DevTut

# Enums

# Basic enumerations

An enum (opens new window) provides a set of related values:

enum Direction {
    case up
    case down
    case left
    case right
}

enum Direction { case up, down, left, right }

Enum values can be used by their fully-qualified name, but you can omit the type name when it can be inferred:

let dir = Direction.up
let dir: Direction = Direction.up
let dir: Direction = .up

// func move(dir: Direction)...
move(Direction.up)
move(.up)

obj.dir = Direction.up
obj.dir = .up

The most fundamental way of comparing/extracting enum values is with a switch (opens new window) statement:

switch dir {
case .up:
    // handle the up case
case .down:
    // handle the down case
case .left:
    // handle the left case
case .right:
    // handle the right case
}

Simple enums are automatically Hashable (opens new window), Equatable (opens new window) and have string conversions:

if dir == .down { ... }

let dirs: Set<Direction> = [.right, .left]

print(Direction.up)  // prints "up"
debugPrint(Direction.up)  // prints "Direction.up"

# Enums with associated values

Enum cases can contain one or more payloads (associated values):

enum Action {
    case jump
    case kick
    case move(distance: Float)  // The "move" case has an associated distance
}

The payload must be provided when instantiating the enum value:

performAction(.jump)
performAction(.kick)
performAction(.move(distance: 3.3))
performAction(.move(distance: 0.5))

The switch statement can extract the associated value:

switch action {
case .jump:
    ...
case .kick:
    ...
case .move(let distance):  // or case let .move(distance):
    print("Moving: \(distance)") 
}

A single case extraction can be done using if case:

if case .move(let distance) = action {
    print("Moving: \(distance)") 
}

The guard case syntax can be used for later use extraction:

guard case .move(let distance) = action else {
    print("Action is not move")
    return
}

Enums with associated values are not Equatable by default. Implementation of the == operator must be done manually:

extension Action: Equatable { }

func ==(lhs: Action, rhs: Action) -> Bool {
    switch lhs {
    case .jump: if case .jump = rhs { return true }
    case .kick: if case .kick = rhs { return true }
    case .move(let lhsDistance): if case .move (let rhsDistance) = rhs { return lhsDistance == rhsDistance }
    }
    return false
}

# Indirect payloads

Normally, enums can't be recursive (because they would require infinite storage):

enum Tree<T> {
    case leaf(T)
    case branch(Tree<T>, Tree<T>)  // error: recursive enum 'Tree<T>' is not marked 'indirect'
}

The indirect keyword makes the enum store its payload with a layer of indirection, rather than storing it inline. You can use this keyword on a single case:

enum Tree<T> {
    case leaf(T)
    indirect case branch(Tree<T>, Tree<T>)
}

let tree = Tree.branch(.leaf(1), .branch(.leaf(2), .leaf(3)))

indirect also works on the whole enum, making any case indirect when necessary:

indirect enum Tree<T> {
    case leaf(T)
    case branch(Tree<T>, Tree<T>)
}

# Raw and Hash values

Enums without payloads can have raw values of any literal type:

enum Rotation: Int {
    case up = 0
    case left = 90
    case upsideDown = 180
    case right = 270
}

Enums without any specific type do not expose the rawValue property

enum Rotation {
    case up
    case right
    case down
    case left
}

let foo = Rotation.up
foo.rawValue //error

Integer raw values are assumed to start at 0 and increase monotonically:

enum MetasyntacticVariable: Int {
    case foo  // rawValue is automatically 0
    case bar  // rawValue is automatically 1
    case baz = 7
    case quux  // rawValue is automatically 8
}

String raw values can be synthesized automatically:

enum MarsMoon: String {
    case phobos  // rawValue is automatically "phobos"
    case deimos  // rawValue is automatically "deimos"
}

A raw-valued enum automatically conforms to RawRepresentable (opens new window). You can get an enum value's corresponding raw value with .rawValue:

func rotate(rotation: Rotation) {
    let degrees = rotation.rawValue
    ...
}

You can also create an enum from a raw value using init?(rawValue:):

let rotation = Rotation(rawValue: 0)  // returns Rotation.Up
let otherRotation = Rotation(rawValue: 45)  // returns nil (there is no Rotation with rawValue 45)

if let moon = MarsMoon(rawValue: str) {
    print("Mars has a moon named \(str)")
} else {
    print("Mars doesn't have a moon named \(str)")
}

If you wish to get the hash value of a specific enum you can access its hashValue, The hash value will return the index of the enum starting from zero.

let quux = MetasyntacticVariable(rawValue: 8)// rawValue is 8
quux?.hashValue //hashValue is 3

# Initializers

Enums can have custom init methods that can be more useful than the default init?(rawValue:). Enums can also store values as well. This can be useful for storing the values they where initialized with and retrieving that value later.

enum CompassDirection {
    case north(Int)
    case south(Int)
    case east(Int)
    case west(Int)

    init?(degrees: Int) {
        switch degrees {
        case 0...45:
            self = .north(degrees)
        case 46...135:
            self = .east(degrees)
        case 136...225:
            self = .south(degrees)
        case 226...315:
            self = .west(degrees)
        case 316...360:
            self = .north(degrees)
        default:
            return nil
        }
    }
    
    var value: Int = {
        switch self {
            case north(let degrees):
                return degrees
            case south(let degrees):
                return degrees
            case east(let degrees):
                return degrees
            case west(let degrees):
                return degrees
        }    
    }
}

Using that initializer we can do this:

var direction = CompassDirection(degrees: 0) // Returns CompassDirection.north
direction = CompassDirection(degrees: 90) // Returns CompassDirection.east
print(direction.value) //prints 90
direction = CompassDirection(degrees: 500) // Returns nil

# Enumerations share many features with classes and structures

Enums in Swift are much more powerful than some of their counterparts in other languages, such as C (opens new window). They share many features with classes (opens new window) and structs (opens new window), such as defining initialisers (opens new window), computed properties (opens new window), instance methods (opens new window), protocol conformances (opens new window) and extensions (opens new window).

protocol ChangesDirection {
    mutating func changeDirection()
}

enum Direction {
    
    // enumeration cases
    case up, down, left, right
    
    // initialise the enum instance with a case
    // that's in the opposite direction to another
    init(oppositeTo otherDirection: Direction) {
        self = otherDirection.opposite
    }
    
    // computed property that returns the opposite direction
    var opposite: Direction {
        switch self {
        case .up:
            return .down
        case .down:
            return .up
        case .left:
            return .right
        case .right:
            return .left
        }
    }
}

// extension to Direction that adds conformance to the ChangesDirection protocol
extension Direction: ChangesDirection {
    mutating func changeDirection() {
        self = .left
    }
}


var dir = Direction(oppositeTo: .down) // Direction.up

dir.changeDirection() // Direction.left

let opposite = dir.opposite // Direction.right

# Nested Enumerations

You can nest enumerations one inside an other, this allows you to structure hierarchical enums to be more organized and clear.

enum Orchestra {
    enum Strings {
        case violin
        case viola
        case cello
        case doubleBasse
    }
    
    enum Keyboards {
        case piano
        case celesta
        case harp
    }
    
    enum Woodwinds {
        case flute
        case oboe
        case clarinet
        case bassoon
        case contrabassoon
    }
}

And you can use it like that:

let instrment1 = Orchestra.Strings.viola
let instrment2 = Orchestra.Keyboards.piano

# Remarks

Like structs and unlike classes, enums are value types and are copied instead of referenced when passed around.

For more information about enums, see The Swift Programming Language (opens new window).

← Structs →