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.