Key down event handler running multiple times in Swift - swift

Question

I am trying to do something when a given key is pressed in a macOS App. First, I ran into a problem where the keyDown event was detected multiple times on each press, therefore executing the handler multiple times. As per a suggestion, I added code to check whether the event is a repeat and it seemed to work at the time. However, this solution seems to work only some of the time, other times the event is getting detected multiple times. Also, I can't seem find a pattern in situations when it works and when it doesn't.What might be the problem and how could I fix it.
Code:
override func viewDidLoad() {
super.viewDidLoad()
NSEvent.addLocalMonitorForEvents(matching: .keyDown, handler: checkKeyDown(event:))
}
func checkKeyDown(event: NSEvent) -> NSEvent{
if event.isARepeat == false{
if event.keyCode == 36{
print("Hello World!")
}
}
return event
}

Answer 1

Removing the event monitor when the window closes seems to have fixed this issue.
var numKeyDown : Any?
override func viewDidLoad() {
super.viewDidLoad()
numKeyDown = NSEvent.addLocalMonitorForEvents(matching: .keyDown, handler: checkKeyDown(event:))
}
override func viewWillDisappear(){
if let numMonitor = self.numKeyDown {
NSEvent.removeMonitor(numMonitor)
}
}
func checkKeyDown(event: NSEvent) -> NSEvent{
if event.isARepeat == false{
if event.keyCode == 36{
print("Hello World!")
}
}
return event
}