Example of how to use a protocol where you might normally use a superclass. This is also more flexible because you can mix it into anything, even if you don't have control of the superclass.

protocol-instead-of-subclass.swift

import UIKit
import XCPlayground

// MARK: Implementation
protocol Page {
    var name: String { get }
    var details: String { get }
    var asViewController: UIViewController { get }
}

extension Page where Self: UIViewController {
    var asViewController: UIViewController {
        return self
    }
}

// MARK: Protocol Conformance

final class ViewController: UIViewController, Page {
    let name: String = "View Controller"
    let details: String = "A regular view controller."
}

final class TableViewController: UITableViewController, Page {
    let name: String = "Table View Controller"
    let details: String = "A table view controller."
}

// MARK: Usage

final class MainViewController: UIViewController {
    let stackView = UIStackView()
    let nameLabel = UILabel()
    let detailsLabel = UILabel()

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

    func loadPage(page: Page) {
        nameLabel.text = page.name
        detailsLabel.text = page.details
        let vc = page.asViewController
        addChildViewController(vc)
        stackView.insertArrangedSubview(vc.view, atIndex: 2)
        vc.didMoveToParentViewController(self)
    }

    private func setupViews() {
        view.backgroundColor = .whiteColor()
        nameLabel.font = .boldSystemFontOfSize(32)
        detailsLabel.font = .systemFontOfSize(20)
        stackView.axis = .Vertical
        stackView.frame = view.frame
        stackView.addArrangedSubview(nameLabel)
        stackView.addArrangedSubview(detailsLabel)
        view.addSubview(stackView)
    }
}

let vc = MainViewController()
vc.view.frame = CGRect(x: 0, y: 0, width: 375, height: 667)
XCPlaygroundPage.currentPage.liveView = vc.view

let regularVC = ViewController()
let tableVC = TableViewController()
// We can pass in the regular view controller, or the table view controller to `loadPage`, which takes the `Page` type.
// Via the protocol, we've also enforced that it can be turned into a view controller, without having to do anything extra
// on the types that implement it.
 vc.loadPage(regularVC)