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

Question

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 observe

Answer 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
}

Answer 2

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.

Answer 3

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];
    }
}

Answer 4

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);
}

Answer 5

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.

Answer 6

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