How to Implement a 'Next' Property to a Caseiterable Enum in Swift

How to implement a 'next' property to a CaseIterable enum in Swift

You can extend CaseIterable constraining Self to Equatable. Then you just need to find the index after the firstIndex of your CaseItareble enumeration and return the element at that position. If the index is equal to the endIndex of all cases just return the first element.

extension CaseIterable where Self: Equatable {
    private var allCases: AllCases { Self.allCases }
    var next: Self {
        let index = allCases.index(after: allCases.firstIndex(of: self)!)
        guard index != allCases.endIndex else { return allCases.first! }
        return allCases[index]
    }
}

Another option is to constrain AllCases to BidirectionalCollection. This will allow you to get the last element of you enumeration, check if it is equal to self and return the first element without the need to iterate your whole collection:

extension CaseIterable where Self: Equatable, AllCases: BidirectionalCollection {
    var allCases: AllCases { Self.allCases }
    var next: Self {
        guard allCases.last != self else { return allCases.first! }
        return allCases[allCases.index(after: allCases.firstIndex(of: self)!)]
    }
}

expanding on CaseIterable next and previous properties:

extension CaseIterable {
    typealias Index = AllCases.Index
    var first: Self { allCases.first! }
    private var allCases: AllCases { Self.allCases }
    private static func index(after i: Index) -> Index { allCases.index(after: i) }
}

extension CaseIterable where AllCases: BidirectionalCollection {
    var last: Self { allCases.last! }
    private static func index(before i: Index) -> Index { allCases.index(before: i) }
}

extension CaseIterable where Self: Equatable {
    var index: Index { Self.firstIndex(of: self) }
    private static func firstIndex(of element: Self) -> Index { allCases.firstIndex(of: element)! }
}

extension CaseIterable where Self: Equatable, AllCases: BidirectionalCollection {
    var previous: Self { first == self ? last : allCases[Self.index(before: index)] }
    var next: Self { last == self ? first : allCases[Self.index(after: index)] }
}

Playground testing;

enum Enum: CaseIterable {
    case a,b,c
}

let value: Enum = .c

let next = value.next  // a
let next2 = next.next  // b
let next3 = next2.next // c

let previous = value.previous      // b
let previous2 = previous.previous  // a
let previous3 = previous2.previous // c

How to get next case of enum(i.e. write a circulating method) in Swift 4.2

Some problems with your approach are:

The Collection protocol does not define a last property.
In order to compare the elements with == they have to be Equatable.
Collection indices are not necessarily integers, they must be incremented
with index(after:).

This seems to be a working solution (tested with Xcode 10.0 beta 2):

extension CaseIterable where Self: Equatable {
    func next() -> Self {
        let all = Self.allCases
        let idx = all.firstIndex(of: self)!
        let next = all.index(after: idx)
        return all[next == all.endIndex ? all.startIndex : next]
    }
}

Example:

enum Direction: CaseIterable {
    case east, south, west, north
}

print(Direction.east.next()) // south
print(Direction.north.next()) // east

Remarks:

Only enumerations without associated values are CaseIterable, and
those are also Equatable (but the compiler does not figure out that
by itself). Therefore Self: Equatable is not a
real restriction.
Self.allCases can be used in Swift 4.2 to access the type property
from an instance method.
The forced unwrapping is safe because we know that the value is
an element of allCases.
Your enum Direction: CaseIterable compiles because the concrete
enum Direction type is Equatable, and its Direction.allCases is an Array – which has integer indices and a last property.

how to use an array as a case of an enum

Just create a static property:

enum Types: String, Codable, CaseIterable {
    case type1, type2, type3, type4, type5
    static let someCases: [Types] = [.type1, .type2, .type3] 
}

Types.someCases  // [type1, type2, type3]

Swift Increment through Enum

Update Starting with Swift 4.2 you can make use of the newly added support CaseIterable protocol, which adds compiler support for generating a list of all cases for an enum. Though @ninestones's comment pointed put that we are not guaranteed for allCases to return the cases in the same order as defined, the synthesized implementation does this, and it's unlikely that definition will change.

Your enum could then look something like this (no more hardcoded start value):

enum CopyState: CaseIterable {
    case binary, hex, both

    mutating func next() {
        let allCases = type(of: self).allCases
        self = allCases[(allCases.index(of: self)! + 1) % allCases.count]
    }
}

You can make this piece of functionality available to all CaseIterable enums:

extension CaseIterable where Self: Equatable {
    mutating func next() {
        let allCases = Self.allCases
        // just a sanity check, as the possibility of a enum case to not be
        // present in `allCases` is quite low
        guard let selfIndex = allCases.index(of: self) else { return }
        let nextIndex = Self.allCases.index(after: selfIndex)
        self = allCases[nextIndex == allCases.endIndex ? allCases.startIndex : nextIndex]
    }
}

enum CopyState: CaseIterable {
    case binary, hex, both
}

var state = CopyState.hex
state.next()
print(state) // both
state.next()
print(state) // binary

Or, a little bit more verbose, but with a better separation of concerns:

extension Collection {
    // adding support for computing indexes in a circular fashion
    func circularIndex(after i: Index) -> Index {
        let nextIndex = index(after: i)
        return nextIndex == endIndex ? startIndex : nextIndex
    }
}

extension Collection where Element: Equatable {
    // adding support for retrieving the next element in a circular fashion
    func circularElement(after element: Element) -> Element? {
        return index(of: element).map { self[circularIndex(after: $0)] }
    }
}

