Question

假设我们有一个枚举，并且想要枚举它:)。如果它具有Int rawValue，则可以使用像这样的计算变量来提供下一个和上一个项目。

enum Fidelity: Int, CaseIterable { 
    case pixel
    case point
    case average
    case datapoint

    var previousFidelity: Fidelity {
        return Fidelity(rawValue: rawValue - 1) ?? .pixel
    }
    var nextFidelity: Fidelity {
        return Fidelity(rawValue: rawValue + 1) ?? .datapoint
    }
}

我走得更远，创建并扩展了CaseIterable，它允许next（）和previous（）适用于多种类型。

// Let's test Swift 4.2 for enumerating enum
// Too complex, not very efficient, but interesting
extension CaseIterable where Self: Equatable  { 
    func next() -> Self? {
        let all = Self.allCases
        let idx = all.index(of: self)!
        let next = all.index(after: idx)
        return (next == all.endIndex) ? nil : all[next]
    }
    func previous() -> Self? {
        let all_reversed = Self.allCases.reversed()
        let idx = all_reversed.index(of: self)!
        let next = all_reversed.index(after: idx)
        return (next == all_reversed.endIndex) ? nil : all_reversed[next]
    }
}

问题是我的解决方案有多高效（即速度，内存）？是否有任何想法可以做相同或相似的事情，也许offset(by: )。

Answer 1

您可以通过以下方式使用previous()来实现offsetBy：

func previous() -> Self? {
    let all = Self.allCases
    var idx = all.index(of: self)!

    if idx == all.startIndex {
        return nil
    } else {
        all.formIndex(&idx, offsetBy: -1)
        return all[idx]
    }
}

您可以将next()和previous()合并到一个更通用的偏移函数中：

extension CaseIterable where Self: Equatable  {
    func advanced(by n: Int) -> Self? {
        let all = Self.allCases
        let idx = all.index(of: self)!
        //An enum with a raw type has at least one case
        let lastIndex = all.index(all.endIndex, offsetBy: -1)
        let limit = n > 0 ? lastIndex : all.startIndex
        if let newIndex = all.index(idx, offsetBy: n, limitedBy: limit) {
            return all[newIndex]
        } else {
            return nil
        }
    }
}

并像这样使用它

let average = Fidelity.average  //average
average.advanced(by: 1)         //datapoint
average.advanced(by: 2)         //nil
average.advanced(by: -3)        //pixel

CaseItearble

1 个答案: