NSOperation KVO problem

Question

Hi all,

I'm using following function to get my application notified after the operation in nsoperationqueue has finished, so that I can schedule the task that's dependent upon the result of the operation. I'm using:

- (void)observeValueForKeyPath:(NSString *)keyPath 
                      ofObject:(id)object 
                        change:(NSDictionary *)change 
                       context:(void *)context
{
  if([keyPath isEqual:@"isFinished"] && _operation == object)
  {  
    NSLog(@"Our Thread Finished!");
    [_operation removeObserver:self forKeyPath:@"isFinished"];
    [self performSelectorOnMainThread:@selector(showDialog) withObject:nil waitUntilDone:YES];
  } 
}

My question is since mostly the tasks assigned to these operations are parsing of data if I try to tap some other button or basically do something that results in action, I get the following exception:

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '<Settings: 0x21b970>: An -observeValueForKeyPath:ofObject:change:context: message was received but not handled.
Key path: isFinished

I perfectly understand that since I try doing other things on main thread, because of which the call to main thread:

[self performSelectorOnMainThread:@selector(showDialog) withObject:nil waitUntilDone:YES];

fails to get executed. But what's the solution to this problem as I want both allow user do any action after making a request and also perform the action scheduled after finishing the task assigned to the operation.

Is it really possible?

Thanx in advance.

Answer 1

If you can require Mac OS X 10.6 Snow Leopard, or (I think) iPhone OS 3.0, you can avoid KVO entirely for this. Just create another operation for the work you want to do on the main thread, add the operation it needs to follow as a dependency, and then put the main-thread operation on the main queue:

NSBlockOperration *mainThreadOp = [NSBlockOperation blockOperationWithBlock:^{
    [self showDialog];
}];
[mainThreadOp addDependency:backgroundOp];
[[NSOperationQueue mainQueue] addOperation:mainThreadOp];

NSOperation supports dependencies between operations on different queues. Just be careful not to make operations on different queues mutually dependent, because that will result in deadlock.