自動繼承初始化程序

Automatic Initializer Inheritance：🤔 一個子類別 (subclass) 何時會繼承母類別 (superclass) 的初始化程序❓

⭐️ 繼承規則

一般來說，subclass 並不會繼承 superclass 的 initializers，但如果符合以下的狀況則會自動繼承：

1. 如果 subclass 為自己所有的屬性 (stored properties) 提供預設值 (default values)，這時 superclass initializers 依然可以正常運作，因此 subclass 就會自動繼承所有的 (designated & convenience) initializers。

2. 如果 subclass 定義了自己的屬性但卻沒有給預設值，這時會造成 superclass initializers 不知如何處理這些屬性 (因為 superclass 根本無從得知這些屬性的存在)，這時 subclass 就會喪失所有的 superclass initializers。

3. 但如果 subclass 又實作 (implement/override) 了所有的 superclass designated initializers，這時 subclass 又會重新繼承所有的 superclass convenience initializers。

Class Initializer Inheritance

⭐️ 重點整理

• 如果 subclass 有定義自己的 convenience initializers，則上述的規則依然成立。

• 在實作 superclass designated initializers 的時候，如果用 override convenience init 的方式，而不是用 override init (override designated initializer) 的方式，可以減少 subclass designated initializers 的數量。 ➡️ convenience override

💈範例

main

// superclass instance
let p = Person(name: "Tom", age: 20)
print(p.age)      // 20

/*  subclass instances  */

// Student
// ---------------------------------------------
// (o) default values for new properties
// (o) no custom designated inits.
//  -> inherits all designated/convenience inits
let s1 = Student(name: "May", age: 17)  // inherited designated init
let s2 = Student(name: "Baby")          // inherited convenience init
let s3 = Student(seatNo: 23)            // custom convenience init
print(s1.seatNo)  // 50
print(s2.age)     // 0
print(s3.name)    // "no name"

// Teacher
// ----------------------------------------------
// (o) default values for new properties
// (x) no custom designated inits.
// (o) implements all superclass designated inits
//  -> inherits all superclass convenience inits.
let t1 = Teacher(name: "Joe", age: 35, title: "Dr.")  // custom designated init
let t2 = Teacher(name: "Joe", age: 35)                // override superclass init
let t3 = Teacher(name: "Babe")                        // inherited convenience init
print(t1.title)   // "Dr."
print(t2.title)   // "Mr./Ms."
print(t3.age)     // 0

// Master
// -----------------------------------------------
// (x) default values
//  -> does not inherit superclass initializers
let m = Master(name: "Sheefu", age: 100, skill: "Kung Fu")  // custom designated init
print(m.skill)    // "Kung Fu"

Person

// superclass
class Person {

  // stored properties
  var name: String
  var age : Int

  // designated: init(name:age:)
  init(name:String, age:Int){
    self.name = name
    self.age  = age
  }

  // convenience: init(name:)
  convenience init(name:String){
    self.init(name: name, age: 0)
  }

}

Student

// (o) default values for new properties
// (o) no custom designated inits.
//  -> inherits all designated/convenience inits
class Student: Person {
  // stored properties with default values
  var seatNo: Int = 50
}

extension Student {
  // custom convenience init
  convenience init(seatNo: Int) {
    // calls inherited designated init
    self.init(name: "no name", age: 16)
    self.seatNo = seatNo
  }
}

Teacher

// (o) default values for new properties
// (x) no custom designated inits.
// (o) implements all superclass designated inits
//  -> inherits all superclass convenience inits.
class Teacher: Person {

  var title: String = "Mr./Ms."

  // custom designated init
  init(name: String, age: Int, title: String) {
    self.title = title
    super.init(name: name, age: age)
  }

  // superclass designated -(override)-> subclass convenience
  override convenience init(name: String, age: Int) {
    self.init(name: name, age: age, title: "Mr./Ms.")
  }

}

Master

// (x) default values
//  -> does not inherit superclass initializers
class Master: Person {
  var skill: String
  // custom designated init
  init(name: String, age: Int, skill: String){
    self.skill = skill
    super.init(name: name, age: age)
  }
}

🔗 原始碼：Repl.it

📘 參考資料

1. Swift in Depth, Ch.5 Demistifying Initializers

2. Initialization - Swift