Decodable 强制 super open class 来实现初始化器

Question

我得到了这个代码：

open class A: Decodable {
    public init() {

    }
}

open class B: A {
    public override init() {
        super.init()
    }

    open required init(from decoder: Decoder) throws {
        fatalError("init(from:) has not been implemented")
    }
}

而且万能的Xcode 9.4.1告诉我需要把requiered关键字前的open改成public。在我将 open 更改为 public 后，编译器告诉我需要将其更改为 open。如果两个 class 都是 open，如果没有我的超级 class A 显式实现所需的初始值设定项，我将无法正常工作，如 class B 中所示。为什么？

Answer 1

Decodable forces super open class to implement the initializer

如果您不继承超类所具有的必需初始化程序，那么您必须自己实现它。

必需的初始化程序

Write the required modifier before the definition of a class initializer to indicate that every subclass of the class must implement that initializer: <..>

You must also write the required modifier before every subclass implementation of a required initializer, to indicate that the initializer requirement applies to further subclasses in the chain.

You do not have to provide an explicit implementation of a required initializer if you can satisfy the requirement with an inherited initializer.

如何避免自己实施：

初始化程序继承

Rule 1

If your subclass doesn’t define any designated initializers, it automatically inherits all of its superclass designated initializers.

来源

• Initialization #Required Initializers
• Initialization #Automatic Initializer Inheritance