NSOperation not being properly cancelled

I am using NSOperation to perform some heavy parsing of data, then return back to the main thread with objects ready to be used by my app. I handle all operations by placing them on a singleton NSOperationQueue. I do this to control how much processing is happening at any point, because each operation temporarily uses a pretty high memory footprint.

So, I have a scenario where I can have several view controllers on screen. Each view controller will create a parsing operation on load and add it to the queue. I allow 2 concurrent processing operations by setting the maxConcurrentOperationCount. Each view controller creates a processing operation, places it on the singleton queue, and retains the operation as a property so it has a handle on it.

If the view contro开发者_运维百科ller needs to go away in response to a user initiated Delete action, I use the NSOperation property in the dealloc method of my view controller to cancel the operation:

-(void)dealloc{
    [self.currentOperation cancel];
    [super dealloc];
}

In my NSOperation subclass, I check the isCancelled property in several places (mostly before significant chunks of long running work) the isCancelled property and attempt to respond to it:

if([self isCancelled]){
    // Perform cleanup
    return;
}

The problem is that the isCancelled property is evaluating to false and the operation continues, eventually calling into Core Data to attempt to retrieve data that has been deleted. I see this happen even when I place an isCancelled check immediately prior to the Core Data fetch request.

I've got a workaround to keep the app from crashing, but I'm thinking I might be going about the implementation wrong. Is there any other way I can maintain a handle on the operation while it is processing so I can cancel it if needed? Is my method not retaining the proper handle on the operation and preventing it from being properly cancelled?

You can't do logic like that in dealloc.

First, dealloc must call super's dealloc as the last line. Once that is done, the object is gone and all subsequent messaging behaviour is undefined ( will crash).

It is also very likely that the queue will retain the operation, thus making cancellation in dealloc meaningless because dealloc cannot be called until the queue releases (unless your memory management is screwed up).

You need to separate your cancellation/invalidation logic from memory management entirely.