An implementation of view controller to choose and present a view conditionally.

ConditionalPresenter.swift

import UIKit

public protocol ConditionalPresenter: class {
    var view: UIView! { get }
    func addChildViewController(childController: UIViewController)

    var current: UIViewController! { get set }
    func generateController() -> UIViewController
}

extension ConditionalPresenter {
    public func updateController() {
        discardCurrent()
        updateCurrent()
    }

    private func discardCurrent() {
        current?.removeFromParentViewController()
        current?.view.removeFromSuperview()
        current = nil
    }

    public func updateFrame() {
        current.view.frame = view.frame
    }

    private func updateCurrent() {
        current = generateController()
        addChildViewController(current)
        view.addSubview(current.view)
        updateFrame()
    }
}

public class ConditionalController: UIViewController, ConditionalPresenter {
    public var current: UIViewController!

    public override func viewWillAppear(animated: Bool) {
        super.viewWillAppear(animated)

        updateController()
    }

    public override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()

        updateFrame()
    }

    public func generateController() -> UIViewController {
        fatalError("Not implemented.")
    }
}