// Reducers

func incReducer(state: ViewController.State) -> ViewController.State {
    var state = state
    state.number += 1
    return state
}

func decReducer(state: ViewController.State) -> ViewController.State {
    var state = state
    state.number -= 1
    return state
}

// Initialization

let vc = ViewController(nibName: "ViewController", bundle: nil)
vc.incHandler = incReducer
vc.decHandler = decReducer

// VC
class ViewController: UIViewController {

    struct State {
        var number: Int
    }

    var state = State(number: 0)

    @IBOutlet weak var numberLabel: UILabel!
    @IBOutlet weak var incButton: UIButton!
    @IBOutlet weak var decButton: UIButton!

    var incHandler: ((_ state: State) -> (State))!
    var decHandler: ((_ state: State) -> (State))!

    override func viewDidLoad() {
        super.viewDidLoad()
        render()
    }

    @IBAction func inc(_ sender: UIButton) {
        state = incHandler(state)
        render()
    }

    @IBAction func dec(_ sender: UIButton) {
        state = decHandler(state)
        render()
    }

    func render() {
        numberLabel.text = String(state.number)
    }
}