How would I pass data from a View Controller to a Container View?

Question

I'm trying to pass data from a ViewController to a Container within it. When I press the button the delegate sends the data to the container but the container resizes. How would I stop this from happening. I'm thinking a prepareForSegue function but I can't figure out how to implement it but I don't know if even that would be a solution.

protocol VCDelegate {

    func passData(theData1:String)

}

class ViewController: UIViewController {

    var delegate : VCDelegate?

    @IBAction func sendTextToContainer(_ sender: Any) {

        let ViewC = self.storyboard!.instantiateViewController(withIdentifier: "ViewController") as! ViewController

        let ContainerV = self.storyboard!.instantiateViewController(withIdentifier: "ContainerView") as! ContainerView

        self.present(ContainerV,animated:true,completion: nil)

        ViewC.delegate = ContainerV

        ViewC.delegate?.passData(theData1:"Hello")
    } 
}

class ContainerView: UIViewController, VCDelegate {

    func passData(theData1: String) {

        print(theData1)
        theText.text = theData1

    }

    @IBOutlet weak var theText: UILabel!

    override func viewWillAppear(_ animated: Bool) {

    }

}

Answer 1

You are instantiating a new, second instance of the child view controller. But if you created a “container” in IB, it's already been instantiated for you.

There are two ways for the parent view controller to pass data to the child. You can pass the initial data in prepare(for:sender:) :

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if let destination = segue.destination as? SecondViewControllerProtocol {
        destination.passData(string: "Initial value")
    }
}

If you want to later update it, you can grab the relevant children :

@IBAction func didTapButton(_ sender: Any) {
    for child in children {
        if let child = child as? SecondViewControllerProtocol {
            child.passData(string: "Updated value")
        }
    }
}

(You obviously could save the reference you captured during prepare(for:sender:) , too, if you wanted.)

The second view controller can then update its label accordingly:

protocol SecondViewControllerProtocol {
    func passData(string: String) 
}

class SecondViewController: UIViewController {
    @IBOutlet weak var label: UILabel!

    private var string: String?

    override func viewDidLoad() {
        super.viewDidLoad()

        // this is in case you passed the data before the label was hooked up

        label.text = string
    }
}

extension SecondViewController: SecondViewControllerProtocol {
    func passData(string: String) {
        self.string = string

        guard let label = label else { return }

        UIView.transition(with: label, duration: 0.25, options: .transitionCrossDissolve, animations: {
            label.text = string
        }, completion: nil)
    }
}

That yields: