One Navigator to Rule them All

A technical talk on moving iOS navigation out of view controllers and into a separate class.

Transcript

1. One Navigator to Rule them All Sommer Panage (@sommer)
4. View Controller A probably has something like this: override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) { let detailData = tableData[indexPath.row] let detailVC = ViewControllerB() detailVC.data = detailData navigationController?.pushViewController(detailVC, animated: true) }
5. View Controller B probably has something like this: init() { // Calls to super, etc. navigationItem.rightBarButtonItem = UIBarButtonItem(title: "Go to C!", style: .plain, target: self, action: #selector(didTapGoButton)) } func didTapGoButton { let nextVC = ViewControllerC() navigationController?.pushViewController(nextVC, animated: true) }
6. What's wrong with that?
7. The omnipotent VC let detailVC = ViewControllerB() detailVC.data = detailData navigationController?.pushViewController(detailVC,animated: true) Our VC is acting like: • Parent • Child • Boss
8. The Implicit Navigation Controller navigationItem.rightBarButtonItem = UIBarButtonItem(title: "Go to C!", style: .plain, target: self, action: #selector(didTapGoButton)) If there's no nav bar, there's no "Go" button!
10. View Controller A might have something like this: override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) { let detailData = tableData[indexPath.row] switch detailData.type { case .b: let bDetailVC = BViewController() bDetailVC.data = detailData navigationController?.pushViewController(bDetailVC, animated: true) case .d: let cDetailVC = DViewController() cDetailVC.data = detailData navigationController?.pushViewController(cDetailVC, animated: true) case .e: let eDetailVC = EViewController() eDetailVC.data = detailData navigationController?.pushViewController(eDetailVC, animated: true) } }
11. More navigation → More code bloat!
12. What are the problems? • VCs are not independent • VCs depend on having parents that can do certain things • VCs are doing too much (flow, presentation, etc.)
13. Solution v1 Delegation
14. Who knows whom??
15. The idea: Use delegation to have a Master View Controller do all the work
16. Who knows whom??
17. How does it work?
18. How does it work? • Each view controller that requires navigation has a delegate • Our root View Controller is MasterNavigationViewController: UINavigationController • MasterNavigationViewController configures every VC and is its delegate
19. Our View Controllers protocol MovieListViewControllerDelegate: class { func didSelectFilm(with id: ID) } final class MovieListViewController: UIViewController { weak let delegate: MovieListViewControllerDelegate? init(delegate: MovieListViewControllerDelegate) { self.delegate = delegate super.init(nibName: nil, bundle: nil) } func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) { tableView.deselectRow(at: indexPath, animated: true) delegate.didSelectFilm(with: data[indexPath.row].id) } }
20. Our View Controllers protocol MovieDetailViewControllerDelegate: class { func didSelectCharacter(with id: ID) } final class MovieDetailViewController: UIViewController { weak let delegate: MovieDetailViewControllerDelegate? init(delegate: MovieDetailViewControllerDelegate) { self.delegate = delegate super.init(nibName: nil, bundle: nil) } }
21. Our View Controllers protocol CharacterViewControllerDelegate: class { func didSelectMovie(with id: ID) func didSelectStarship(with id: ID) } final class CharacterViewController: UIViewController { weak let delegate: CharacterViewControllerDelegate? init(delegate: CharacterViewControllerDelegate) { self.delegate = delegate super.init(nibName: nil, bundle: nil) } }
22. The MasterNavigationViewController final class MasterNavigationViewController: UINavigationController { } extension MasterNavigationViewController: MovieListViewControllerDelegate { func didSelectFilm(with id: ID) { let detailVC = MovieDetailViewController(delegate: self) pushViewController(detailVC, animated: true) } }
23. The MasterNavigationViewController extension MasterNavigationViewController: MovieDetailViewControllerDelegate { func didSelectCharacter(with id: ID) { /* Configure & Present CharacterVC */ } func didSelectPlanet(with id: ID) { /* Configure & Present PlanetVC */ } }
24. What does this fix? • Individual VCs don't control flow/presentation • Individual VCs don't need to know about their parents • Individual VCs do not create other non-child VCs
25. What isn't so great? • Doing all our magic in a view controller • Code duplication and/or implicit dependencies between delegates • Generally lots of boilerplate; everything has a navigation delegate!
26. MasterNavigationViewController is still a view controller • As the app grows, this top level VC will likely house other logic control logic • Still tied to all the VC lifetime magic of UIKit • The VC delegate can easily get bloated with all kinds of tasks, not just nav
27. Code duplication
28. Code duplication protocol MovieListViewControllerDelegate: class { func didSelectFilm(with id: ID) } protocol CharacterViewControllerDelegate: class { func didSelectMovie(with id: ID) }
29. Code duplication extension MasterNavigationViewController: CharacterViewControllerDelegate { func didSelectMovie(with id: ID) { let detailVC = MovieDetailViewController(delegate: self) pushViewController(detailVC, animated: true) } } extension MasterNavigationViewController: MovieListViewControllerDelegate { func didSelectFilm(with id: ID) { let detailVC = MovieDetailViewController(delegate: self) pushViewController(detailVC, animated: true) } }
30. Code duplication protocol CharacterViewControllerDelegate: class { func didSelectFilm(with id: ID) } extension MasterNavigationViewController: CharacterViewControllerDelegate { // didSelectFilm handled by MovieListViewControllerDelegate implementation } extension MasterNavigationViewController: MovieListViewControllerDelegate { func didSelectFilm(with id: ID) { let detailVC = MovieDetailViewController(delegate: self) pushViewController(detailVC, animated: true) } }
