Code that listens for changes on screens in Swift.

ScreenObserver.swift

class EventObserverDemo {
    var lastCount = NSScreen.screens.count

    init() {
        NotificationCenter.default.addObserver(
            self,
            selector: #selector(trigger),
            name: NSApplication.didChangeScreenParametersNotification,
            object: nil
        )
    }

    @objc private func trigger(notification: NSNotification) {
        let newCount = NSScreen.screens.count

        if newCount != lastCount {
            print("Switched from \(lastCount) to \(newCount) displays")
            lastCount = newCount
        }
    }
}