Key-value observing is a mechanism that allows objects to be notified of changes to specified properties of other objects. You can use key-value observing with a Swift class, as long as the class inherits from the NSObject class. You can use these two steps to implement key-value observing in Swift.

1. Add the dynamic modifier and @objc attribute to any property you want to observe. For more information on dynamic, see Requiring Dynamic Dispatch.

class MyObjectToObserve: NSObject {
    @objc dynamic var myDate = NSDate()
    func updateDate() {
        myDate = NSDate()
    }
}

1. Create an observer for the key path and call the observe(_:options:changeHandler) method like description on Apple doc. Or create a observer on observable(MyObjectToObserve) directly

var myContext = 0

class MyObserver: NSObject {
    @objc var objectToObserve: MyObjectToObserve
    var observation: NSKeyValueObservation?
    
    init(object: MyObjectToObserve) {
        objectToObserve = object
        super.init()
        objectToObserve.addObserver(self, forKeyPath: "myDate", options: .new, context: &myContext)
    }
    deinit {
        objectToObserve.removeObserver(self, forKeyPath: "myDate")
    }
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        //print("here , \(keyPath), \(object), \(change?[NSKeyValueChangeKey.newKey])")
        guard context == &myContext else { return }
        if let value = change?[NSKeyValueChangeKey.newKey] {
            print("Observation,  myDate changed: \(value)")
        }
    }
}

1. Verifiy observer:

let observed = MyObjectToObserve()
let observer = MyObserver(object: observed)

observed.updateDate()
sleep(3)
observed.updateDate()

print("finished")

You will see output in Xcode debug area:

Observation:  myDate changed: 2018-01-30 18:15:41 +0000
Observation:  myDate changed: 2018-01-30 18:15:44 +0000
finished