Possible to pass an enum type name as an argument in Swift?

Question

I am trying to figure out if it's possible to pass around enum's type in the same way that you can pass Class objects in Swift.

My actual use case is a bit more complicated than this, but for discussion, let's say I have two Int enums:

enum Foo: Int, CustomStringConvertible {
    case firstFoo = 0
    case anotherFoo = 1
    var description: String {
        switch self {
        case .firstFoo:
            return "Hello Foo"
        case .anotherFoo:
            return "Goodbye Foo"
        }
    }
}

enum Bar: Int, CustomStringConvertible {
    case firstBar = 0
    case anotherBar = 1
    var description: String {
        switch self {
        case . firstBar:
            return "Hello Bar"
        case . anotherBar:
            return "Goodbye Bar"
        }
    }
}

I would like to be able to write a function like this:

func justAnExample(whichEnum: enum) {
    let val = whichEnum(rawValue: 0)
    print("description: \(String(val))")
}

And then use it like this:

justAnExample(Foo)
// prints: "description: Hello Foo"
justAnExample(Bar)
// prints: "description: Hello Bar"

Is this possible? If so, what is the signature of whichEnum in the function declaration?

Answer 1

You can take advantage of the fact that enums with raw values automatically conform to the RawRepresentable protocol. You can define a generic function that takes a metatype argument of a given type T (spelled as T.Type), where that T is RawRepresentable and, also in your case, its RawValue is Int.

This will allow you to pass in the metatypes of both Foo and Bar, spelled Foo.self and Bar.self respectively:

func justAnExample<T : RawRepresentable>(_ enumType: T.Type) where T.RawValue == Int {

  // Note that an explicit use of init is required when creating an instance from a
  // metatype. We're also using a guard, as `init?(rawValue:)` is failable.
  guard let val = enumType.init(rawValue: 0) else { return }
  print("description: \(val)")
}

justAnExample(Foo.self) // prints: "description: Hello Foo"
justAnExample(Bar.self) // prints: "description: Hello Bar"