快速访问按类型枚举

Question

I've got an enum in Swift. It's kind of like

enum LegalArgs {
    case AsString(String)
    case AsBool(Bool)
    ... etc
}

I want to access this enum conditionally by type . So if I have an instance of LegalArgs, I can pass T and get back a T? if the instance was of that type. Otherwise I will have to duplicate a bunch of code for different cases.

My current code looks a bit like this:

String? maybeAsString(arg: LegalArgs) {
    switch arg {
    case .AsString(let str):
        return str;
    default:
        return nil;
    }
}

The problem is that I've got to duplicate this function for every case in the enum.

Answer 1

You can use a generic asType function:

enum LegalArgs {
    case AsString(String)
    case AsBool(Bool)
    case AsNumber(Int)

    func asType<T>(type: T.Type) -> T? {
        switch self {
        case AsString(let str): return str as? T
        case AsBool(let bol): return bol as? T
        case AsNumber(let num): return num as? T
        }
    }
}

// usage
LegalArgs.AsBool(true).asType(Bool.self) // true
LegalArgs.AsBool(true).asType(String.self) // nil