How can I use a Swift enum as a Dictionary key? (C

问题:

I've defined an enum to represent a selection of a "station"; stations are defined by a unique positive integer, so I've created the following enum to allow negative values to represent special selections:

enum StationSelector : Printable {
    case Nearest
    case LastShown
    case List
    case Specific(Int)

    func toInt() -> Int {
        switch self {
        case .Nearest:
            return -1
        case .LastShown:
            return -2
        case .List:
            return -3
        case .Specific(let stationNum):
            return stationNum
        }
    }

    static func fromInt(value:Int) -> StationSelector? {
        if value > 0 {
            return StationSelector.Specific(value)
        }
        switch value {
        case -1:
            return StationSelector.Nearest
        case -2:
            return StationSelector.LastShown
        case -3:
            return StationSelector.List
        default:
            return nil
        }
    }

    var description: String {
    get {
        switch self {
        case .Nearest:
            return "Nearest Station"
        case .LastShown:
            return "Last Displayed Station"
        case .List:
            return "Station List"
        case .Specific(let stationNumber):
            return "Station #\(stationNumber)"
        }
    }
    }
}

I'd like to use these values as keys in a dictionary. Declaring a Dictionary yields the expected error that StationSelector doesn't conform to Hashable. Conforming to Hashable is easy with a simple hash function:

var hashValue: Int {
get {
    return self.toInt()
}
}

However, Hashable requires conformance to Equatable, and I can't seem to define the equals operator on my enum to satisfy the compiler.

func == (lhs: StationSelector, rhs: StationSelector) -> Bool {
    return lhs.toInt() == rhs.toInt()
}

The compiler complains that this is two declarations on a single line and wants to put a ; after func, which doesn't make sense, either.

Any thoughts?

回答1:

Info on Enumerations as dictionary keys:

From the Swift book:

Enumeration member values without associated values (as described in Enumerations) are also hashable by default.

However, your Enumeration does have a member value with an associated value, so Hashable conformance has to be added manually by you.

Solution

The problem with your implementation, is that operator declarations in Swift must be at a global scope.

Just move:

func == (lhs: StationSelector, rhs: StationSelector) -> Bool {
    return lhs.toInt() == rhs.toInt()
}

outside the enum definition and it will work.

Check the docs for more on that.

回答2:

I struggled for a little trying to make an enum with associated values conform to Hashable.

Here's I made my enum with associated values conform to Hashable so it could be sorted or used as a Dictionary key, or do anything else that Hashable can do.

You have to make your associated values enum conform to Hashable because associated values enums cannot have a raw type.

public enum Components: Hashable {
    case None
    case Year(Int?)
    case Month(Int?)
    case Week(Int?)
    case Day(Int?)
    case Hour(Int?)
    case Minute(Int?)
    case Second(Int?)

    ///The hashValue of the `Component` so we can conform to `Hashable` and be sorted.
    public var hashValue : Int {
        return self.toInt()
    }

    /// Return an 'Int' value for each `Component` type so `Component` can conform to `Hashable`
    private func toInt() -> Int {
        switch self {
        case .None:
            return -1
        case .Year:
            return 0
        case .Month:
            return 1
        case .Week:
            return 2
        case .Day:
            return 3
        case .Hour:
            return 4
        case .Minute:
            return 5
        case .Second:
            return 6
        }

    }

}

Also need to override the equality operator:

/// Override equality operator so Components Enum conforms to Hashable
public func == (lhs: Components, rhs: Components) -> Bool {
    return lhs.toInt() == rhs.toInt()
}

回答3:

For more readability, let's reimplement StationSelector with Swift 3:

enum StationSelector {
    case nearest, lastShown, list, specific(Int)
}

extension StationSelector: RawRepresentable {

    typealias RawValue = Int

    init?(rawValue: RawValue) {
        switch rawValue {
        case -1: self = .nearest
        case -2: self = .lastShown
        case -3: self = .list
        case (let value) where value >= 0: self = .specific(value)
        default: return nil
        }
    }

    var rawValue: RawValue {
        switch self {
        case .nearest: return -1
        case .lastShown: return -2
        case .list: return -3
        case .specific(let value) where value >= 0: return value
        default: fatalError("StationSelector is not valid")
        }
    }

}

---

The Apple developer API Reference states about Hashable protocol:

When you define an enumeration without associated values, it gains Hashable conformance automatically, and you can add Hashable conformance to your other custom types by adding a single hashValue property.

Therefore, because StationSelector implements associated values, you must make StationSelector conform to Hashable protocol manually.

---

The first step is to implement == operator and make StationSelector conform to Equatable protocol:

extension StationSelector: Equatable {

    static func == (lhs: StationSelector, rhs: StationSelector) -> Bool {
        return lhs.rawValue == rhs.rawValue
    }

}

Usage:

let nearest = StationSelector.nearest
let lastShown = StationSelector.lastShown
let specific0 = StationSelector.specific(0)

// Requires == operator
print(nearest == lastShown) // prints false
print(nearest == specific0) // prints false

// Requires Equatable protocol conformance
let array = [nearest, lastShown, specific0]
print(array.contains(nearest)) // prints true

---

Once Equatable protocol is implemented, you can make StationSelector conform to Hashable protocol:

extension StationSelector: Hashable {

    var hashValue: Int {
        return self.rawValue.hashValue
    }

}

Usage:

// Requires Hashable protocol conformance
let dictionnary = [StationSelector.nearest: 5, StationSelector.lastShown: 10]

---

The following code shows the required implementation for StationSelector to make it conform to Hashable protocol using Swift 3:

enum StationSelector: RawRepresentable, Hashable {

    case nearest, lastShown, list, specific(Int)

    typealias RawValue = Int

    init?(rawValue: RawValue) {
        switch rawValue {
        case -1: self = .nearest
        case -2: self = .lastShown
        case -3: self = .list
        case (let value) where value >= 0: self = .specific(value)
        default: return nil
        }
    }

    var rawValue: RawValue {
        switch self {
        case .nearest: return -1
        case .lastShown: return -2
        case .list: return -3
        case .specific(let value) where value >= 0: return value
        default: fatalError("StationSelector is not valid")
        }
    }

    static func == (lhs: StationSelector, rhs: StationSelector) -> Bool {
        return lhs.rawValue == rhs.rawValue
    }

    var hashValue: Int {
        return self.rawValue.hashValue
    }

}

回答4:

Just for emphasising what Cezar said before. If you can avoid having a member variable, you don't need to implement the equals operator to make enums hashable – just give them a type!

enum StationSelector : Int {
    case Nearest = 1, LastShown, List, Specific
    // automatically assigned to 1, 2, 3, 4
}

That's all you need. Now you can also initiate them with the rawValue or retrieve it later.

let a: StationSelector? = StationSelector(rawValue: 2) // LastShown
let b: StationSelector = .LastShown

if(a == b)
{
    print("Selectors are equal with value \(a?.rawValue)")
}

For further information, check the documentation.