NEW: Got a question? Get help on our new forums! >>

What are property observers?

Swift version: 5.1

Paul Hudson    @twostraws   

Property observers are Swift's way of letting you attach functionality to changes in property values. For example, you might want to say, "whenever the player's score changes, update this label to show their new score." Here's a basic example that prints message to the debug console when a variable changes:

var score = 0 {
    willSet {
        print("Score is about to change to \(newValue)")
    }

    didSet {
        print("Score just changed from \(oldValue) to \(score)")
    }
}

score = 10
Hacking with Swift is sponsored by Instabug

SPONSORED Catch bugs as soon as they happen and know exactly why a crash occurred by integrating Instabug's SDK in one minute. You will automatically receive device data, network logs, and reproduction steps with every bug and crash report.

Learn more and get started for free

Sponsor Hacking with Swift and reach the world's largest Swift community!

Cascable unleashes the power of your camera and unlocks powerful workflows for shooting, managing, and geotagging your photos.

Available from iOS 7.0 – see Hacking with Swift tutorial 1

Similar solutions…

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.

BUY OUR BOOKS
Buy Pro Swift Buy Swift Design Patterns Buy Testing Swift Buy Hacking with iOS Buy Swift Coding Challenges Buy Swift on Sundays Volume One Buy Server-Side Swift (Vapor Edition) Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Advanced iOS Volume Three Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with macOS Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Server-Side Swift (Kitura Edition) Buy Beyond Code

Was this page useful? Let us know!

Average rating: 4.0/5