31. Code duplication protocol CharacterViewControllerDelegate: class { func didSelectFilm(with id: ID) } extension MasterNavigationViewController: CharacterViewControllerDelegate { // didSelectFilm handled by MovieListViewControllerDelegate implementation } extension MasterNavigationViewController: MovieListViewControllerDelegate { func didSelectFilm(with id: ID) { let crawlVC = MovieCrawlViewController(delegate: self) pushViewController(crawlVC, animated: true) } }
32. So much boilerplate • Every view controller that can navigate must now have a delegate. • Boilerplate grows as a function of how much navigation is possible.
33. Solution v2 The Navigator
34. What's so great about the Navigator? • A standalone object • Not tied to View Controller life cycle or responsibilites • We can use Navigator Protocols to not have to repeat boilerplate... • ...and to group functionality
35. How does it work? • Navigator is a base protocol • All navigator protocols inherit form Navigator • Navigator protocols are grouped by functionality, not per VC
36. Protocols are functionality based, not VC based protocol Navigator { // Any navigation functionalities we want available to all VCs can live here } protocol MovieNavigator: Navigator { func viewController(_ viewController: UIViewController, didSelectMovie id: ID) } protocol CharacterNavigator: Navigator { func viewController(_ viewController: UIViewController, didSelectCharacter id: ID) } protocol StarshipNavigator: Navigator { func viewController(_ viewController: UIViewController, didSelectStarship id: ID) }
37. Each VC declares which functionality it needs by selecting navigators final class MoviesListViewController: UIViewController { typealias MyNavigator = MovieNavigator let navigator: MyNavigator init(navigator: MyNavigator) { self.navigator = navigator super.init(nibName: nil, bundle: nil) } } final class CharacterViewController: UIViewController { typealias MyNavigator = MovieNavigator & StarshipNavigator let navigator: MyNavigator init(navigator: MyNavigator) { self.navigator = navigator super.init(nibName: nil, bundle: nil) } }
38. Adding functionality is as simple as requiring another navigator final class CharacterViewController: UIViewController { typealias MyNavigator = MovieNavigator & StarshipNavigator let navigator: MyNavigator init(navigator: MyNavigator) { self.navigator = navigator super.init(nibName: nil, bundle: nil) } }
39. Adding functionality is as simple as requiring another navigator final class CharacterViewController: UIViewController { typealias MyNavigator = MovieNavigator & StarshipNavigator & ErrorNavigator let navigator: MyNavigator init(navigator: MyNavigator) { self.navigator = navigator super.init(nibName: nil, bundle: nil) } }
40. Implementing the AppNavigator • The AppNavigator implements all Navigator protocols in extensions • The AppDelegate creates and runs the AppNavigator • AppNavigator sets up your rootVC and keeps a reference to it • AppNavigator has one job; manage app wide navigation • AppNavigator is not a view controller, just a class
41. The AppNavigator takes control navigation final class AppNavigator: Navigator { fileprivate let navigationController: UINavigationController init(navigationController: UINavigationController) { self.navigationController = navigationController // configuration if navigationController goes here } func run() { let listVC = MoviesListViewController(navigator: self) navigationController.setViewControllers([listVC], animated: false) } }
42. The AppNavigator implements all Navigator protocols extension AppNavigator: MovieNavigator { func viewController(_ viewController: UIViewController, didSelectMovie id: ID) { let movieVC = MovieDetailViewController(navigator: self) navigationController.pushViewController(movieVC, animated: true) } } extension AppNavigator: CharacterNavigator { func viewController(_ viewController: UIViewController, didSelectCharacter id: ID) { let characterVC = CharacterViewController(navigator: self) navigationController.pushViewController(characterVC, animated: true) } } extension AppNavigator: StarshipNavigator { func viewController(_ viewController: UIViewController, didSelectStarship id: ID) { let shipVC = StarshipViewController(navigator: self) navigationController.pushViewController(shipVC, animated: true) } }
43. We can use the viewController param to differentiate different flows when needed extension AppNavigator: MovieNavigator { func viewController(_ viewController: UIViewController, didSelectMovie id: ID) { let movieVC = MovieDetailViewController(navigator: self) // Decomp this out of course if viewController is CharacterViewController { movieVC.navigationItem.rightBarButtonItem = UIBarButtonItem( title: "...", style: .plain, target: self, action: #selector(didTapMore)) } navigationController.pushViewController(movieVC, animated: true) } }
44. Did we solve our problems? • By defining our navigator protocols by functionality rather than by VC, we've avoided lots of boilerplate... • ...and code duplication. • We are no longer coupled to the VC lifecycle.
45. Benefits beyond our iPhone app right now • Apple TV? iPad? • "The big rewrite" • Move fast and don't break things
46. Adding complexity • Top level AppNavigator can have sub-navigators. • Each layer of navigation can be managed by its own navigator.
47. Adding a layer movieListVC.navigationItem.rightBarButtonItem = UIBarButtonItem(title: "...", style: .plain, target: self, action: #selector(didTapMore))
48. A stack of subnavigators @objc private func didTapMore() { let crawlNavVC = UINavigationController() let newNavigator = CrawlNavigator(crawlNavVC, api: api) { [unowned self] in crawlNavVC.dismiss(animated: true, completion: nil) self.removeNavigator(crawlNav) } addNavigator(newNavigator) newNavigator.run() navigationController.present(navVC, animated: true, completion: nil) }
49. Hat tip to the Coordinator Pattern • Coordinator handles navigation and model mutation • Delegation is per VC
50. Summary • View Controllers should be reusable and reorderable easily • View Controllers should avoid any assumptions about where they live in the app • Navigators capture the "flow" of these individual entities • Navigators allow us to separate the what from the how
51. Thank you! Contact me: @Sommer on Twitter Thanks to... • www.swapi.com (i.e. the Star Wars API) • iOS Conf SG