Property observers (willSet, didSet) in Swift

Introduction

Property observers let you watch and respond when a value changes. They help you run code right before or after a value updates.

You want to update the user interface when a value changes.

You need to validate or adjust a value before it is saved.

You want to log changes to a property for debugging.

You want to trigger other actions automatically when data changes.

Syntax

Swift

var propertyName: Type {
    willSet(newValue) {
        // code to run before the value changes
    }
    didSet(oldValue) {
        // code to run after the value changes
    }
}

willSet runs just before the new value is stored.

didSet runs right after the new value is stored.

Examples

This example prints messages before and after the score changes.

Swift

var score: Int = 0 {
    willSet {
        print("Score will change to \(newValue)")
    }
    didSet {
        print("Score changed from \(oldValue) to \(score)")
    }
}

This example checks the temperature after it changes and prints a warning if it's too hot.

Swift

var temperature: Double = 20.0 {
    didSet {
        if temperature > 30 {
            print("It's too hot!")
        }
    }
}

Sample Program

This program creates a light switch that prints messages before and after it changes state.

Swift

class LightSwitch {
    var isOn: Bool = false {
        willSet {
            print("Light will turn \(newValue ? "ON" : "OFF")")
        }
        didSet {
            print("Light was \(oldValue ? "ON" : "OFF"), now is \(isOn ? "ON" : "OFF")")
        }
    }
}

let light = LightSwitch()
light.isOn = true
light.isOn = false

Important Notes

You don't have to provide both willSet and didSet; you can use just one.

Inside willSet, the new value is available as newValue by default.

Inside didSet, the old value is available as oldValue by default.

Summary

Property observers let you run code before and after a property changes.

Use willSet to react before the value changes.

Use didSet to react after the value changes.