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I just read long introduction to NSOperationQueues and NSOperation here.

My question is the following. I need to run two operations is the same time. When both those tasks finished I need to make another calculations based on results from two finished operations. If one of the operations fails then whole operation should also fails. Those two operations does not have dependencies and are completely independent from each other so we can run them in parallel.

How to wait for this 2 operation to finish and then continue with calculations? I don't want to block UI Thread. Should I make another NSOperation that main method is creating two NSOperations add them to some local (for this operation) queue and wait with waitUntilAllOperationsAreFinished method. Then continue calculations?

I don't like in this approach that I need to create local queue every time I creating new operation. Can I design it that way that I can reuse one queue but wait only for two local operations? I can imagine that method waitUntilAllOperationsAreFinished can wait until all tasks are done so it will blocks when a lot of tasks will be performed in parallel. Any design advice? Is creating NSOperationQueue expensive? Is there any better ways to do it in iOS without using NSOperation & NSOperationQueue? I'm targeting iOS 9+ devices.

In Swift 4, you can do it this way:

let group = DispatchGroup()
// async
DispatchQueue.global().async 
 // enter the group
 group.enter()
 taskA(onCompletion: (_) in
 // leave the group
 group.leave()
 )
 group.enter()
 taskB(onCompletion: (_) in
 group.leave()
 )


group.notify(queue: DispatchQueue.main) 
 // do something on task A & B completion

And there is an excellent tutorial on GCD from raywenderlich.com.