swift - 在一个循环中初始化多个不可变数组

Question

这是我的代码:

class Birthgiver {}
class Son: Birthgiver {}
class Daughter: Birthgiver {}

class BirthgiverHolder {
    let sons: [Son]
    let daughters: [Daughter]

    init(birthGivers: [Birthgiver]) {
        // How to initializer both sons and daugthers in 1 loop?
        // This is my current way (looping twice):
        sons = birthGivers.compactMap { $0 as? Son }
        daughters = birthGivers.compactMap { $0 as? Daughter }
    }
}

我在数组 birthGivers 上循环两次。有什么方法可以初始化儿子和女儿而只循环一次 birthGivers？我不想将数组标记为 vars。

Answer 1

选项 1:拥有本地变量并在完成后填充常量:

init(birthGivers: [Birthgiver]) {
    var sons: [Son] = []
    var daughters: [Daughter] = []

    for child in birthGivers {
        switch child {
            case let son as Son: sons.append(son)
            case let daughter as Daughter: daughters.append(daughter)
            default: break
        }
    }

    self.sons = sons
    self.daughters = daughters
}

选项 2:您也可以使用 reduce(into:) 实现它(尽管我个人认为上面的代码更具可读性):

init(birthGivers: [Birthgiver]) {
    (sons, daughters) = birthGivers.reduce(into: ([], [])) {
        switch $1 {
        case let son as Son: $0.0.append(son)
        case let daughter as Daughter: $0.1.append(daughter)
        default: break
        }
    }
}

选项 3:坚持使用 compactMap 方法:

init(birthGivers: [Birthgiver]) {
    sons = birthGivers.compactMap { $0 as? Son }
    daughters = birthGivers.compactMap { $0 as? Daughter }
}

在大多数情况下，最后一个选项就足够了。您需要大量记录才能观察到性能差异。