How to get the name of enumeration value in Swift?

问题:

If I have an enumeration with raw Integer values:

enum City: Int {
  case Melbourne = 1, Chelyabinsk, Bursa
}

let city = City.Melbourne

How can I convert a city value to a string Melbourne? Is this kind of a type name introspection available in the language?

Something like (this code will not work):

println("Your city is \(city.magicFunction)")
> Your city is Melbourne

回答1:

As of Xcode 7 beta 5 you can now print type names and enum cases by default using print(_:), or convert to String using String's init(_:) initializer or string interpolation syntax. So for your example:

enum City: Int {
    case Melbourne = 1, Chelyabinsk, Bursa
}
let city = City.Melbourne

print(city)
// prints "Melbourne"

let cityName = "\(city)"   // or `let cityName = String(city)`
// cityName contains "Melbourne"

So there is no longer a need to define & maintain a convenience function that switches on each case to return a string literal. In addition, this works automatically for any enum, even if no raw-value type is specified.

debugPrint(_:) & String(reflecting:) can be used for a fully-qualified name:

debugPrint(city)
// prints "App.City.Melbourne" (or similar, depending on the full scope)

let cityDebugName = String(reflecting: city)
// cityDebugName contains "App.City.Melbourne"

Note that you can customise what is printed in each of these scenarios:

extension City: CustomStringConvertible {
    var description: String {
        return "City \(rawValue)"
    }
}

print(city)
// prints "City 1"

extension City: CustomDebugStringConvertible {
    var debugDescription: String {
        return "City (rawValue: \(rawValue))"
    }
}

debugPrint(city)
// prints "City (rawValue: 1)"

(I haven't found a way to call into this "default" value, for example, to print "The city is Melbourne" without resorting back to a switch statement. Using \(self) in the implementation of description/debugDescription causes an infinite recursion.)

The comments above String's init(_:) & init(reflecting:) initializers describe exactly what is printed, depending on what the reflected type conforms to:

extension String {
    /// Initialize `self` with the textual representation of `instance`.
    ///
    /// * If `T` conforms to `Streamable`, the result is obtained by
    ///   calling `instance.writeTo(s)` on an empty string s.
    /// * Otherwise, if `T` conforms to `CustomStringConvertible`, the
    ///   result is `instance`'s `description`
    /// * Otherwise, if `T` conforms to `CustomDebugStringConvertible`,
    ///   the result is `instance`'s `debugDescription`
    /// * Otherwise, an unspecified result is supplied automatically by
    ///   the Swift standard library.
    ///
    /// - SeeAlso: `String.init<T>(reflecting: T)`
    public init<T>(_ instance: T)

    /// Initialize `self` with a detailed textual representation of
    /// `subject`, suitable for debugging.
    ///
    /// * If `T` conforms to `CustomDebugStringConvertible`, the result
    ///   is `subject`'s `debugDescription`.
    ///
    /// * Otherwise, if `T` conforms to `CustomStringConvertible`, the result
    ///   is `subject`'s `description`.
    ///
    /// * Otherwise, if `T` conforms to `Streamable`, the result is
    ///   obtained by calling `subject.writeTo(s)` on an empty string s.
    ///
    /// * Otherwise, an unspecified result is supplied automatically by
    ///   the Swift standard library.
    ///
    /// - SeeAlso: `String.init<T>(T)`
    public init<T>(reflecting subject: T)
}

See the release notes for info about this change.

回答2:

There is no introspection on enum cases at the moment. You will have to declare them each manually:

enum City : String, Printable {
  case Melbourne = "Melbourne"
  case Chelyabinsk = "Chelyabinsk"
  case Bursa = "Bursa"

  var description : String {
    get {
        return self.rawValue
    }
  }
}

Note: The Printable protocol does not currently work in Playgrounds. If you want to see the string in a playground you will have to call toRaw() manually

If you need the raw type to be an Int, you will have to do a switch yourself:

enum City : Int, Printable {
  case Melbourne = 1, Chelyabinsk, Bursa

  var description : String {
    get {
      switch(self) {
        case Melbourne:
          return "Melbourne"
        case Chelyabinsk:
          return "Chelyabinsk"
        case Bursa:
          return "Bursa"
      }
    }
  }
}

回答3:

In Swift-3 (tested with XCode 8.1) you can add the following methods in your enum:

/**
 * The name of the enumeration (as written in case).
 */
var name: String {
    get { return String(describing: self) }
}

/**
 * The full name of the enumeration
 * (the name of the enum plus dot plus the name as written in case).
 */
var description: String {
    get { return String(reflecting: self) }
}

You can then use it as a normal method call on your enum instance. It might also work in previous Swift versions, but I haven't tested it.

