swift - 如何避免子类中的默认初始化参数冗余？

Question

我想为某些初始化参数提供默认值。我希望能够在子类中重用相同的默认值，但没有找到这样做的方法。

首先尝试 - 参数默认值:

class A { 

    typealias Mapper = (A) -> String

    let mapper: Mapper

    init(mapper: Mapper = {a in "foo"}) {
        self.mapper = mapper
    }

}

class B: A {

    let myVar: Int

    init(myVar: Int, mapper: Mapper = {a in "foo"}) {
        self.myVar = myVar

    }
}

let b: B = B(myVar: 1)
let str = b.mapper(b)
let b2: B = B(myVar: 2, mapper: {a in "bar"})

我必须指定默认值两次(在 A 和 B init 中)才能使用或不使用默认值初始化 B。

我也试过使用便利初始化器，同样的问题:

class A {

    typealias Mapper = (A) -> String

    let mapper: Mapper

    convenience init() {
        self.init(mapper: {a in "foo"})
    }

    init(mapper: Mapper) {
        self.mapper = mapper
    }

}

class B: A {

    let myVar: Int

    convenience init(myVar: Int) {
        self.init(myVar: myVar, mapper: {a in "foo"})
    }

    init(myVar: Int, mapper: Mapper) {
        self.myVar = myVar

        super.init(mapper: mapper)
    }
}

我想也许至少我把默认值放在一个静态变量中，像这样:

class A {

    typealias Mapper = (A) -> String

    static let defMapper: Mapper = {a in "foo"}

    let mapper: Mapper

    convenience init() {
        self.init(mapper: A.defMapper)
    }

    init(mapper: Mapper) {
        self.mapper = mapper
    }

}

class B: A {

    let myVar: Int

    convenience init(myVar: Int) {
        self.init(myVar: myVar, mapper: A.defMapper)
    }

    init(myVar: Int, mapper: Mapper) {
        self.myVar = myVar

        super.init(mapper: mapper)
    }
}

(也可以将变量定义为惰性变量，以避免在不使用默认参数时进行初始化)。

这一直有效，直到我向 A 添加一个泛型类型。然后我得到“泛型类型尚不支持的静态属性”。所以我必须将默认值放在类之外，这导致还必须将类型别名定义放在类之外，如果我将泛型添加到混合中，这将是一团糟。

有办法解决吗？

Answer 1

如何使用计算属性:

class A<T> {

typealias Mapper = (A) -> String

static var defMapper: Mapper {
    return { a in "foo" }
}

let mapper: Mapper

convenience init() {
    self.init(mapper: A.defMapper)
}

init(mapper: Mapper) {
    self.mapper = mapper
}

}

class B<T>: A<T> {

    let myVar: Int

    convenience init(myVar: Int) {
        self.init(myVar: myVar, mapper: A.defMapper)
    }

    init(myVar: Int, mapper: Mapper) {
        self.myVar = myVar

        super.init(mapper: mapper)
    }
}

let b: B = B<String>(myVar: 1)

let str = b.mapper(b)

let b2: B = B<String>(myVar: 2, mapper: {a in "bar"})

A<Int>(mapper: {a in "bar"})

A<Int>()

或类型方法:

class A<T> {

    typealias Mapper = (A) -> String

    static func defMapper() -> Mapper {
        return { a in "foo" }
    }

    let mapper: Mapper

    convenience init() {
        self.init(mapper: A.defMapper())
    }

    init(mapper: Mapper) {
        self.mapper = mapper
    }

}

class B<T>: A<T> {

    let myVar: Int

    convenience init(myVar: Int) {
        self.init(myVar: myVar, mapper: A.defMapper())
    }

    init(myVar: Int, mapper: Mapper) {
        self.myVar = myVar

        super.init(mapper: mapper)
    }
}

let b: B = B<String>(myVar: 1)

let str = b.mapper(b)

let b2: B = B<String>(myVar: 2, mapper: {a in "bar"})

A<Int>(mapper: {a in "bar"})

A<Int>()