statemachine.swift

class ViewControllerStandIn {
    var stateMachine: StateMachine<State>
    
    init(withSomething: Bool) {
        if withSomething {
            let delegate = FirstDelegate()
            self.stateMachine = StateMachine(withInitialState: .A, callback: delegate.didTransitionFrom(from:to:))
        } else {
            let delegate = SecondDelegate()
            self.stateMachine = StateMachine(withInitialState: .B, callback: delegate.didTransitionFrom(from:to:))
        }
    }
}

enum State {
    case A
    case B
    
    func toString() -> String {
        switch self {
        case .A:
            return "State A"
        case .B:
            return "State B"
        }
    }
}

class StateMachine<StateType> {
    private let callback: (StateType, StateType) -> Void
    
    var state: StateType {
        didSet {
            callback(oldValue, state)
        }
    }
    
    init(withInitialState initialState: StateType, callback: @escaping (_ oldState: StateType, _ newState: StateType) -> Void) {
        self.state = initialState
        self.callback = callback
    }
}

class FirstDelegate {
    func didTransitionFrom(from: State, to: State) {
        print("First Delegate going from " + from.toString() + " to " + to.toString())
    }
}

class SecondDelegate {
    func didTransitionFrom(from: State, to: State) {
        print("Second Delegate going from " + from.toString() + " to " + to.toString())
    }
}