Im trying to detect events of focus change(when the active window changed) in swift, by adding observer like this:  func createObserver() {              var observer = NSWorkspace.shared.notificationCenter.addObserver(                forName: NSWorkspace.didActivateApplicationNotification, object: NSWorkspace.shared,                queue: nil,                using: {_ in                    print("got focus change event"))} } this is just an example how Im using this code in my project this method(createObserver() runs from a background thread). In my project, Im using dispatch_main() in the main function of the project(who is in c++,Xcode). Im only getting the handler(the print), when Im switching the dispatch_main() into CFRunLoopRun() on the main thread, but i do not know if i can do it and what it can cause? I want to know why i do not get the handler called if Im running RunLoop.current.run() on the thread which runs the createObserver function ? why i must run the run loop from the main thread? I have tried almost everything. I also tried to add a source to this specific run loop, but i dont see a way to connect between the source and the specific notification. Is there any other way to get the notifications, and not to delete the dispatch_main ? I want to run the RunLoop from a background thread, for not to block my program. Thank you !!