Swift `toggle()` extension for enums

CaseTogglable.swift

protocol CaseTogglable {}

extension CaseTogglable where Self: CaseIterable, Self: Equatable {
  mutating func toggle() {
    assert(Self.allCases.isEmpty == false, "CaseTogglable cannot be applied to an enum with no cases!")
    let index = Self.allCases.firstIndex(of: self)!
    let nextIndex = Self.allCases.index(after: index)
    if nextIndex < Self.allCases.endIndex {
      self = Self.allCases[nextIndex]
    } else {
      self = Self.allCases[Self.allCases.startIndex]
    }
  }

  func toggled() -> Self {
    var copy = self
    copy.toggle()
    return copy
  }
}

// MARK: - Example

enum MyEnum: CaseIterable, CaseTogglable {
  case one
  case two
  case three
}

var value = MyEnum.one

value.toggle() // → two
value.toggle() // → three
value.toggle() // → one

let toggled1 = value.toggled()  // → two
let toggled2 = toggled1.toggled()  // → three
let toggled3 = toggled2.toggled()  // → one

Check out CaseSwitchable.swift as well