Trying to model a sequence of view controllers.

ViewControllerSequence.swift

struct ViewControllerSequence {
    typealias ViewControllerProducer = Void -> UIViewController

    private var storage = TypeDictionary<ViewControllerProducer>()

    private var producers: [ViewControllerProducer]

    init(_ producers: [ViewControllerProducer]) {
        self.producers = producers
    }

    mutating func next(previous: UIViewController? = nil) -> UIViewController? {
        if let previous = previous, producer = storage.get(previous.dynamicType) {
            return producer()
        }
        else {
            return produceAndCacheNextViewController()
        }
    }

    mutating func produceAndCacheNextViewController() -> UIViewController? {
        guard !producers.isEmpty else { return nil }

        let producer = producers.removeFirst()

        let controller = producer()

        if let nextProducer = producers.first {
            storage.set(controller.dynamicType, value: nextProducer)
        }

        return controller
    }
}

struct TypeDictionary<T> {
    private var storage = [Int: T]()

    mutating func set(type: AnyClass, value: T) {
        storage[hashFromType(type)] = value
    }

    func get(type: AnyClass) -> T? {
        return storage[hashFromType(type)]
    }

    private func hashFromType(type: AnyClass) -> Int {
        return "\(type)".hashValue
    }
}

private lazy var viewControllerSequence: ViewControllerSequence = ViewControllerSequence([
    { [unowned self] in PhoneNumberFormViewController(delegate: self) },
    { [unowned self] in VerifyNumberFormViewController(delegate: self) },
    { [unowned self] in SignUpFormViewController(delegate: self) }
])

private func showNextViewController(previousViewController: UIViewController) {
    if let nextViewController = viewControllerSequence.next(previousViewController) {
        previousViewController.showViewController(nextViewController, sender: nil)
    }
    else {
        // The flow is complete!
    }
}