Queueing and Handling multiple requests on iphone - iphone

Question

In my application, I'm using NSOperationQueue and NSInvocationOperation objects to execute all the operations. I have multiple queues in my application and I am using KVO "isFinished" on call of which I am performing the next operation on main thread.
The problem is: whenever I perform two operations one after another my application crashes saying:
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '<myViewController: 0x481b200>: An -observeValueForKeyPath:ofObject:change:context: message was received but not handled.
Key path: isFinished
Observed object: <NSInvocationOperation: 0x45d9ea0>
Change: {
kind = 1;
}
Context: 0x0'
My general code is as follows:
operation = [[NSInvocationOperation alloc] initWithTarget:self selector:#selector(somemethod) object:nil];
[operation addObserver:self forKeyPath:#"isFinished" options:0 context:nil];
[operationQueue addOperation:operation];
[operation release];
- (void)observeValueForKeyPath:(NSString *)keyPath
ofObject:(id)object
change:(NSDictionary *)change
context:(void *)context
{
if([keyPath isEqual:#"isFinished"] && operation == object)
{
[operation removeObserver:self forKeyPath:#"isFinished"];
[self performSelectorOnMainThread:#selector(newerPostLoadingNumberOfUdates) withObject:nil waitUntilDone:YES];
}
else
{
[super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
}
}
I want to know wheather what is the best practice to enqueue operations that are requested and execute them accordingly?
Thanx in advance.

Answer 1

This will be difficult to answer w/o seeing more of your code as you mention using multiple queues. I see a few things to consider:
1) you have [operation release] and then you compare operation to object in your KVO handler. you should not release `operation until you are done with it. In the KVO handler in this case.
2) as mentioned above, you should do string compares using isEqualToString
3) it sounds like you are trying to run your queue sequentially such that only one operation is executed at a time and you queue another operation after the first has finished. Instead you could set your queue to only perform one operation at a time with:
operationQueue.maxConcurrentOperationCount = 1;
and then add all the operations at once. If order of execution matters, you can use Grand Central Dispatch directly and use dispatch_sync to execute blocks in a specific sequence on a queue.