Experimenting with using variable enums as state machines, inspired by http://github.com/schwa/SwiftStateMachine

turnstile.swift

// Playground - noun: a place where people can play

import UIKit

enum Turnstile {
    case Locked
    case Unlocked
    
    mutating func push() -> Bool {
        switch self {
        case .Unlocked:
            self = .Locked
            return true
        default:
            return false
        }
    }
    
    mutating func coin() -> Bool {
        switch self {
        case .Locked:
            self = .Unlocked
            return true
        default:
            return false
        }
    }
}

class SubwayStation {
    var turnstile: Turnstile = .Locked {
        willSet {
            switch (turnstile, newValue) {
            case (.Unlocked, .Locked):
                println("Turnstile is locking!")
            case (.Locked, .Unlocked):
                println("Turnstile is unlocking!")
            default:
                break
            }
        }
    }
    
    func enter() {
        if turnstile.push() {
            println("Entered the station!")
        }
        else {
            println("Have to pay first!")
        }
    }
    
    func pay() {
        if turnstile.coin() {
            println("Thank you for your patronage!")
        }
        else {
            println("Fare already paid! Please enter.")
        }
    }
}

var MontgomeryStreet = SubwayStation()

MontgomeryStreet.enter()
// => Have to pay first!

MontgomeryStreet.pay()
// => Turnstile is unlocking!
//    Thank you for your patronage!

MontgomeryStreet.enter()
// => Turnstile is locking!
//    Entered the station!

MontgomeryStreet.pay()
// => Turnstile is unlocking!
//    Thank you for your patronage!

MontgomeryStreet.pay()
// => Fare already paid! Please enter.

MontgomeryStreet.pay()
// => Fare already paid! Please enter.

MontgomeryStreet.enter()
// => Turnstile is locking!
//    Entered the station!