NSEvent leak for key down in macOS

Question

In Xcode 10.1 with Swift 4.2 I'm having a memory leak when I add a local monitor for key down events in my NSViewController, that it is be instanced as minimal version (without nib and xib).

override func loadView() {
    self.view = NSView()
    self.view.wantsLayer = true
}

override func viewDidLoad(){
    super.viewDidLoad
    NSEvent.addLocalMonitorForEvents(matching: .keyDown, handler: handler)
}

lazy var handler:(NSEvent)->NSEvent? = { [ weak self ,unowned picker = picker] event in
    picker.keyDown(with: event)
    return event
}

This memory leak does not have much information:Memory leak

EDIT

In deinit method removeMonitor is called

deinit {
   NSEvent.removeMonitor(self)
}

EDIT 2

Issue solved :

    override func loadView() {
    self.view = NSView()
    self.view.wantsLayer = true
}
var monitor:Any? // This is essential

override func viewDidLoad(){
    super.viewDidLoad
    monitor = NSEvent.addLocalMonitorForEvents(matching: .keyDown, handler: handler)
}

lazy var handler:(NSEvent)->NSEvent? = { [ weak self ,unowned picker = picker] event in
    picker.keyDown(with: event)
    return event
}

deinit {
   NSEvent.removeMonitor(monitor)
}

Do you remove the event monitor when the view controller is dismissed? — KSigWyatt
addLocalMonitorForEvents Note The monitor Block is called for all future events that match mask. You must call removeMonitor: to stop the monitor. Under garbage collection, the monitor (and everything the Block references) will not be collected until removeMonitor: is invoked. — Marek H
I remove it in the deinit method with NSEvent.removeMonitor (self). That's right?. Leaks persist — Jesús Mateos Gomez

KSigWyatt KSigWyatt · Accepted Answer · 2019-03-11T15:07:37

From the Apple Docs;

Note

The monitor Block is called for all future events that match mask. You must call removeMonitor(_:) to stop the monitor. Under garbage collection, the monitor (and everything the Block references) will not be collected until removeMonitor(_:) is invoked.

Meaning that the monitor will continue to look for matching events until removeMonitor() is invoked. So your system is using extra memory to keep looking for events, and if you never call this - it could lead to a fairly large memory leak. As it says even with garbage collection, this object is still allocated - because it is looking for events that could take place at any time (so it is not guaranteed that this will be collected). Make sure you call this when you want the system to stop looking for events.

You could also do something like this in your handler.

You can return the event unmodified, create and return a new NSEvent object, or return nil to stop the dispatching of the event.

NSEvent leak for key down in macOS

1 Answers