Dispatch group and NSNotification

Question

My issue is simple. I have 3 tasks, one is triggered by an NSNotification. How to I wait for all task to be completed before proceeding.

So far, I tried using dispatch group but can't find a way to add the task triggered by NSNotification. (I tried adding the dispatch command within the method triggered by the NSNotification, but if the notification comes after task 1 and 2, it's too late to add to the group as it is already completed.)

_asyncDispatch = dispatch_group_create();


dispatch_group_async(_asyncDispatch,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ {
    [self doTask1];
});

dispatch_group_async(_asyncDispatch,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ {
    [self doTask2];
});

dispatch_group_notify(_asyncDispatch,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ {

    // We're done!
});

Answer 1

Thanks @Sega-Zero for your guidance. Here is the solution I implemented.

_operationQueue = [[NSOperationQueue alloc] init];
_semaphore = dispatch_semaphore_create(0);

NSOperation *uploadOperation = [NSBlockOperation blockOperationWithBlock:^{
    [self doFirstTask];
}];

NSOperation *downloadOperation = [NSBlockOperation blockOperationWithBlock:^{
    dispatch_semaphore_wait(_semaphore, DISPATCH_TIME_FOREVER);
}];
NSOperation *completionOperation = [NSBlockOperation blockOperationWithBlock:^{
    [self doNextMethod];
}];

[completionOperation addDependency:uploadOperation];
[completionOperation addDependency:downloadOperation];

[_operationQueue addOperations:@[uploadOperation, downloadOperation, completionOperation] waitUntilFinished:NO];

And then, in the method triggered by the NSNotification

-(void)methodTriggeredByNotif:(NSNotification *)notification {

// doing some serious stuff here and when done
dispatch_semaphore_signal(_semaphore);}

Voila. Thanks to all.