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Swift Academy · Lesson

Stored vs computed properties, observers

Define stored vs computed properties, create read-only and get/set computed properties, and react to changes with willSet/didSet observers.

Overview

This lesson contrasts stored vs computed properties and shows willSet/didSet observers to react to changes.

Stored vs computed

Stored properties keep data. Computed properties calculate a value when accessed.

struct Rectangle {
    // stored properties (hold data)
    var width: Double
    var height: Double

    // computed property (derived on access)
    var area: Double {
        return width * height   // read-only computed
    }
}
let r = Rectangle(width: 3, height: 4)
print("area =", r.area)  // 12

All lessons in this course

  1. Structs vs Classes, properties & inits
  2. Stored vs computed properties, observers
  3. Initializers & deinit (classes)
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