In your example:

enum City: Int {
    case Melbourne = 1, Chelyabinsk, Bursa
    var name: String {
        get { return String(describing: self) }
    }
    var description: String {
        get { return String(reflecting: self) }
    }
}
let city = City.Melbourne

print(city.name)
// prints "Melbourne"

print(city.description)
// prints "City.Melbourne"

If you want to provide this functionality to all your enums, you can make it an extension:

/**
 * Extend all enums with a simple method to derive their names.
 */
extension RawRepresentable where RawValue: Any {
  /**
   * The name of the enumeration (as written in case).
   */
  var name: String {
    get { return String(describing: self) }
  }

  /**
   * The full name of the enumeration
   * (the name of the enum plus dot plus the name as written in case).
   */
  var description: String {
    get { return String(reflecting: self) }
  }
}

This only works for Swift enums.

回答4:

For Objective-C enums the only way currently seems to be, for example, to extend the enum with CustomStringConvertible ending up with something like:

extension UIDeviceBatteryState: CustomStringConvertible {
    public var description: String {
        switch self {
        case Unknown:
            return "Unknown"
        case Unplugged:
            return "Unplugged"
        case Charging:
            return "Charging"
        case Full:
            return "Full"
        }
    }
}

And then casting the enum as String:

String(UIDevice.currentDevice().batteryState)

回答5:

This is so disappointing.

For the case when you need those names (that the compiler perfectly knows the exact spelling of, but refuses to let access -- thank you Swift team!! --) but do not want or cannot make String the base of your enum, a verbose, cumbersome alternative is as follows:

enum ViewType : Int, Printable {

    case    Title
    case    Buttons
    case    View

    static let all = [Title, Buttons, View]
    static let strings = ["Title", "Buttons", "View"]

    func string() -> String {
        return ViewType.strings[self.rawValue]
    }

    var description:String {
        get {
            return string()
        }
    }
}

You can use the above as follows:

let elementType = ViewType.Title
let column = Column.Collections
let row = 0

println("fetching element \(elementType), column: \(column.string()), row: \(row)")

And you'll get the expected result (code for the Column similar, but not shown)

fetching element Title, column: Collections, row: 0

In the above, I have made the description property refer back to the string method, but this is a matter of taste. Also note that so called static variables need to be scope qualified by the name of their enclosing type, as the compiler is too amnesic and cannot recall the context all by itself...

The Swift team must really be commanded. They created enum that you cannot enumerate and that which you can use enumerate on are "Sequences" but not enum!

回答6:

On top of the String(…) (CustomStringConvertible) support for enums in Swift 2.2, there's also somewhat broken reflection support for them. For enum cases with associated values it is possible to get the label of the enum case using reflection:

enum City {
    case Melbourne(String)
    case Chelyabinsk
    case Bursa

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

print(City.Melbourne("Foobar").label) // prints out "Melbourne"

By being broken, I however meant that for "simple" enums, the above reflection based label computed property just returns nil (boo-hoo).

print(City.Chelyabinsk.label) // prints out nil

The situation with reflection should be getting better after Swift 3, apparently. The solution for now though is String(…), as suggested in one of the other answers:

print(String(City.Chelyabinsk)) // prints out Cheylabinsk

回答7:

Simple but works...

enum ViewType : Int {
    case    Title
    case    Buttons
    case    View
}

func printEnumValue(enum: ViewType) {

    switch enum {
    case .Title: println("ViewType.Title")
    case .Buttons: println("ViewType.Buttons")
    case .View: println("ViewType.View")
    }
}

回答8:

Swift now has what are known as Implicitly Assigned Raw Value. Basically if you don't give raw values to each case and the enum is of type String, it deduces that the case's raw value is itself in string format. Go on give it a try.

enum City: String {
  case Melbourne, Chelyabinsk, Bursa
}

let city = City.Melbourne.rawValue

// city is "Melbourne"

回答9:

For swift :

extension UIDeviceBatteryState: CustomStringConvertible {
    public var description: String {
        switch self {
        case .unknown:
            return "unknown"
        case .unplugged:
            return "unplugged"
        case .charging:
            return "charging"
        case .full:
            return "full"
        }
    }
}

if your variable "batteryState" then call :

self.batteryState.description