How to make a Swift String enum available in Objective-C?

I have this enum with String values, which will be used to tell an API method that logs to a server what kind of serverity a message has. I'm using Swift 1.2, so enums can be mapped to Objective-C

@objc enum LogSeverity : String {
    case Debug = "DEBUG"
    case Info = "INFO"
    case Warn = "WARN"
    case Error = "ERROR"
}

I get the error

> @objc enum raw type String is not an integer type

I haven't managed to find anywhere which says that only integers can be translated to Objective-C from Swift. Is this the case? If so, does anyone have any best-practice suggestion on how to make something like this available in Objective-C?

One of the solutions is to use the RawRepresentable protocol.

It's not ideal to have to write the init and rawValue methods but that allows you to use this enum as usual in both Swift and Objective-C.

@objc public enum LogSeverity: Int, RawRepresentable {
    case debug
    case info
    case warn
    case error

    public typealias RawValue = String

    public var rawValue: RawValue {
        switch self {
            case .debug:
                return "DEBUG"
            case .info:
                return "INFO"
            case .warn:
                return "WARN"
            case .error:
                return "ERROR"
        }
    }

    public init?(rawValue: RawValue) {
        switch rawValue {
            case "DEBUG":
                self = .debug
            case "INFO":
                self = .info
            case "WARN":
                self = .warn
            case "ERROR":
                self = .error
            default:
                return nil
        }
    }
}

From the Xcode 6.3 release notes (emphasis added):

> Swift Language Enhancements
>
> ...
> Swift enums can now be exported to Objective-C using the @objc > attribute. @objc enums must declare an integer raw type, and cannot be > generic or use associated values. Because Objective-C enums are not > namespaced, enum cases are imported into Objective-C as the > concatenation of the enum name and case name.

Here's a solution that works.

@objc public enum ConnectivityStatus: Int {
    case Wifi
    case Mobile
    case Ethernet
    case Off
    
    func name() -> String {
        switch self {
        case .Wifi: return "wifi"
        case .Mobile: return "mobile"
        case .Ethernet: return "ethernet"
        case .Off: return "off"
        }
    }
}

Here is work around if you really want to achieve the goal. However, you can access the enum values in objects that Objective C accepts, not as actual enum values.

enum LogSeverity : String {

    case Debug = "DEBUG"
    case Info = "INFO"
    case Warn = "WARN"
    case Error = "ERROR"

    private func string() -> String {
        return self.rawValue
    }
}

@objc
class LogSeverityBridge: NSObject {

    class func Debug() -> NSString {
        return LogSeverity.Debug.string()
    }

    class func Info() -> NSString {
        return LogSeverity.Info.string()
    }

    class func Warn() -> NSString {
        return LogSeverity.Warn.string()
    }

    class func Error() -> NSString {
        return LogSeverity.Error.string()
    }
}

To call :

NSString *debugRawValue = [LogSeverityBridge Debug]

If you don't mind to define the values in (Objective) C, you can use the NS_TYPED_ENUM macro to import constants in Swift.

For example:

.h file

typedef NSString *const ProgrammingLanguage NS_TYPED_ENUM;

FOUNDATION_EXPORT ProgrammingLanguage ProgrammingLanguageSwift;
FOUNDATION_EXPORT ProgrammingLanguage ProgrammingLanguageObjectiveC;

.m file

ProgrammingLanguage ProgrammingLanguageSwift = "Swift";
ProgrammingLanguage ProgrammingLanguageObjectiveC = "ObjectiveC";

In Swift, this is imported as a struct as such:

struct ProgrammingLanguage: RawRepresentable, Equatable, Hashable {
    typealias RawValue = String

    init(rawValue: RawValue)
    var rawValue: RawValue { get }
    
    static var swift: ProgrammingLanguage { get }
    static var objectiveC: ProgrammingLanguage { get }
}

Although the type is not bridged as an enum, it feels very similar to one when using it in Swift code.

