Swift：在编译时将枚举值映射到类型

Question

I've got an enum in a library target. Like this:

enum SomeEnum {
  case EnumValue1
  case EnumValue2
  case EnumValue3
}

I've got some structs in my app. Like this:

struct Struct1 {...}
struct Struct2 {...}
struct Struct3 {...}

My aim to map this structs to enum values in my app at compile time. Something like this:

extension SomeEnum {
  case EnumValue1(Struct1)
  case EnumValue2(Struct2)
  case EnumValue3(Struct2)
}

But apparently I can't add associated values in an extension.

Original enum in a library shouldn't know anything about my app structs. I'd like to perform this mapping on the app side. So I can't modify my original enum. How can I achieve this?

EDIT: Sorry, I've missed a use case example. I want to create an object of corresponding struct type for each enum value:

func createStruct<T>(type: T.Type) {
  return T()
}

Usage:

let enumValue = SomeEnum.EnumValue2
//here I should use an invented way of mapping enum value to struct:
let object = createStruct(enumValue.struct)

Answer 1

It's not possible. Qute from here

the raw-value type must conform to the Equatable protocol and one of the following literal-convertible protocols: IntegerLiteralConvertible for integer literals, FloatingPointLiteralConvertible for floating-point literals, StringLiteralConvertible for string literals that contain any number of characters, and ExtendedGraphemeClusterLiteralConvertible for string literals that contain only a single character.

even if you try to realize Equatable protocol - it won't work.

struct Struct1 : Equatable {}

func ==(lhs: Struct1, rhs: Struct1) -> Bool {
    return true // some code here
}

enum SomeEnum {
    case EnumValue1 = Struct1
}

This code won't work. Maybe you should create some String that will match your struct.

UPD: You can use associated values like it's said in docs . And it will be during compilation not runtime.

struct Struct1 {}
struct Struct2 {}
struct Struct3 {}

enum SomeEnum {
    case EnumValue1(Struct1)
    case EnumValue2(Struct2)
    case EnumValue3(Struct3)
}

Answer 2

You can do it with:

extension SomeEnum {
    func getStruct() -> Any.Type {
        switch self {
        case .EnumValue1:
            return Struct1.self
        case .EnumValue2:
            return Struct2.self
        case .EnumValue3:
            return Struct3.self
        }
    }
}

let structType = SomeEnum.EnumValue1.getStruct()

But it seems useless because you do not know type of structType and have to use switch...case again to determine it.