Swift 枚举可以有多个原始值吗？

Question

I want to associate two raw values to an enum instance (imagine an enum representing error types, I want Error.Teapot to have an Int type property code with value 418, and a String property set to I'm a teapot .)

Note the difference between raw values and associated values here—I want all Teapot instances to have a code of 418, I don't want a unique associated value for each Teapot instance.

Is there a better way than adding computed properties to the enum that switch ed on self to look up the appropriate value?

Answer 1

No, an enum cannot have multiple raw values - it has to be a single value, implementing the Equatable protocol, and be literal-convertible as described in the documentation .

I think the best approach in your case is to use the error code as raw value, and a property backed by a prepopulated static dictionary with the error code as key and the text as value.

Answer 2

You have a couple options. But neither of them involve raw values. Raw values are just not the right tool for the task.

Option 1 (so-so): Associated Values

I personally highly recommend against there being more than one associated value per enum case. Associated values should be dead obvious (since they don't have arguments/names), and having more than one heavily muddies the water.

That said, it's something the language lets you do. This allows you to have each case defined differently as well, if that was something you needed. Example:

enum ErrorType {
    case teapot(String, Int)
    case skillet(UInt, [CGFloat])
}

Option 2 (better): Tuples! And computed properties!

Tuples are a great feature of Swift because they give you the power of creating ad-hoc types. That means you can define it in-line. Sweet!

If each of your error types are going to have a code and a description, then you could have a computed info property (hopefully with a better name?). See below:

enum ErrorType {
    case teapot
    case skillet

    var info: (code: Int, description: String) {
        switch self {
        case .teapot:
            return (418, "Hear me shout!")
        case .skillet:
            return (326, "I'm big and heavy.")
        }
    }
}

Calling this would be much easier because you could use tasty, tasty dot syntax :

let errorCode = myErrorType.info.code

Answer 3

I created a way of simulating this (No different than what Marcos Crispino suggested on his answer). Far from a perfect solution but allows us to avoid those nasty switch cases for every different property we want to get.

The trick is to use a struct as the "properties/data" holder and using it as a RawValue in the enum itself.

It has a bit of duplication but it's serving me well so far. Every time you want to add a new enum case, the compiler will remind you to fill in the extra case in the rawValue getter, which should remind you to update the init? which would remind you to create the new static property on the struct.

Gist

Code to the Gist:

enum VehicleType : RawRepresentable {

    struct Vehicle : Equatable {
        let name: String
        let wheels: Int

        static func ==(l: Vehicle, r: Vehicle) -> Bool {
            return l.name == r.name && l.wheels == r.wheels
        }

        static var bike: Vehicle {
            return Vehicle(name: "Bicycle", wheels: 2)
        }

        static var car: Vehicle {
            return Vehicle(name: "Automobile", wheels: 4)
        }

        static var bus: Vehicle {
            return Vehicle(name: "Autobus", wheels: 8)
        }
    }

    typealias RawValue = Vehicle

    case car
    case bus
    case bike

    var rawValue: RawValue {
        switch self {
        case .car:
            return Vehicle.car
        case .bike:
            return Vehicle.bike
        case .bus:
            return Vehicle.bus
        }
    }

    init?(rawValue: RawValue) {
        switch rawValue {
        case Vehicle.bike:
            self = .bike
        case Vehicle.car:
            self = .car
        case Vehicle.bus:
            self = .bus
        default: return nil
        }
    }
}

VehicleType.bike.rawValue.name
VehicleType.bike.rawValue.wheels
VehicleType.car.rawValue.wheels

VehicleType(rawValue: .bike)?.rawValue.name => "Bicycle"
VehicleType(rawValue: .bike)?.rawValue.wheels => 2
VehicleType(rawValue: .car)?.rawValue.name => "Automobile"
VehicleType(rawValue: .car)?.rawValue.wheels => 4
VehicleType(rawValue: .bus)?.rawValue.name => "Autobus"
VehicleType(rawValue: .bus)?.rawValue.wheels => 8

Answer 4

No, you cannot have multiple raw values associated with an enum.

In your case, you could have the raw value to be equal to the code, and have an associated value with the description. But I think the computed properties approach is the best option here.

Answer 5

One workaround if you wanted to have many static properties for a YourError could be to import a property list; you could set the root object to a dictionary, with your enum raw value as the key for each object, allowing you to easily retrieve static structured data for the object.

This has an example of importing and using a plist: http://www.spritekitlessons.com/parsing-a-property-list-using-swift/

That might be overkill for simply an error description, for which you could just use a hardcoded static function with a switch statement for your enum values, that returns the error string you need. Simply place the static function in the same .swift file as your enum.

