Swift: How to observe if screen is locked in macOS

I want to detect if the user has locked his screen (in macOS) using Swift.

Based on this answer I’ve created the following code:

import Cocoa
import Quartz

if let dict = Quartz.CGSessionCopyCurrentDictionary() as? [String : Any] {
    let locked = dict["CGSSessionScreenIsLocked"]
    print(locked as? String ?? "")
}

...which seems to work fine if I explicitly run the code.

But how is it possible to observe the value so I get notified when the value got changed?

You can observe distributed notifications. They are not documented.

let dnc = DistributedNotificationCenter.default()

let lockObserver = dnc.addObserver(forName: .init("com.apple.screenIsLocked"),
                               object: nil, queue: .main) { _ in
    NSLog("Screen Locked")
}

let unlockObserver = dnc.addObserver(forName: .init("com.apple.screenIsUnlocked"),
                                 object: nil, queue: .main) { _ in
    NSLog("Screen Unlocked")
}