A Simple Swift State Machine

StateMachine.swift

import Foundation

class StateMachine<P:StateMachineDelegateProtocol>{
  private unowned let delegate:P

  private var _state:P.StateType{
    didSet{
      delegate.didTransitionFrom(oldValue, to:_state)
    }
  }

  var state:P.StateType{
    get{
      return _state
    }
    set{ //Can't be an observer because we need the option to CONDITIONALLY set state
      delegateTransitionTo(newValue)
    }
  }
  
  
  init(initialState:P.StateType, delegate:P){
    _state = initialState //set the primitive to avoid calling the delegate.
    self.delegate = delegate
  }
  
  
  private func delegateTransitionTo(to:P.StateType){
    switch delegate.shouldTransitionFrom(_state, to:to){
    case .Continue:
      _state = to
    case .Redirect(let newState):
      _state = to
      state = newState
    case .Abort:
      break;
    }
  }
}



protocol StateMachineDelegateProtocol: class{
  typealias StateType
  func shouldTransitionFrom(from:StateType, to:StateType)->Should<StateType>
  func didTransitionFrom(from:StateType, to:StateType)
}



enum Should<T>{
  case Continue, Abort, Redirect(T)
}

~Example.swift

import UIKit

class Example : UIView{
  private var machine:StateMachine<Example>!
  enum TrafficLight{
    case Stop, Go, Caution
  }

  
  required init(coder aDecoder: NSCoder) {
    super.init(coder: aDecoder)
    machine = StateMachine(initialState: .Stop, delegate: self)
  }

  
  @IBAction func tappedGo(sender:AnyObject){
    machine.state = .Go
  }
  
  
  @IBAction func tappedCaution(sender:AnyObject){
    machine.state = .Caution
  }
}



extension Example : StateMachineDelegateProtocol{
  typealias StateType = TrafficLight

  
  func shouldTransitionFrom(from: StateType, to: StateType) -> Should<StateType> {
    switch (from, to){
    case (.Stop, .Go), (.Caution, .Stop):
      return .Continue
    case (.Go, .Caution):
      return .Redirect(.Stop)
    default:
      return .Abort
    }
  }
  
  
  func didTransitionFrom(from: StateType, to: StateType) {
    switch to{
    case .Stop:
      backgroundColor = UIColor.redColor()
    case .Go:
      backgroundColor = UIColor.greenColor()
    case .Caution:
      backgroundColor = UIColor.yellowColor()
    }
  }
}