How do you share data between view controllers and

Question

Say I have multiple view controllers in my Swift app and I want to be able to pass data between them. If I'm several levels down in a view controller stack, how do I pass data to another view controller? Or between tabs in a tab bar view controller?

(Note, this question is a "ringer".) It gets asked so much that I decided to write a tutorial on the subject. See my answer below.

Answer 1

Swift 4

There are so many approaches for data passing in swift. Here I am adding some of the best approaches of it.

1) Using StoryBoard Segue

Storyboard segues are very much useful for passing data in between Source and Destination View Controllers and vice versa also.

// If you want to pass data from ViewControllerB to ViewControllerA while user tap on back button of ViewControllerB.
        @IBAction func unWindSeague (_ sender : UIStoryboardSegue) {
            if sender.source is ViewControllerB  {
                if let _ = sender.source as? ViewControllerB {
                    self.textLabel.text = "Came from B = B->A , B exited"
                }
            }
        }

// If you want to send data from ViewControllerA to ViewControllerB
        override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
            if  segue.destination is ViewControllerB {
                if let vc = segue.destination as? ViewControllerB {
                    vc.dataStr = "Comming from A View Controller"
                }
            }
        }

2) Using Delegate Methods

ViewControllerD

//Make the Delegate protocol in Child View Controller (Make the protocol in Class from You want to Send Data)
    protocol  SendDataFromDelegate {
        func sendData(data : String)
    }

    import UIKit

    class ViewControllerD: UIViewController {

        @IBOutlet weak var textLabelD: UILabel!

        var delegate : SendDataFromDelegate?  //Create Delegate Variable for Registering it to pass the data

        override func viewDidLoad() {
            super.viewDidLoad()
            // Do any additional setup after loading the view.
            textLabelD.text = "Child View Controller"
        }

        @IBAction func btnDismissTapped (_ sender : UIButton) {
            textLabelD.text = "Data Sent Successfully to View Controller C using Delegate Approach"
            self.delegate?.sendData(data:textLabelD.text! )
            _ = self.dismiss(animated: true, completion:nil)
        }
    }

ViewControllerC

    import UIKit

    class ViewControllerC: UIViewController , SendDataFromDelegate {

        @IBOutlet weak var textLabelC: UILabel!

        override func viewDidLoad() {
            super.viewDidLoad()
            // Do any additional setup after loading the view.
        }

        @IBAction func btnPushToViewControllerDTapped( _ sender : UIButton) {
            if let vcD = self.storyboard?.instantiateViewController(withIdentifier: "ViewControllerD") as?  ViewControllerD  {
                vcD.delegate = self // Registring Delegate (When View Conteoller D gets Dismiss It can call sendData method
    //            vcD.textLabelD.text = "This is Data Passing by Referenceing View Controller D Text Label." //Data Passing Between View Controllers using Data Passing
                self.present(vcD, animated: true, completion: nil)
            }
        }

        //This Method will called when when viewcontrollerD will dismiss. (You can also say it is a implementation of Protocol Method)
        func sendData(data: String) {
            self.textLabelC.text = data
        }

    }

Answer 2

It depends when you want to get data.

If you want to get data whenever you want, can use a singleton pattern. The pattern class is active during the app runtime. Here is an example of the singleton pattern.

class AppSession: NSObject {

    static let shared = SessionManager()
    var username = "Duncan"
}

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()

        print(AppSession.shared.username)
    }
}

If you want to get data after any action, can use NotificationCenter.

extension Notification.Name {
    static let loggedOut = Notification.Name("loggedOut")
}

@IBAction func logoutAction(_ sender: Any) {
    NotificationCenter.default.post(name: .loggedOut, object: nil)
}

NotificationCenter.default.addObserver(forName: .loggedOut, object: nil, queue: OperationQueue.main) { (notify) in
    print("User logged out")
}