swift - CaseItearble的通用next()和previous()方法

假设我们有一个枚举，并且要在它上面枚举：）。
如果它有Int rawValue，我们可以使用这样的计算变量来提供下一个和上一个项。

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
    }
}

我更进一步，为caseiiterable创建并扩展了它，它允许next（）和previous（）用于各种类型。

// 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或低效（即速度、内存）？
是否有做相同或类似事情的想法，可能offset(by: )。

最佳答案

您可以通过以下方式使用previous()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]
    }
}

您可以在更通用的偏移函数中组合next()和previous()：

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
        }
    }
}

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

关于swift - CaseItearble的通用next()和previous()方法，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/55267612/