使用关联值时获取枚举名称

Question

I have an enum defined as follows

enum Fruit {
    case Apple(associatedValue: String)
    case Orange(associatedValue: String)
}

I have a function that takes an argument of type Fruit

func printNameOnly(fruit: Fruit) {

}

In this function I want to get the enum case as a string, ie I want to get the string "Apple" or "Orange" without regard to whatever the associated value is. Is this possible with Swift?

I could obviously write a function which takes the fruit enum and returns a string using a case statement, but I am trying to find a way to avoid that since the string I want is the enum case name itself.

Answer 1

Try this (Swift 3.1). Covers associated or regular cases.

enum Fruit {
    case banana
    case apple(String)
    case orange(String)

    var label:String {
        let mirror = Mirror(reflecting: self)
        if let label = mirror.children.first?.label {
            return label
        } else {
            return String(describing:self)
        }
    }
}

print(Fruit.banana.label) // "banana"
print(Fruit.apple("yum").label) // "apple"

Answer 2

So you want a Swift enum with a RawValue and an associated value.

You can't

The best solution I can think of is adding a computed property to your enum (similarly to what you suggested in your own question).

enum Fruit {
    case Apple(name:String)
    case Orange(name:String)

    var fruitDesc: String {
        switch self {
        case .Apple: return "Apple"
        case .Orange: return "Orange"
        }
    }
}

let fruit = Fruit.Apple(name: "McIntosh")
print(fruit.fruitDesc) // Apple

Answer 3

When enums have associated values, printing them will list all of the values along with the enum name. For example:

let anApple = Fruit.Apple("myApple")
print(anApple)

This will produce:

Apple("myApple")

Therefore to get just "Apple", extract the part up to the first "(".

Answer 4

If you want to get the enumeration case label for debugging purposes, this neat private reflection function from the Swift runtime might come in handy:

/// Returns the case label for the given enumeration value.
public func getEnumCaseName<T>(_ value: T) -> String? {
    return __getEnumCaseName(value).flatMap { String(validatingUTF8: $0) }
}

/// A private Swift function from the compiler which returns the case
/// label of the given enumeration value, represented as a C string.
@_silgen_name("swift_EnumCaseName")
fileprivate func __getEnumCaseName<T>(_ value: T) -> UnsafePointer<CChar>?

I've tested this approach with Swift 4 only. As mentioned above, I would not recommend using this for production code but rather only for debugging utilities as it is private/undocumented API and it might break in a future Swift version.

Answer 5

Yes, reflection API can help. This one will work both in debugging and in production.

enum Fruit {
    case apple(associatedValue: String)
    case orange(associatedValue: String)
}

func fruitNameOnly(fruit: Fruit) -> String {
    return Mirror(reflecting: fruit).children.first!.label!
}

let greenApple = Fruit.apple(associatedValue: "Green")

print(fruitNameOnly(fruit: greenApple)) // apple

An extended version making use of both associated and raw values:

enum Fruit: RawRepresentable {
    case apple(associatedValue: String)
    case orange(associatedValue: String)

   typealias RawValue = String
    var rawValue: String {

        //return Mirror(reflecting: self).children.first!.label!
        // we rather use a regular switch this time which allows for custom values
        switch self {
        case .apple:     return "apple"
        case .orange(let av):   return "orange " + av // Edge case, careful!
        // Normally rawValues should form one to one relationship.
        // RawRepresentable protocol indicates, that you can "switch back and forth between a custom type and an associated RawValue type without losing the value of the original RawRepresentable type" (developer.apple.com/documentation/swift/rawrepresentable)

        }
    }

    init?(rawValue: String) {
        switch rawValue {
        case "apple":
            self = .apple(associatedValue: "")
        case "orange":
            self = .orange(associatedValue: "")
        default:
            return nil
        }
    }
}

func fruitNameOnly(fruit: Fruit) -> String {
    return Mirror(reflecting: fruit).children.first!.label!
}

let greenApple = Fruit.apple(associatedValue: "green")
print(fruitNameOnly(fruit: greenApple)) // apple

if let yellowOrange = Fruit.init(rawValue: "orange") {
   print(yellowOrange) // orange(associatedValue: "")
}
print(greenApple.rawValue) //apple
let redOrange = Fruit.orange(associatedValue: "red")
print(redOrange.rawValue) //orange red