Swift StateMachine

StateMachine.swift

protocol StateMachine {
    typealias State
    
    var state: State { get }
    
    func canTransitionToState(newState: State) -> Bool
    func transitionToState(newState: State) // throws,  in Swift 2
}

Transmission.swift

class Transmission : StateMachine {
    typealias State = TransmissionState
    
    internal private(set) var state: State = .Parked
    
    internal func canTransitionToState(newState: State) -> Bool {
        switch newState {
        case .Parked, .Drive, .Reverse:
            return state == .Neutral
        case .Neutral:
            return state != .Neutral
        }
    }
    
    internal func transitionToState(newState: State) {
        precondition(canTransitionToState(newState), "Unable to transition from \(state) to \(newState)")
        state = newState
    }
}