Observing Changes with Swift KVO

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

by The Captain

September 5, 2023
Title: Swift Key-Value Observing

Swift Key-Value Observing

Swift provides a powerful mechanism called Key-Value Observing (KVO) that allows an object to observe changes to the value of a key in another object. With KVO, you can be notified whenever a specific property or value is modified, enabling you to take action or update your UI accordingly. This tutorial will walk you through the process of implementing KVO in Swift with a code snippet.

Getting started

To use KVO in Swift, follow the steps below: 1. Define a class that will be observed and make sure the properties you want to observe are marked with the `@objc dynamic` attribute. For example, let's create a `Person` class with a property `name` that will be observed:
class Person: NSObject {
    @objc dynamic var name: String
    // ...
2. Create an observer class that conforms to the `NSObject` protocol. This observer class will be responsible for handling the changes observed. Here's an example of an `Observer` class:
class Observer: NSObject {
    @objc var person: Person
    init(person: Person) {
        self.person = person
        // Start observing the changes
        person.addObserver(self, forKeyPath: #keyPath(Person.name), options: .new, context: nil)
    deinit {
        // Stop observing the changes
        person.removeObserver(self, forKeyPath: #keyPath(Person.name))
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if keyPath == #keyPath(Person.name) {
            // Handle the observed change
            if let newName = change?[.newKey] as? String {
                print("Name changed to: \(newName)")
3. Finally, create an instance of your observer class and start observing the changes. Changes to the observed key path will trigger the `observeValue(forKeyPath:of:change:context:)` method in the observer class:
let person = Person(name: "John")
let observer = Observer(person: person)

person.name = "Mike" // This will trigger the observer's observeValue method}
4. Remember to stop observing the changes when you're done. This can be done in the deinitializer of your observer class:
person = nil // This will remove the observer and deallocate the observer instance}


Key-Value Observing (KVO) in Swift is a useful mechanism to observe changes to properties in an object. By implementing the steps outlined in this tutorial, you can easily observe and respond to changes in your Swift applications. Remember to remove the observer when you no longer need to observe the changes. Now that you have a basic understanding of KVO in Swift, you can explore more advanced use cases and leverage this powerful feature in your own projects. Happy coding!