Cocoa - Return information from NSOperation

I've personally done this in two ways. Both worked quite well.

First Way

You can have a function that returns the "output", and you call that function after the operation has finished. You can observe the key isFinished on the NSOperation object, or use -[NSOperationQueue waitUntilAllOperationsAreFinished] to determine when the operation is finished.

Second Way

You can use a delegate or target/action from the NSOperation. Just make sure that when you call the delegate function, or trigger the target/action, you do so on the main thread by using -[NSObject performSelectorOnMainThread:withObject:waitUntilDone:]

It depends on your application. How many of the operations do you expect to run concurrently? What are you doing with the result? Are results aggregated or do they need to display in the UI immediately?

Sending notifications on the main thread is fairly heavyweight. Depending on what you're doing with the result, you could choke up your UI. From a design perspective, are the objects that you would be sending from the notification something that the observing class should know about? It may make more sense to make changes to your model objects from the operation and have a controller object observe those changes instead.

You could use Key Value Observing but you must be careful. The observer observes in the same thread that the change occurs on, so you should not make UI changes directly when observing isFinished.

If the object is only owned and used by the operation, then yes, it should be safe to send at the end of main. It will be retained by the notification.

The doc says it's better to make the operation member variable when you want get the result from operation. But I do not think the solution will adapt you.

I think notification is good and There is another way to get the result is make a completion block for your own operation.See here: Return data from NSOperation? I tried it's work well.

I tried another way before.set the operation completion block,and make a weak reference to operation and make the result an property of operation. so you can get the operation result when you finished.

But completion is not reliable,when you cancel the operation the completion excuted too. so choose the best for you.