Title: "Key-Value Observing in Swift: A Beginner's Guide

A portrait painting style image of a pirate holding an iPhone.

by The Captain

May 16, 2024

Working with Key-Value Observing in Swift

Key-Value Observing (KVO) is a mechanism in Cocoa and Cocoa Touch frameworks that allows objects to be notified of changes to properties of other objects. In Swift, KVO can be implemented using the NSObject class and the observeValue(forKeyPath:) method.

To use KVO in Swift, start by creating a class that subclasses NSObject and declares the properties you want to observe. Next, override the observeValue(forKeyPath:) method to handle the changes in the observed properties.

import Foundation

class MyClass: NSObject {
    @objc dynamic var value: String = ""

    override init() {
        self.addObserver(self, forKeyPath: "value", options: [.new, .old], context: nil)

    deinit {
        self.removeObserver(self, forKeyPath: "value")

    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if keyPath == "value" {
            if let newValue = change?[.newKey] as? String {
                print("Value changed to: \(newValue)")

let myObject = MyClass()
myObject.value = "Hello, World!"}

In this code snippet, we have a class MyClass that subclasses NSObject and declares a property value that is marked as @objc dynamic to enable KVO. In the observeValue(forKeyPath:) method, we check if the observed property is "value" and extract the new value from the change dictionary to print it out when it changes.

Remember to call addObserver in the init method and removeObserver in the deinit method to avoid memory leaks when using KVO in Swift.