NEW! Master Swift design patterns with my latest book! >>

What are property observers?

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

Available from iOS 7.0 – see Hacking with Swift tutorial 1

Did this solution work for you? Please pass it on!

Other people are reading…

About the Swift Knowledge Base

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

Swift on the server is here

Get ahead of the game and learn server-side Swift with my latest book – build real-world projects while you learn!

MASTER SWIFT NOW
Buy Practical iOS 12 Buy Pro Swift Buy Swift Design Patterns Buy Practical iOS 11 Buy Swift Coding Challenges Buy Server-Side Swift (Vapor Edition) Buy Server-Side Swift (Kitura Edition) Buy Hacking with macOS Buy Advanced iOS Volume One Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with Swift Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Beyond Code

Was this page useful? Let me know!

Click here to visit the Hacking with Swift store >>