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

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by The Captain

April 2, 2024
Working with Key-Value Observing in Swift

Working with Key-Value Observing in Swift

Key-Value Observing (KVO) is a mechanism provided by Cocoa frameworks that allows objects to be notified of changes to the properties of other objects. In Swift, KVO can be implemented using the `observe` method provided by `NSObject`.

Using Key-Value Observing

To work with KVO in Swift, first, you need to subclass `NSObject` and observe the changes on a specific property of another object. Here's an example code snippet demonstrating how to observe changes on a property of an object:

import Foundation

class MyObject: NSObject {
    @objc dynamic var myProperty: String = ""

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

    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if keyPath == "myProperty" {
            print("Value changed: \(myProperty)")

let obj = MyObject()
obj.myProperty = "Hello World"

Cleaning Up Observers

It's important to clean up the observers when they are no longer needed to prevent memory leaks. You can remove the observer using `removeObserver` method in the `deinit` or before the object is deallocated.

deinit {
    removeObserver(self, forKeyPath: "myProperty")


Key-Value Observing is a powerful mechanism in Swift that allows objects to observe changes in the properties of other objects. By implementing KVO, you can create more dynamic and responsive applications. Remember to clean up the observers to avoid memory leaks.