下面是一个协议“Sortable”，用于描述排序、by和ascending。By是一个未知枚举。
Sortable跟在RawRepresentable后面，需要实现from/to rawValue。协议的rawValue是“（按.rawValue），（升序.intValue）"，但是当我需要从rawValue中获取self时，“按（rawValue：Int（list[0]）？？0）”是一个可选的枚举值，如何为其提供一个默认的枚举值？我不知 prop 体的枚举值，我只知道“By：原始值可表示，其中原始值==整数”

protocol Sortable: RawRepresentable where RawValue == String {
    associatedtype By: RawRepresentable where By.RawValue == Int
    
    var by: By { get set }
    var ascending: Bool { get set }
    
    init(by: By, ascending: Bool)
}

extension Sortable {
    init?(rawValue: RawValue) {
        let list = rawValue.split(separator: ",").compactMap { $0.isEmpty ? nil : "\($0)" }
        guard list.count == 2 else { return nil }
        let by = By(rawValue: Int(list[0]) ?? 0) ??                      <===== HERE
        let ascending = Int(list[1])?.boolValue ?? true

        self.init(by: by, ascending: ascending)
    }

    var rawValue: RawValue {
        return "\(by.rawValue),\(ascending.intValue)"
    }
}

更新
我得到了一个解决方案，添加一个“静态变量defaultBy：By { get }"，但不是很好。有什么想法吗？

protocol Sortable: RawRepresentable where RawValue == String {
    associatedtype By: RawRepresentable where By.RawValue == Int
    
    var by: By { get set }
    var ascending: Bool { get set }
    static var defaultBy: By { get }
    
    init(by: By, ascending: Bool)
}

extension Sortable {
    init?(rawValue: RawValue) {
        let list = rawValue.split(separator: ",").compactMap { $0.isEmpty ? nil : "\($0)" }
        guard list.count == 2 else { return nil }
        let by = By(rawValue: Int(list[0]) ?? 0) ?? Self.defaultBy
        let ascending = Int(list[1])?.boolValue ?? true

        self.init(by: by, ascending: ascending)
    }

    var rawValue: RawValue {
        return "\(by.rawValue),\(ascending.intValue)"
    }
}