🔸required initializer

╱🚧 under construction -> subclass, override, designated initializer, inherited initializer, convenience init

swift ⟩ type ⟩ init ⟩ initializers ⟩ required

1️⃣ write required before the definition of a class initializer to indicate that every subclass of the class must implement that initializer. ( 👉 💈範例 (1) )

2️⃣ write required before every subclass implementation of a required initializer, don’t write override when overriding a required designated initializer.

You don’t have to provide an explicit implementation of a required initializer if you can satisfy the requirement with an inherited initializer. ( 👉 💈範例 (2) )

💾 語法

// superclass
class SuperClass {
    required init() { ... }    // 1️⃣ required (designated) init
}

// subclass
class Subclass: SuperClass {
    required init() { ... }    // 2️⃣ required init (No `override` here❗️)
}

💈範例

1

下面的例子說明：如果 subclass 沒有提供 required init 會發生什麼錯誤，以及如何解決它。

// superclass
class BasePoint {

    var x: Double
    var y: Double
    
    // ⭐️ required (designated) init
    required init(x: Double, y: Double) {
        self.x = x
        self.y = y
    }
}

// subclass
class ColoredPoint: BasePoint {

    var color: String
    
    // ⛔ error: 'required' initializer 'init(x:y:)' must be provided 
    //    by subclass of 'BasePoint'.
    
    // required (designated) init
    required init(x: Double, y: Double, color: String = "black") {
        self.color = color
        super.init(x: x, y: y)
    }
}

我們可以在 subclass 提供一個 required init (require designated initializer)，來解決這個問題：

// 方法一：required designated init
// ⭐️ subclass must implement required init
required init(x: Double, y: Double) {
    self.color = "black"    // ⭐️ init self's properties first
    super.init(x: x, y: y)  // ⭐️ init super's properties next
}

或者也可以提供一個 required convenience init，然後將真正初始化的工作交給自己的 required init：

// 方法二：required convenience init
required convenience init(x: Double, y: Double) {
    // ⭐️ delegates to self's required init
    self.init(x: x, y: y, color: "black")
}

但千萬不能用 required init (require designated initializer)，然後將初始化的工作交給自己的另一個 required init❗

// required (designated) init but delegates❗
required init(x: Double, y: Double) {
    // ⛔ error: 
    // designated initializer cannot delegate (with 'self.init')
    self.init(x: x, y: y, color: "black")
}

2

當子類別 1️⃣ 沒有新屬性 2️⃣ 也不需修改父類別的初始化邏輯時，則不需實作自己的 required init，用繼承來的就可以了。

// BaseClass
class BaseClass {

    var x: Int
    var y: Int
    
    required init(x: Int, y: Int) {
        self.x = x
        self.y = y
    }
}

// SubClass
class SubClass: BaseClass {
    // ⭐️ 子類別沒有新屬性，不需實作 `required init`。
    func idle() { /* do nothing */ }
}

let obj = SubClass(x: 10, y: 20)

print(obj.x)    // 10
print(obj.y)    // 20

📘 手冊

• Swift ⟩ Initialization ⟩

  • Required Initializers