What's the easiest way to pass data between two simultaneously displayed view controllers?

Question

I'm getting in a muddle trying to pass data between two viewcontrollers that are displayed on one screen via container views.

Minimal example below - top view (TopVC) has a textInput field. When I press the button, I want the label on the bottom view (BottomVC) to display the inputted text. Moreover, I want it to pass back a message to the TopVC and update the topVC label with the message "Successfully contacted bottom VC"

Storyboard set up

I have no idea now to reference the view controllers from each other basically.

class TopViewController: UIViewController {

    @IBOutlet weak var textInput: UITextField!
    @IBOutlet weak var textOutput: UILabel!

    @IBAction func go(_ sender: UIButton) {
        // ??????????? 
    }

    override func viewDidLoad() {  
        super.viewDidLoad()
        self.view.backgroundColor = UIColor.blue
    }

    func callMeBaby(greeting: String) {
        textOutput.text = greeting
    }
}

In the ????? placeholder, I want to put something that basically works like BottomVC.test(textInput.text, callmebaby) - but obviously I need to put in some extra code to 'introduce' the two ViewControllers, and I'm not sure what to do.

class BottomViewController: UIViewController {

    @IBOutlet weak var textLabel: UILabel!

    override func viewDidLoad() {
        super.viewDidLoad()
        self.view.backgroundColor = UIColor.yellow
    }

    func test(input: String, completion: (String) -> Void) {
        textLabel.text = input
        completion("Successfully contacted bottom VC")
    }
}

Answer 1

Creating Delegates

Start with creating delegates for both of your container ViewControllers. Don't forget to add : class. If you didn't do it, you wouldn't be able to create weak delegate variable:

protocol TopViewControllerDelegate: class {
    func sendMessage(_ string: String)
}
protocol BottomViewControllerDelegate: class {
    func sendMessage(_ string: String)
}

Now for every container ViewController create weak delegate variable

class TopViewController: UIViewController {
    weak var delegate: TopViewControllerDelegate?
    ...
}

class BottomViewController: UIViewController {
    weak var delegate: BottomViewControllerDelegate?
    ...
}

then for TopVC implement Bottom's delegate and for BottomVC Top's.

extension TopViewController: BottomViewControllerDelegate {
    func sendMessage(_ string: String) {
        // do something
    }
}
extension BottomViewController: TopViewControllerDelegate {
    func sendMessage(_ string: String) {
        // do something
    }
}

Assigning Delegates

Your segues between main ViewController and containers should have their own identifiers: EmbedTop, EmbedBottom.

So in your WrapperViewController create variable for your Top and Bottom ViewController and override method prepare(for:sender:) and inside assign these variables

class WrapperViewController: UIViewController {

    var topVC: TopViewController?
    var bottomVC: BottomViewController?

    ...

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if segue.identifier == "EmbedTop" {
            topVC = segue.destination as! TopViewController
        } else if segue.identifier == "EmbedBottom" {
            bottomVC = segue.destination as! BottomViewController
        }
    }

}

finally in viewDidAppear set delegate of TopVC's as BottomVC and of BottomVC's as TopVC

override func viewDidAppear(_ animated: Bool) {
    topVC.delegate = bottomVC
    bottomVC.delegate = topVC
}

Now your two ViewControllers can speak with each other! :-)

---

Example:

class BottomViewController: UIViewController {
    ...
    func speakToTop() {
        delegate?.sendMessage("Hi Top!")
    }
}