject:(id)object {
    if ([_observedObjects containsObject:object]) {
        [_observedObjects removeObject:object];
    }
}

回答5:

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

回答6:

In my opinion - this works similar to retainCount mechanism. You can't be sure that at the current moment you have your observer. Even if you check: self.observationInfo - you can't know for sure that you will have/won't have observers in future.

Like retainCount. Maybe the observationInfo method is not exactly that kind of useless, but I only use it in debug purposes.

So as a result - you just have to do it like in memory management. If you added an observer - just remove it when you don't need it. Like using viewWillAppear/viewWillDisappear etc. methods. E.g:

-(void) viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
    [self addObserver:nil forKeyPath:@"" options:NSKeyValueObservingOptionNew context:nil];
}

-(void) viewWillDisappear:(BOOL)animated
{
    [super viewWillDisappear:animated];
    [self removeObserver:nil forKeyPath:@""];
}

And it you need some specific checks - implement your own class that handles an array of observers and use it for your checks.

回答7:

[someObject observationInfo] return nil if there is no observer.

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

}

回答8:

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.

回答9:

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:

@interface ObjectA : NSObject
-(void)subscribeToKeyboardNotifications;
-(void)unsubscribeToKeyboardNotifications;
@end

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

@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

回答10:

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

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

example of usage

- (void)dealloc {
    SafeRemoveObserver(someObject, self, somePath);
}