For instance,

static func codeForError(error : YourErrorType) -> Int {
    switch(error) {
        case .Teapot:
            return "I'm a Teapot"
        case .Teacup:
            return "I'm a Teacup"
        ...
        default:
            return "Unknown Teaware Error"
    }
}

This has the benefit (compared to the .plist solution) of better accomodating localization. However, a .plist could just contain a key used for retrieving the proper localization, instead of a error string, for this purpose.

Answer 6

For beginning, assuming you want to store a code and a message, you can use a struct for RawValue

struct ErrorInfo {
    let code: Int
    let message: String
}

Next step is to define the enum as being RawRepresentable , and use ErrorInfo as the raw value:

enum MyError: RawRepresentable {
    typealias RawValue = ErrorInfo

    case teapot

What remains is to map between instances of MyError and ErrorInfo :

static private let mappings: [(ErrorInfo, MyError)] = [
        (ErrorInfo(code: 418, message: "I'm a teapot"), .teapot)
    ]

With the above, let's build the full definition of the enum:

enum MyError: RawRepresentable {
    static private let mappings: [(ErrorInfo, MyError)] = [
    (ErrorInfo(code: 418, message: "I'm a teapot"), .teapot)
    ]

    case teapot

    init?(rawValue: ErrorInfo) {
        guard let match = MyError.mappings.first(where: { $0.0.code == rawValue.code && $0.0.message == rawValue.message}) else {
            return nil
        }
        self = match.1
    }

    var rawValue: ErrorInfo {
        return MyError.mappings.first(where: { $0.1 == self })!.0
    }
}

Some notes:

• you could use only the error code for matching, however this might result in inconsistent raw values if the messages differ
• the amount of boilerplate code required to have raw values of some custom type might not outcome the benefits of using associated values.

Answer 7

Possible work around may to associate custom functions with enum

 enum ToolbarType : String{
        case Case = "Case", View="View", Information="Information"
        static let allValues = [Case, View, Information]

        func ordinal() -> Int{
            return ToolbarType.allValues.index(of: self)!
        }
 }

Can be used as

 for item in ToolbarType.allValues {
        print("\(item.rawValue): \(item.ordinal())")
 }

Output

Case: 0
View: 1
Information: 2

Possibly you can have additional functions to associate enum type to different values

Answer 8

This doesn't particularly answer your question, which was asking to find a better way than switch ing through self to look up the appropriate value but this answer may still be useful for someone looking in the future that needs a simple way to get a string from an enum which is defined as an integer type.

enum Error: UInt {
    case Teapot = 418
    case Kettle = 419

    static func errorMessage(code: UInt) -> String {
        guard let error = Error(rawValue: code) else {
            return "Unknown Error Code"
        }

        switch error {
        case .Teapot:
            return "I'm a teapot!"
        case .Kettle:
            return "I'm a kettle!"
        }
    }
}

This way, we can get the errorMessage two ways:

1. With an integer (eg. that was returned as an error code from a server)
2. With an enum value (the rawValue we define for the enum)

Option 1:

let option1 = Error.errorMessage(code: 418)
print(option1)  //prints "I'm a teapot!"

Option 2:

let option2 = Error.errorMessage(code: Error.Teapot.rawValue)
print(option2)  //prints "I'm a teapot!"    

Answer 9

In modern versions of Swift it's possible to get the string value of an enum case label, even without that enum being declared with a : String rawValue.

How to get the name of enumeration value in Swift?

So there is no longer a need to define and 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.

This, at least, allows you to have "multiple raw values" by having both a real : Int rawValue as well as the string used as the case label.

Answer 10

I think it just tricky, and I have create my own idea like below:

enum Gender:NSNumber
{
    case male = 1
    case female = 0

    init?(strValue: String?) {
        switch strValue {
        case Message.male.value:
            self = .male
        case Message.female.value:
            self = .female
        default: return nil
        }
    }

    var strValue: String {
        switch self {
        case .male:
            return Message.male.value
        case .female:
            return Message.female.value
        }
    }
}

Answer 11

First of all, enums should only have one raw value. However if you want to have something that can use multiple raw values... there is a way to 'hack' this, but you have to make it codable and hashable yourself, implement custom init's etc.

enum MyCustomEnum: Codable, Hashable {

// duplicate every case with associated value of Codable.Type
case myFirstCase, _myFirstCase(Codable.Type)
case mySecondCase, _mySecondCase(Codable.Type)
case myThirdCase, _myThirdCase(Codable.Type)
case unknown(Any), _unknown(Codable.Type, Any) // handles unknown values

