Get notification when NSOperationQueue finishes all tasks

Question

NSOperationQueue has waitUntilAllOperationsAreFinished, but I don't want to wait synchronously for it. I just want to hide progress indicator in UI when queue finishes.

What's the best way to accomplish this?

I can't send notifications from my NSOperations, because I don't know which one is going to be last, and [queue operations] might not be empty yet (or worse - repopulated) when notification is received.

Answer 1

Use KVO to observe the operations property of your queue, then you can tell if your queue has completed by checking for [queue.operations count] == 0.

Somewhere in the file you're doing the KVO in, declare a context for KVO like this (more info):

static NSString *kQueueOperationsChanged = @"kQueueOperationsChanged";

When you setup your queue, do this:

[self.queue addObserver:self forKeyPath:@"operations" options:0 context:&kQueueOperationsChanged];

Then do this in your observeValueForKeyPath:

- (void) observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object 
                         change:(NSDictionary *)change context:(void *)context
{
    if (object == self.queue && [keyPath isEqualToString:@"operations"] && context == &kQueueOperationsChanged) {
        if ([self.queue.operations count] == 0) {
            // Do something here when your queue has completed
            NSLog(@"queue has completed");
        }
    }
    else {
        [super observeValueForKeyPath:keyPath ofObject:object 
                               change:change context:context];
    }
}

(This is assuming that your NSOperationQueue is in a property named queue)

At some point before your object fully deallocs (or when it stops caring about the queue state), you'll need to unregister from KVO like this:

[self.queue removeObserver:self forKeyPath:@"operations" context:&kQueueOperationsChanged];

Addendum: iOS 4.0 has an NSOperationQueue.operationCount property, which according to the docs is KVO compliant. This answer will still work in iOS 4.0 however, so it's still useful for backwards compatibility.