Question

I’m trying to create a NSOperationQueue and add a NSOperation to it. Later I wan’t to check if the queue is running and maybe abort it. All of that is supposed to be called from within different functions. What’s the best approach to do this? I would be glad for a code example. Thanks!

Answer 1

I would create an operation queue that’s managed by a singleton.

First, create your singleton class. It will provide access to the NSOperationQueue. Let’s call the singleton MyGlobalQueueManager.

It will have an ivar called myGlobalQueue:

@property (nonatomic) NSOperationQueue* myGlobalQueue;

In the .m file of MyGlobalQueueManager, create a fairly standard init method that will set up the operation queue:

- (id)init
{
    self = [super init];
    if (self)
    {
        myGlobalOperationQueue = [[NSOperationQueue alloc] init];
    }
    return self;
}

Now, the method that provides itself as a singleton. Again, this is pretty standard stuff:

 + (MyGlobalQueueManager *)sharedInstance
{
    static MyGlobalQueueManager *sharedInstance = nil;
    static dispatch_once_t isDispatched;

    dispatch_once(&isDispatched, ^
                  {
                      sharedInstance = [[MyGlobalQueueManager alloc] init];
                  });

    return sharedInstance;
}

Let’s access that queue from wherever you want to use it:

MyGlobalQueueManager* myGlobalQueueManager = [MyGlobalQueueManager sharedInstance];
NSOperationQueue *myGlobalQueue = myGlobalQueueManager.myGlobalOperationQueue;

You can then add operations to that queue as you fancy.

How to know if anythings queued?

NSUInteger count = [myGlobalQueue operationCount];

How to abort? Cancel everything as follows:

[myGlobalQueue cancelAllOperations];

Cancelling of course depends on the operations. If you’re writing custom NSOperation classes, you’ll need to handle that yourself.

I find NSOperation and NSOperationQueue to be fairly easy to use and quite straightforward.

A great document to read for all this is the Concurrency Programming Guide. Specifically, have a look at Operation Queues