// define an allCases value to determine the only values your app 'sees'.
static var allCases: [Self] {
    return [
        .myFirstCase,
        .mySecondCase,
        .myThirdCase
        // unknown(String) // you can add unknown as well, but this is too mask any unknown values.
    ]
}

static func == (lhs: MyCustomEnum, rhs: MyCustomEnum) -> Bool {
    return lhs.stringValue == rhs.stringValue // can be either one of your custom raw values.
}

// add this per raw value. In this case one for Int and one for String
init(rawValue: Int) {
    guard let value = Self.allCases.first(where:{ $0.intValue == rawValue }) else {
        self = ._unknown(Int.self, rawValue)
        return
    }
    switch value {
    case .myFirstCase: self = ._myFirstCase(Int.self)
    case .mySecondCase: self = ._mySecondCase(Int.self)
    case .myThirdCase: self = ._myThirdCase(Int.self)
    default: self = ._unknown(Int.self, rawValue)
    }
}

init(rawValue: String) {
    guard let value = Self.allCases.first(where:{ $0.stringValue == rawValue }) else {
        self = ._unknown(String.self, rawValue)
        return
    }
    switch value {
    case .myFirstCase: self = ._myFirstCase(String.self)
    case .mySecondCase: self = ._mySecondCase(String.self)
    case .myThirdCase: self = ._myThirdCase(String.self)
    default: self = ._unknown(Int.self, rawValue)
    }
}

// add this per raw value. In this case one for Int and one for String
var intValue: Int {
    switch self {
    case .myFirstCase, ._myFirstCase(_): return 1
    case .mySecondCase, ._mySecondCase(_): return 2
    case .myThirdCase, ._myThirdCase(_): return 3
    case .unknown(let value), ._unknown(_, let value): return value as? Int ?? -1 // you can also choose to let intValue return optional Int.
    }
}

var stringValue: String {
    switch self {
    case .myFirstCase, ._myFirstCase(_): return "my first case"
    case .mySecondCase, ._mySecondCase(_): return "my second case"
    case .myThirdCase, ._myThirdCase(_): return "my third case"
    case .unknown(let value), ._unknown(_, let value): return value as? String ?? "not a String" // you can also choose to let stringValue return optional String.
    }
}

// determine the codable type using Mirror
private func getCodableType() -> Codable.Type? {
    let mirrorOfModuleType = Mirror.init(reflecting: self)
    guard let childOfModuleType = mirrorOfModuleType.children.first else { // no children, means no associated values.
        return nil
    }
    let value = childOfModuleType.value // can be either Codable.Type, String or (Codable.Type & String)
    if let rawValue = value as? Codable.Type {
        return rawValue
    } else {
        guard let rawValue = value as? (Codable.Type, String) else {
            // unknown(String), we don't know the rawValue as given, but try in this part of the code to guess what type fits best.
            if self.stringValue != "\(self.intValue)" { // e.g. "1" might match 1 but "1.0" and 1 don't match
                return String.self
            } else {
                return Int.self // return either a default value, or nil. It's your choice.
            }
        }
        return rawValue.0
    }
}

// confine to hashable using getCodableType
func hash(into hasher: inout Hasher) {
    if self.getCodableType() is String.Type {
        hasher.combine(self.stringValue)
    } else { // if you don't call hasher.combine at all, you can expect strange issues. If you do not know the type, choose one that is most common.
        hasher.combine(self.intValue)
    }
}

// confine to Decodable
init(from decoder: Decoder) throws {
    if let rawValue = try? Int.init(from: decoder) {
        self.init(rawValue: rawValue)
    } else if let rawValue = try? String.init(from: decoder) {
        self.init(rawValue: rawValue)
    } else {
        throw DecodingError.valueNotFound(Self.self, DecodingError.Context(codingPath: [], debugDescription: "no matching value was found"))
    }
}

// confine to Encodable using getCodableType
func encode(to encoder: Encoder) throws {
    let rawValue = self.getCodableType()
    if rawValue is String.Type {
        try self.stringValue.encode(to: encoder)
    } else if rawValue is Int.Type {
        try self.intValue.encode(to: encoder)
    } else {
        // getCodableType returns nil if it does not know what value it is. (e.g. myFirstCase without associated value) If you want to support this as well, you can encode using one of your rawValues to the encoder.
        throw EncodingError.invalidValue(Self.self, EncodingError.Context.init(codingPath: [], debugDescription: "this enum does not have a correct value", underlyingError: nil))
    }
}

}

this code is scalable to any number of raw value as long as they are Codable