Sundell's Swift

Iterating through enum members

2015-05-21

Sometimes you want to perform a sequence of operations for each member of an enum. Say you have the following:

enum Profession: Int {
    case Engineer
    case Designer
    case Tester
}

Now, say you want to list all the professions available; loop through each member and turn it into a string, like this:

Engineer
Designer
Tester

What you could do, is add a static method to Profession that returns all its members, like this:

enum Profession: Int {
    case Engineer
    case Designer
    case Tester

    static func allProfessions() -> [Profession] {
        return [.Engineer, .Designer, .Tester]
    }
}

This, while simple, has the big downside of being error-prone each time a new member is added. There’s no way to enforce at compile time that allProfessions() actually returns all possible values.

Introducing EnumIterator

I chose to solve this by creating a class - EnumIterator that loops through all the members of en enum that conforms to IterableEnum by constantly incrementing an integer, and stopping whenever that integer cannot be converted into a member of the enum in question. Like this:

protocol IterableEnum {
    var rawValue: Int { get }
    init?(rawValue: Int)
}

class EnumIterator<T: IterableEnum> {
    class func iterate(forEachCase: T -> Void) {
        var currentRawValue = 0

        while let enumCase = T(rawValue: currentRawValue) {
            forEachCase(enumCase)
            currentRawValue++
        }
    }
}

This is nice, because it doesn’t require the enum that you want to iterate through to implement any additional methods, it simply has to be an Int type enum.

Let’s give it a spin:

EnumIterator<Profession>.iterate() {
    switch $0 {
        case .Engineer:
            println("Engineer")
        case .Designer:
            println("Designer")
        case .Tester:
            println("Tester")
    }
}

And we get the result we were looking for!

But, what happens if the Enum is not linear?

One problem with the current implementation of EnumIterator is that it will stop mid-way if the enum is not exactly linear. Say we add the following to Profession:

enum Profession: Int, IterableEnum {
    case Engineer
    case Designer
    case Tester
    case TeamLeader = 9
}

Now, we still get the same output as before, that’s because our EnumIterator will attempt to initialize Profession with the raw integer value of 3, and fail - because our 4th member has a raw value of 9 - and thus stop. Not cool.

But, we can fix this! Let’s break out the sequencing (i++) logic from the iterate() method, and add a new method that lets our API user override the default sequencing:

class EnumIterator<T: IterableEnum> {
    class func iterate(forEachCase: T -> Void) {
        self.iterateWithSequenceOverride({ Int in
            return nil
        }, forEachCase: forEachCase)
    }

    class func iterateWithSequenceOverride(sequenceOverride: Int -> T?, forEachCase: T -> Void) {
        var currentRawValue = 0

        while true {
            if let enumCase = sequenceOverride(currentRawValue) {
                forEachCase(enumCase)
                currentRawValue = enumCase.rawValue + 1
                continue
            } else if let enumCase = T(rawValue: currentRawValue) {
                forEachCase(enumCase)
                currentRawValue++
                continue
            }

            return
        }
    }
}

Our API user can now choose to return a custom value T for a given index. Let’s implement it for Profession:

enum Profession: Int, IterableEnum {
    case Engineer
    case Designer
    case Tester
    case TeamLeader = 9

    static func iterationSequenceOverride() -> (Int -> Profession?) {
        return {
            if $0 == 3 {
                return .TeamLeader
            }

            return nil
        }
    }
}

Now, we will get the correct expected output by calling:

EnumIterator<Profession>.iterateWithSequenceOverride(Profession.iterationSequenceOverride()) {
    switch $0 {
    case .Engineer:
        println("Engineer")
    case .Designer:
        println("Designer")
    case .Tester:
        println("Tester")
    case .TeamLeader:
        println("TeamLeader")
    }
}

That’s it! We can now iterate through both linear and non-linear Int based enums with ease!

Documented, final implementation, as part of my SwiftKit library:
SwiftKit/EnumIterator on GitHub