How to Get Next Case of Enum(I.E. Write a Circulating Method) in Swift 4.2

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.

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.

Swift: Index of an element in enum (CaseIterable)

Couple of changes are necessary:

1. The return type needs to be Self.AllCases.Index? rather than Int?. In practice, these types will be equivalent, as seen below.
2. 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.
3. (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 do you check elegantly case enum in the .first(where:) closure in Swift?

first won't do anything useful. I think you want contains.

items.contains { Item.foo ~= $0 }

Regardless, you'll need this:

/// Match `enum` cases with associated values, while disregarding the values themselves.
/// - Parameter case: Looks like `Enum.case`.
public func ~= <Enum: Equatable, AssociatedValue>(
  case: (AssociatedValue) -> Enum,
  instance: Enum
) -> Bool {
  Mirror.associatedValue(of: instance, ifCase: `case`) != nil
}

/// Match `enum` cases with associated values, while disregarding the values themselves.
/// - Parameter case: Looks like `Enum.case`.
public func ~= <Enum, AssociatedValue>(
  case: (AssociatedValue) -> Enum,
  instance: Enum
) -> Bool {
  Mirror.associatedValue(of: instance, ifCase: `case`) != nil
}

/// Match non-`Equatable` `enum` cases without associated values.
public func ~= <Enum>(pattern: Enum, instance: Enum) -> Bool {
  guard (
    [pattern, instance].allSatisfy {
      let mirror = Mirror(reflecting: $0)
      return
        mirror.displayStyle == .enum
        && mirror.children.isEmpty
    }
  ) else { return false }

  return .equate(pattern, to: instance) { "\($0)" }
}

public extension Mirror {
  /// Get an `enum` case's `associatedValue`.
  static func associatedValue<AssociatedValue>(
    of subject: Any,
    _: AssociatedValue.Type = AssociatedValue.self
  ) -> AssociatedValue? {
    guard let childValue = Self(reflecting: subject).children.first?.value
    else { return nil }

    if let associatedValue = childValue as? AssociatedValue {
      return associatedValue
    }

    let labeledAssociatedValue = Self(reflecting: childValue).children.first
    return labeledAssociatedValue?.value as? AssociatedValue
  }

  /// Get an `enum` case's `associatedValue`.
  /// - Parameter case: Looks like `Enum.case`.
  static func associatedValue<Enum: Equatable, AssociatedValue>(
    of instance: Enum,
    ifCase case: (AssociatedValue) throws -> Enum
  ) rethrows -> AssociatedValue? {
    try associatedValue(of: instance)
      .filter { try `case`($0) == instance }
  }

  /// Get an `enum` case's `associatedValue`.
  /// - Parameter case: Looks like `Enum.case`.
  static func associatedValue<Enum, AssociatedValue>(
    of instance: Enum,
    ifCase case: (AssociatedValue) throws -> Enum
  ) rethrows -> AssociatedValue? {
    try associatedValue(of: instance).filter {
      .equate(try `case`($0), to: instance) {
        Self(reflecting: $0).children.first?.label
      }
    }
  }
}

public extension Optional {
  /// Transform `.some` into `.none`, if a condition fails.
  /// - Parameters:
  ///   - isSome: The condition that will result in `nil`, when evaluated to `false`.
  func filter(_ isSome: (Wrapped) throws -> Bool) rethrows -> Self {
    try flatMap { try isSome($0) ? $0 : nil }
  }
}

public extension Equatable {
  /// Equate two values using a closure.
   static func equate<Wrapped, Equatable: Swift.Equatable>(
    _ optional0: Wrapped?, to optional1: Wrapped?,
    using transform: (Wrapped) throws -> Equatable
  ) rethrows -> Bool {
    try optional0.map(transform) == optional1.map(transform)
  }
}

How can I get similar print result of NSLog for int enum in swift?

print uses String.init(describing:) under the hood to convert whatever you give it to a String, so you can do that too:

NSLog("%@", String(describing: type))

But really though, the enum should conform to CustomStringConvertible:

enum ImportType: Int, CustomStringConvertible {
    
    case First = 1
    case None
    case Original
    
    var description: String {
        switch self {
        case .First:
            return "First"
        case .None:
            return "None"
        case .Original:
            return "Original"
        }
    }
}

and you should not rely on this default behaviour of String(describing:), because its behaviour is not specified unless the type conforms to TextOutputStreamable, CustomStringConvertible or CustomDebugStringConvertible. See here for more info.

Swift's enum with one case causes BAD_ACCESS at runtime

This is definitely a bug with the compiler. Oddly enough, adding a second case to your enum works perfectly:

enum OneCaseEnum {
    case X
    case Y
}

class OneEnumWrapper {
    var myVar: OneCaseEnum

    init(v: OneCaseEnum)
    {
        self.myVar = v
    }
}

let e = OneCaseEnum.X
print(e)
let x = OneEnumWrapper(v: e) // no crash
print(x)

You can use this as a workaround until this crash is identified and fixed.

rdar://25314388

SR-1035

Sample project

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