// Protocol to allow iterating in place (similar to a type conforming to both Sequence and IteratorProtocol)
protocol InPlaceIterable {
    mutating func next()
}

extension InPlaceIterable where Self: CaseIterable, Self: Equatable {
    // adding default implementation for enums
    mutating func next() {
        self = type(of: self).allCases.circularElement(after: self)!
    }
}

// now the enums need only the protocol conformances, they get the
// functionalities for free
enum CopyState: CaseIterable, InPlaceIterable {
    case binary, hex, both
}

You could use Int as raw value for your enum (note that this is also the default raw value if you don't specify it), and use it like this:

enum CopyState: Int {
    case binary, hex, both

    mutating func next(){
        self = CopyState(rawValue: rawValue + 1) ?? .binary
    }
}

var state = CopyState.hex
state.next()
print(state) // both
state.next()
print(state) // binary

This works fine as long as you have the raw values of the enum cases in consecutive order. By default the compiler assigns consecutive raw values.

You'd also need to keep in mind to update the next() method if the first case changes, otherwise it will no longer correctly work.

An alternative to the above limitation, suggested by @MartinR, is to force unwrap the raw value zero:

mutating func next(){
    self = CopyState(rawValue: rawValue + 1) ?? CopyState(rawValue: 0)!
}

The above code won't require updating the method when the first enum case changes, however it has the potential of crashing the app if the starting raw value of the enum changes.

iOS Swift, Enum CaseIterable extension

Not all enumeration types have an associated RawValue, and if they have then it is not necessarily a String.

Therefore you need to restrict the extension to enumeration types which are RawRepresentable, and define the return value as an array of RawValue:

extension CaseIterable where Self: RawRepresentable {

    static var allValues: [RawValue] {
        return allCases.map { $0.rawValue }
    }
}

Examples:

enum TypeOptions: String, CaseIterable {
    case all
    case article
    case show
    case unknown = "?"
}
print(TypeOptions.allValues) // ["all", "article", "show", "?" ]

enum IntOptions: Int, CaseIterable {
    case a = 1
    case b = 4
}
print(IntOptions.allValues) // [1, 4]

enum Foo: CaseIterable {
    case a
    case b
}
// This does not compile:
print(Foo.allValues) // error: Type 'Foo' does not conform to protocol 'RawRepresentable'

Extending a Protocol property to provide a default implementation in Swift

A possible solution is to locate the current value in the allCases collection,
and the return the next element (or wrap around to the first element):

public protocol CycleValue: CaseIterable, Equatable {
    var nextValue: Self { get }
}

public extension CycleValue {
    var nextValue: Self {
        var idx = Self.allCases.index(of: self)!
        Self.allCases.formIndex(after: &idx)
        return idx == Self.allCases.endIndex ? Self.allCases.first! : Self.allCases[idx]
    }
}

(Note that both forced unwraps are safe!)
Example:

public enum AppThemeAttributes: CycleValue  {
    case classic, darkMode // etc.
}

let a = AppThemeAttributes.classic
print(a) // classic
let b = a.nextValue
print(b) // darkMode
let c = b.nextValue
print(c) // classic

The protocol must conform to Equatable for this to compile, but that
is not a real restriction: A CaseIterable protocol cannot have
associated values, so that the compiler can always synthesize the
Equatable conformance.

Swift: Index of an element in enum (CaseIterable)

Couple of changes are necessary:

The return type needs to be Self.AllCases.Index? rather than Int?. In practice, these types will be equivalent, as seen below.
You also need to constrain any types to Equatable, because you need to be equatable in order to use firstIndex(of:). Again, in practice, any CaseIterable will usually be an enum without associated values, meaning it will be equatable automatically.
(Optional change) This function will never return nil, because you're finding one case in a CaseIterable. So you can remove the optionality on the return type (Self.AllCases.Index) and force unwrap.

Example:

public extension CaseIterable where Self: Equatable {

    public func ordinal() -> Self.AllCases.Index {
        return Self.allCases.firstIndex(of: self)!
    }

}

enum Example: CaseIterable {

    case x
    case y

}

Example.y.ordinal()  // 1
type(of: Example.y.ordinal()) // Int

Personally, I'd add that "Ordinal" usually means something different than what you're doing, and I'd recommend changing the function name to elementIndex() or something. But that's an aside.

How to enumerate an enum with String type?

Swift 4.2+

Starting with Swift 4.2 (with Xcode 10), just add protocol conformance to CaseIterable to benefit from allCases. To add this protocol conformance, you simply need to write somewhere:

extension Suit: CaseIterable {}

If the enum is your own, you may specify the conformance directly in the declaration:

enum Suit: String, CaseIterable { case spades = "♠"; case hearts = "♥"; case diamonds = "♦"; case clubs = "♣" }

Then the following code will print all possible values:

Suit.allCases.forEach {
    print($0.rawValue)
}

Compatibility with earlier Swift versions (3.x and 4.x)

If you need to support Swift 3.x or 4.0, you may mimic the Swift 4.2 implementation by adding the following code:

#if !swift(>=4.2)
public protocol CaseIterable {
    associatedtype AllCases: Collection where AllCases.Element == Self
    static var allCases: AllCases { get }
}
extension CaseIterable where Self: Hashable {
    static var allCases: [Self] {
        return [Self](AnySequence { () -> AnyIterator<Self> in
            var raw = 0
            var first: Self?
            return AnyIterator {
                let current = withUnsafeBytes(of: &raw) { $0.load(as: Self.self) }
                if raw == 0 {
                    first = current
                } else if current == first {
                    return nil
                }
                raw += 1
                return current
            }
        })
    }
}
#endif

How to Implement a 'Next' Property to a Caseiterable Enum in Swift