an example of NSOperation addDependency for this SO question: http://stackoverflow.com/q/30851799/1254172

addDependency.m

//assume NSOperationQueue *operationQueue is created somewhere
//assume dispatch_semaphore_t semaphore is created somewhere

NSOperation *firstOperation = [NSBlockOperation blockOperationWithBlock:^{
    [self doTask1];
}];
NSOperation *secondOperation = [NSBlockOperation blockOperationWithBlock:^{
    [self doTask2];
}];
NSOperation *notificationOperation = [NSBlockOperation blockOperationWithBlock:^{
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
}];
NSOperation *completionOperation = [NSBlockOperation blockOperationWithBlock:^{
    [self completionTask];
}];

[completionOperation addDependency:firstOperation];
[completionOperation addDependency:secondOperation];
[completionOperation addDependency:notificationOperation];

[operationQueue addOperations:@[firstOperation, secondOperation, notificationOperation, completionOperation] waitUntilFinished:NO];