Removing observers for an NSOperation - iphone

Question

I have a view which loads data via an NSOperation within an NSOperationQueue. I want to allow users to leave this view before the operation has completed. My problem is that I can't seem to consistently do this without crashing. Here is my code to start the operation:
NSOperationQueue* tmpQueue = [[NSOperationQueue alloc] init];
self.queue = tmpQueue;
[tmpQueue release];
SportsLoadOperation* loadOperation = [[SportsLoadOperation alloc] init];
[loadOperation addObserver:self forKeyPath:#"isFinished" options:0 context:NULL];
[self.queue addOperation:loadOperation];
[loadOperation release];
If I leave the view while the operation is still executing, I often get this error:
[SportsViewController retain]: message sent to deallocated instance 0x38b5a0
If I try to remove the observers so that this doesn't occur, like this:
-(void)viewWillDisappear:(BOOL)animated {
if (self.isLoadingData) {
for (NSOperation *operation in [self.queue operations]) {
if([operation isExecuting]) {
[operation cancel];
[operation removeObserver:self forKeyPath:#"isFinished"];
}
}
}
[super viewWillDisappear:animated];
}
Then I sometimes get this error:
Terminating app due to uncaught exception 'NSRangeException', reason:
'Cannot remove an observer <SportsViewController 0x661c730> for the key path "isFinished" from <SportsLoadOperation 0x66201a0> because it is not registered as an observer.'
How can I avoid these problems?

Answer 1

The 2nd error message says it all.
Have you tried to not removeObserver after [operation cancel] and see what happens then?
Have you tried to first removeObserver and only then cancel the operation?
These might help to narrow down the conditions that trigger the error. Also, you might want to add log output to the code to see when it actually executes.
And, like freespace's answer says, adding & removing observers is best done in the construction / destruction methods of the observed instances. This generally yields more stable code.

Answer 2

Looks like you have an instance of SportsLoadOperation that doesn't have SportsViewController as an observer. Are you inserting SportsLoadOperation anywhere else in your code? If this is the case, consider writing an initWithObserver method for SportsLoadOperaion that will do the observing automatically. This avoids errors caused by forgetting to set the observer on isFinished.
Also, it is probably better to do the removal of observer in dealloc then in viewWillDisappear because viewWillDisappear is called in many circumstances, e.g. when displaying a modal view controller. Thus you might be prematurely stopping your operations.
Edit
Instead of checking against [operation isExecuting] check against [operation isCancelled]. This is because [operation cancel] doesn't stop an operation - it can and will continue executing if you don't actually check for isCancelled in your main method. This means that if viewWillDisappear is called twice, you could end up attempting to call removeObserver twice on the same instance of SportsLoadOperation, with the second attempt failing.
Add some debugging output statements in the following places:
when you create a SportsLoadOperation instance and insert it into the queueu
when you are cancelling a SportsLoadOperation instance and removing from it observers.

Answer 3

I ended up solving this by overriding the observed operation's addObserver and removeObserver methods, to keep track of observers so I could remove them in the [cancel] method.
All I have to do now is call the operation queue to cancel all operations before I dismiss the controller that was observing the operations.
Below, _observers is an NSMutableDictionary.
- (void)addObserver:(NSObject*)observer
forKeyPath:(NSString*)keyPath
options:(NSKeyValueObservingOptions)options context:(void*)context
{
[super addObserver:observer forKeyPath:keyPath options:options context:context];
[_observers setValue:observer forKey:keyPath];
}
- (void)removeObserver:(NSObject*)observer forKeyPath:(NSString*)keyPath
{
[super removeObserver:observer forKeyPath:keyPath];
[_observers removeObjectForKey:keyPath];
}
- (void)cancel
{
[super cancel];
for(id key in _observers)
{
id object = [_observers valueForKey:key];
[super removeObserver:object forKeyPath:key];
}
[_observers removeAllObjects];
}