You can read more about this technique in the "Interacting with C APIs" of the Using Swift with Cocoa and Objective-C documentation

Code for Xcode 8, using the fact that Int works but other methods aren't exposed to Objective-C. This is pretty horrible as it stands...

class EnumSupport : NSObject {
    class func textFor(logSeverity severity: LogSeverity) -> String {
        return severity.text()
    }
}

@objc public enum LogSeverity: Int {
    case Debug
    case Info
    case Warn
    case Error
    
    func text() -> String {
        switch self {
            case .Debug: return "debug"
            case .Info: return "info"
            case .Warn: return "warn"
            case .Error: return "error"
        }
    }
}

This is my use case:

• I avoid hard-coded Strings whenever I can, so that I get compile warnings when I change something
• I have a fixed list of String values coming from a back end, which can also be nil

Here's my solution that involves no hard-coded Strings at all, supports missing values, and can be used elegantly in both Swift and Obj-C:

@objc enum InventoryItemType: Int {
    private enum StringInventoryItemType: String {
        case vial
        case syringe
        case crystalloid
        case bloodProduct
        case supplies
    }
    
    case vial
    case syringe
    case crystalloid
    case bloodProduct
    case supplies
    case unknown
    
    static func fromString(_ string: String?) -> InventoryItemType {
        guard let string = string else {
            return .unknown
        }
        guard let stringType = StringInventoryItemType(rawValue: string) else {
            return .unknown
        }
        switch stringType {
        case .vial:
            return .vial
        case .syringe:
            return .syringe
        case .crystalloid:
            return .crystalloid
        case .bloodProduct:
            return .bloodProduct
        case .supplies:
            return .supplies
        }
    }
    
    var stringValue: String? {
        switch self {
        case .vial:
            return StringInventoryItemType.vial.rawValue
        case .syringe:
            return StringInventoryItemType.syringe.rawValue
        case .crystalloid:
            return StringInventoryItemType.crystalloid.rawValue
        case .bloodProduct:
            return StringInventoryItemType.bloodProduct.rawValue
        case .supplies:
            return StringInventoryItemType.supplies.rawValue
        case .unknown:
            return nil
        }
    }
}

Here's what I came up with. In my case, this enum was in the context providing info for a specific class, ServiceProvider.

class ServiceProvider {
    @objc enum FieldName : Int {
        case CITY
        case LATITUDE
        case LONGITUDE
        case NAME
        case GRADE
        case POSTAL_CODE
        case STATE
        case REVIEW_COUNT
        case COORDINATES
        
        var string: String {
            return ServiceProvider.FieldNameToString(self)
        }
    }
    
    class func FieldNameToString(fieldName:FieldName) -> String {
        switch fieldName {
        case .CITY:         return "city"
        case .LATITUDE:     return "latitude"
        case .LONGITUDE:    return "longitude"
        case .NAME:         return "name"
        case .GRADE:        return "overallGrade"
        case .POSTAL_CODE:  return "postalCode"
        case .STATE:        return "state"
        case .REVIEW_COUNT: return "reviewCount"
        case .COORDINATES:  return "coordinates"
        }
    }
}

From Swift, you can use .string on an enum (similar to .rawValue). From Objective-C, you can use [ServiceProvider FieldNameToString:enumValue];

You can create an private Inner enum. The implementation is a bit repeatable, but clear and easy. 1 line rawValue, 2 lines init, which always look the same. The Inner has a method returning the "outer" equivalent, and vice-versa.

Has the added benefit that you can directly map the enum case to a String, unlike other answers here.

Please feel welcome to build on this answer if you know how to solve the repeatability problem with templates, I don't have time to mingle with it right now.

@objc enum MyEnum: NSInteger, RawRepresentable, Equatable {
    case
    option1,
    option2,
    option3
    
    // MARK: RawRepresentable
    
    var rawValue: String {
        return toInner().rawValue
    }
    
    init?(rawValue: String) {
        guard let value = Inner(rawValue: rawValue)?.toOuter() else { return nil }
        self = value
    }
    
    // MARK: Obj-C support
    
    private func toInner() -> Inner {
        switch self {
        case .option1: return .option1
        case .option3: return .option3
        case .option2: return .option2
        }
    }
    
    private enum Inner: String {
        case
        option1 = "option_1",
        option2 = "option_2",
        option3 = "option_3"
        
        func toOuter() -> MyEnum {
            switch self {
            case .option1: return .option1
            case .option3: return .option3
            case .option2: return .option2
            }
        }
    }
}

I think @Remi 's answer crashes in some situations as I had this:

My error's screesshot. so I post my edition for @Remi 's answer:

@objc public enum LogSeverity: Int, RawRepresentable {
    case debug
    case info
    case warn
    case error

    public typealias RawValue = String

    public var rawValue: RawValue {
        switch self {
            case .debug:
                return "DEBUG"
            case .info:
                return "INFO"
            case .warn:
                return "WARN"
            case .error:
                return "ERROR"
        }
    }

    public init?(rawValue: RawValue) {
        switch rawValue {
            case "DEBUG":
                self = .debug
            case "INFO":
                self = .info
            case "WARN":
                self = .warn
            case "ERROR":
                self = .error
            default:
                return nil
        }
    }
}