Big idea here is to create a centralized place to receive and dispatch navigation events.
The advantage of having a main view controller like this is you have:
- one central place to receive and handle all navigation events
- you are are free to dispatch and navigate where ever you like
This demo creates a MainViewController sitting at the top of the application, which contains all the subviews the app is able to display (HomeViewController and ScanViewController). When the scan button is pressed, an event is fired up via the Responder Chain which the MainViewController is configured to handle. When it gets it, it programmatically selects the ScanViewController in the tab bar and updates the via.
So there are two challenges we need to overcome here:
- How are we going to get a message from a sub view controller all the way up to our
MainViewController. - Once we get than message, what are we going to do with it.
There are lots of ways we could send that message up to our parent view controller (protocol-delegate, closures, Notification Center), but the once I have chosen to use here is Responder chain. Why? Because the responder chain is built directly into every UIView. Every view is a UIResponder. So by simply setting a button target action to nil
button.addTarget(nil, action: .didTapScan, for: .primaryActionTriggered)I get a convenient way to send a message from way down in the application, all the way up to the top for free. No extra plumbing required. All I need to do is implement that action method in my MainViewController and I can intercept any Responder chain event from anywhere in the app (because MainViewController will always be a part of the hierarchy).
extension MainViewController: StarBucksResponder {
func didTapScan(sender: UIButton?) {
// remove currently displayed children VCs
let existingChildren = children
for child in existingChildren {
child.view.removeFromSuperview()
child.removeFromParent()
}
// add new one we want to present
sbTabBarController.selectedIndex = 1 // Scan
add(sbTabBarController)
}
}And once I get it, I am free to present to do whatever I like (in this case show the ScanViewController by programmatically selecting it in the tab bar).
AppDelegate.swift
import UIKit
@main
class AppDelegate: UIResponder, UIApplicationDelegate {
var window: UIWindow?
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
window = UIWindow(frame: UIScreen.main.bounds)
window?.makeKeyAndVisible()
window?.backgroundColor = .systemBackground
window?.rootViewController = MainViewController()
return true
}
}Here we simply start by displaying our MainViewController at startup.
MainViewController.swift
import Foundation
import UIKit
class MainViewController: UIViewController {
let sbTabBarController = UITabBarController()
override init(nibName nibNameOrNil: String?, bundle nibBundleOrNil: Bundle?) {
super.init(nibName: nil, bundle: nil)
setup()
present()
}
required init?(coder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}
extension MainViewController {
func setup() {
let homeVC = HomeViewController()
let scanVC = ScanViewController()
let homeNC = UINavigationController(rootViewController: homeVC)
let scanNC = UINavigationController(rootViewController: scanVC)
sbTabBarController.viewControllers = [homeNC, scanNC]
}
func present() {
// This is how you add child VC to parent - x3 things
addChild(sbTabBarController)
view.addSubview(sbTabBarController.view)
didMove(toParent: self)
}
}
// MARK: Responder chain events
extension MainViewController: StarBucksResponder {
func didTapScan(sender: UIButton?) {
// remove currently displayed children VCs
let existingChildren = children
for child in existingChildren {
child.view.removeFromSuperview()
child.removeFromParent()
}
// add new one we want to present
sbTabBarController.selectedIndex = 1 // Scan
add(sbTabBarController)
}
}The MainViewController is responsible for setting up all the views in the app. In this case via a tab bar.
StarBucksResponder.swift
import Foundation
import UIKit
/*
This is where we can capture all the responder chain events for our app.
*/
@objc protocol StarBucksResponder {
@objc optional func didTapScan(sender: UIButton?)
}If you have a lot of navigation events going on, it can be convenient to group them all in one place.
HomeViewController.swif
import Foundation
import UIKit
class HomeViewController: UIViewController {
let button = makeButton(withText: "Scan")
override init(nibName nibNameOrNil: String?, bundle nibBundleOrNil: Bundle?) {
super.init(nibName: nil, bundle: nil)
title = "Home"
view.backgroundColor = .systemRed
setTabBarImage(imageName: "house.fill", title: "Home")
}
required init?(coder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
override func loadView() {
super.loadView()
setup()
layout()
}
}
extension HomeViewController {
func setup() {
/*
If I wanted to handle this target action myself, would make `self` target
and handle like any other button action. Need to define selector here
*/
button.addTarget(self, action: #selector(performScanAction(sender:)), for: .primaryActionTriggered)
/*
But if I want to fire it up the responder chain, and let someone else handle it,
I can define a protocol and add it to selector via extension, and set target = `nil`.
In this case I want the `MainViewController` to intercept and handle this request
and present the `ScanViewController` as a result of this button press.
So I need to go implement this method there.
By leaving both commented in the event will be handled in both places.
*/
button.addTarget(nil, action: .didTapScan, for: .primaryActionTriggered)
}
func layout() {
view.addSubview(button)
NSLayoutConstraint.activate([
button.centerXAnchor.constraint(equalTo: view.centerXAnchor),
button.centerYAnchor.constraint(equalTo: view.centerYAnchor),
])
}
@objc func performScanAction(sender: Any?) {
print("I got this.")
}
}
// Syntactic sugar to make our code look nicer.
private extension Selector {
static let didTapScan = #selector(StarBucksResponder.didTapScan(sender:))
}This view controller doesn't do much. It simply displays a button, and when the button is pressed it fires an event up the responder chain by setting the target on the button to nil.
ScanViewController.swift
import Foundation
import UIKit
class ScanViewController: UIViewController {
override init(nibName nibNameOrNil: String?, bundle nibBundleOrNil: Bundle?) {
super.init(nibName: nil, bundle: nil)
title = "Scan"
view.backgroundColor = .systemIndigo
setTabBarImage(imageName: "qrcode", title: "Scan")
}
required init?(coder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}This view controller doesn't do anything.
ViewController+Ext.swift
import Foundation
import UIKit
extension UIViewController {
func setTabBarImage(imageName: String, title: String) {
let configuration = UIImage.SymbolConfiguration(scale: .large)
let image = UIImage(systemName: imageName, withConfiguration: configuration)
tabBarItem = UITabBarItem(title: title, image: image, tag: 0)
}
func add(_ child: UIViewController) {
addChild(child)
view.addSubview(child.view)
child.didMove(toParent: self)
}
func remove() {
guard parent != nil else {
return
}
willMove(toParent: nil)
view.removeFromSuperview()
removeFromParent()
}
}
func makeButton(withText text: String) -> UIButton {
let button = UIButton()
button.translatesAutoresizingMaskIntoConstraints = false
button.setTitle(text, for: .normal)
button.titleLabel?.adjustsFontSizeToFitWidth = true
button.contentEdgeInsets = UIEdgeInsets(top: 8, left: 16, bottom: 8, right: 16)
button.backgroundColor = .systemBlue
button.layer.cornerRadius = 8
return button
}Some extensions to help add and remove view controllers and make buttons and tab bar items.