What are property observers?

Swift version: 5.0

Paul Hudson @twostraws May 28th 2019

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

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