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

You can create a new NSThread, or execute a selector in background, and wait in there. When the NSOperationQueue finishes, you can send a notification of your own.

I'm thinking on something like:

- (void)someMethod {
    // Queue everything in your operationQueue (instance variable)
    [self performSelectorInBackground:@selector(waitForQueue)];
    // Continue as usual
}

...

- (void)waitForQueue {
    [operationQueue waitUntilAllOperationsAreFinished];
    [[NSNotificationCenter defaultCenter] postNotification:@"queueFinished"];
}

Answer 2

How about adding an NSOperation that is dependent on all others so it will run last?

Answer 3

What about using KVO to observe the operationCount property of the queue? Then you'd hear about it when the queue went to empty, and also when it stopped being empty. Dealing with the progress indicator might be as simple as just doing something like:

[indicator setHidden:([queue operationCount]==0)]

Answer 4

This is how I do it.

Set up the queue, and register for changes in the operations property:

myQueue = [[NSOperationQueue alloc] init];
[myQueue addObserver: self forKeyPath: @"operations" options: NSKeyValueObservingOptionNew context: NULL];

...and the observer (in this case self) implements:

- (void) observeValueForKeyPath:(NSString *) keyPath ofObject:(id) object change:(NSDictionary *) change context:(void *) context {
    if (
        object == myQueue
        &&
        [@"operations" isEqual: keyPath]
    ) {
        NSArray *operations = [change objectForKey:NSKeyValueChangeNewKey];

        if ( [self hasActiveOperations: operations] ) {
            [spinner startAnimating];
        } else {
            [spinner stopAnimating];
        }
    }
}

- (BOOL) hasActiveOperations:(NSArray *) operations {
    for ( id operation in operations ) {
        if ( [operation isExecuting] && ! [operation isCancelled] ) {
            return YES;
        }
    }

    return NO;
}

In this example "spinner" is a UIActivityIndicatorView showing that something is happening. Obviously you can change to suit...

Answer 5

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.

When you setup your queue, do this:

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

Then do this in your observeValueForKeyPath:

- (void) observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object 
                         change:(NSDictionary *)change context:(void *)context
{
    if (object == self.queue && [keyPath isEqual:@"operations"]) {
        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)

EDIT: iOS 4.0 now 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.