🔰computed property (get/set)

accessor╱getter/setter╱🚧 -> keyword "let"

Swift ⟩ type ⟩ property ⟩ computed property

⚖️ stored vs. computed

• 屬性沒有實際儲存的值，值是動態計算的。

computed property 的優勢：

• get ：可動態計算並傳回屬性值。

• set ：可攔截屬性變更，檢查或調整其值，保護內部狀態。

⚖️ willSet/didSet vs. get/set

• computed property 也稱為 accessor。

• computed property 一定要用 var 宣告。

💈範例

struct Circle {

    var radius: Double
    
    // ⭐️ computed property
    var area: Double { .pi * radius * radius }    // ⭐️ (implicit) get only
}

// 利用 computed property 來確保屬性值的範圍
struct NoLessThanThree {

    // inner state
    private var _value: Int = 3

    // ⭐️ computed property
    var value: Int {
        get { _value }
        set { _value = max(newValue, 3) }    // 確保數值不小於 3
    }
}

// 註：如果初始化後屬性值就不變，可考慮以下做法：
struct Example {

    // (stored property) private set/internal get (get OK/no set!)
    private(set) var value: Int

    init(value: Int) {
        self.value = max(value, 3)          // 確保數值不小於 3
    }
}

📁 測試

// 測試
var num = NoLessThanThree()
num.value = 5      
num.value = 2      // value 被 setter 調整為 3 ⭐️ 
print(num.value)   // 3

📗 參考

• Computed properties in Swift: A basic feature for safer and cleaner code
• Develop with Swift ⟩ Customize views with properties

🔴 主題

• local computed variables

• mutating getter / nonmutating setter

👥 相關

• property observer (willSet/didSet)： you can add in computed properties that you inherit.

📘 手冊

• Swift ⟩ Properties ⟩

  • Stored Properties
  • Computed Properties