Enums

Enums are one of the basic constructs that the Swift language offers. At the same level as classes, structs, and functions, they are used to represent values that can only have a finite amount of states.

Take the Optional enum, for example; it is represented by an enum perfectly. It represents a value that can have two, and only two, states, represented by the two members of the Optional enum. It can either be initialized to .none or filled with a value, .wrapped(value).

Enums are incredibly powerful in Swift. From very simple cases to generics, they are among the most powerful tools that we have for writing our programs.

Simple enums

Let's say you're building a smart light remote control; you can easily represent the state of this light with the following enum:

enum State {
    case on
    case off
}

let anOnLight = State.on

This is a very simple example, and we could have used a Boolean value, but with the enum, we set ourselves up for expansion.

Adding methods

Now, we may want to add a method to this State enumeration. After all, it's very common to just toggle the switch on and off without thinking:

extension State {
    mutating func toggle() {
        self = self == .off ? .on : .off
    }
}

var state: State = .on
state.toggle()
state == .off // true

As in the previous section, we can just extend the State enum to add the toggle functionality. Enums follow value semantics; therefore, we have to mark the toggle method as mutating.

Associating values

Enums can also contain associated values. In our scenario, we can leverage this to represent a dimmer. A dimmer changes the intensity of the light, so we can represent it with a third member-the dimmed member:

enum State: Equatable {
    case on
    case off
    case dimmed(value: Double)
}

You may have noticed that we needed to add the Equatable conformance. This is required, as otherwise, the compiler can't synthesize equality anymore without our hint. This implementation works, but we lack a few things. First, not all Double values are valid; we'd probably like to keep these in a reasonable span (between 0 and 1, for example). But perhaps not all of our lights support such values between 0 and 1. Others may want to support between 0 and a 100 or integers between 0 and 255.

Generic enums

In the following example, we will build a fully generic light:

enum State<T>: Equatable where T: Equatable {
    case on
    case off
    case dimmed(T)
}

struct Bits8Dimming: Equatable {
    let value: Int
    init(_ value: Int) {
        assert(value > 0 && value < 256)
        self.value = value
    }
}

struct ZeroOneDimming: Equatable {
    let value: Double
    init(_ value: Double) {
        assert(value > 0 && value < 1)
        self.value = value
    }
}

let nostalgiaState: State<Bits8Dimming> = .dimmed(.init(10))
let otherState: State<ZeroOneDimming> = .dimmed(.init(0.4))

The dim type is now specified as a part of the State type. This gives us a lot of flexibility, as well as validation. Wrapping the value into a small struct adds very little overhead in terms of performance, and allows us to ensure that the values are sane before being set into our enum. 

Raw type enums

A raw type is a base type for all enumeration members; in our example, we can hardcode presets for our dimming, as follows:

enum LightLevel: String {
    case quarter
    case half
    case threequarters
}

let state: State<LightLevel> = .dimmed(.half)

Thanks to the generic implementation and the fact that String is equatable, we can use this raw value in our dimmed state.

With the LightLevel enum, which has a raw type of String, the compiler will use the member name as the underlying raw value:

LightLevel.half.rawValue == “half” // == true

You can override these by specifying them, as follows:

enum LightLevel: String {
    case quarter = “1/4”
    case half = “1/2”
    case threequarters = “3/4”
}

When using Int as a raw type, the underlying raw values will follow the order of the cases:

enum Level: Int {
    case base // == 0
    case more // == 1
    case high = 100
    case higher // == 101
}

Switching the state of light

With our final case, let's look at how to interpret the current state of the light:

switch state {
case .on:
    doSomething()
case .off:
    doSomething()
case .dimmed(let value):
    switch value {
    case .quarter:
        doSomething()
    case .half:
        doSomething()
    case .threeQuarters:
        doSomething()
    }
}

The switch statement in Swift is very different from the one in Objective-C. First, the cases do not fall through each other, so there's no need to add the break statement after each case.

If you want multiple cases to be handled with the same code, you can use the following strategy:

switch state {
case .on, .off:
    doSomething()
default:
    break
}

Falling through is somehow not encouraged in Swift, so always try to adapt your code in order not to leverage this. If you can't avoid it, the following code shows how it should be implemented:

switch state {
case .off:
    doSomethingOff()
    fallthrough
case .on:
    doSomething()
default:
    break
}

If state is off, both doSomethingOff and doSomething will be called. If state is on, only doSomething will be called.