将数据传递给 TabBar 控制器

Question

I want pass data between two ViewControllers throw the TabBarController. Inside the first ViewController I located textField and button. Inside the second I located Label. When I write some text in textField and push button, I expect that this text appear in the Label in the second ViewController. But nothing happens.

And my code:

First ViewController:

import UIKit

class FirstViewController: UIViewController {
    @IBOutlet weak var textField: UITextField!

    @IBAction func enter(_ sender: Any) {
        if textField.text != "" {
            if let window = UIApplication.shared.delegate?.window, let tabBarController = window?.rootViewController as? UITabBarController, let second = tabBarController.viewControllers?.first as? SecondViewController {
                second.label.text = textField.text
                tabBarController.selectedIndex = 0
            }
        }
    }

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

    }

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


}

Second ViewController:

import UIKit

class SecondViewController: UIViewController {

    @IBOutlet weak var label: UILabel!
    var myString = String()

    override func viewDidLoad() {
        super.viewDidLoad()

        label.text = myString
        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


}

Answer 1

I think the problem is in

tabBarController.viewControllers?.first as? SecondViewController

You probably want to do this instead:

tabBarController.viewControllers?[1] as? SecondViewController

Don't forget arrays are indexed from 0, so viewControllers?[1] actually returns the second element in the array.

Answer 2

First i think your view controllers that are representing tabs should be embedded into navigation controllers which would then be linked to the TabBarController.

Secondly, the preferred and recommended way to send data between controllers is through protocols (delegates). Here is a good example you can check out, step by step: https://medium.com/@jamesrochabrun/implementing-delegates-in-swift-step-by-step-d3211cbac3ef

However if you're looking for a quick fix of your solution i think that Bruno Phillipe's answer gets it to some extent, but not quite. We can't really be sure what controller is at what index in the view controller list. I think this should work:

@IBAction func enter(_ sender: Any) {
    if textField.text != "" {
        if let window = UIApplication.shared.delegate?.window, let tabBarController = window?.rootViewController as? UITabBarController {

            //check if there are view controllers in the tabBarController
            guard let vcList = tabBarController.viewControllers else {

                return
            }

            for controller in vcList {

                if let second = controller as? SecondViewController {
                    //this will be executed only when a controller is SeconfViewController
                    second.label.text = textField.text
                    tabBarController.selectedIndex = 0
                }
            }

        }
    }
}

EDIT:

I tried it myself and the problem is that you were trying to set the label.text when in fact the label component was never initialised. I think if you simply stored the textField value into myString variable in SecondViewController it would work (not sure).

However here's the solution using a protocol (delegate) which is the right way to send data between controllers. Ask any questions you might have. This should work:

FirstViewController:

import Foundation
import UIKit

protocol LabelChangeDelegate: class {

    func changeLabelWithText(_ text: String?)
}

class FirstViewController: UIViewController {

    weak var delegate: LabelChangeDelegate?

    @IBOutlet weak var textField: UITextField!

    @IBAction func enter(_ sender: UIButton) {

        if textField.text != "" {
            if let window = UIApplication.shared.delegate?.window, let tabBarController = window?.rootViewController as? UITabBarController {

                //check if there are view controllers in the tabBarController
                guard let vcList = tabBarController.viewControllers else {

                    return
                }

                for controller in vcList {

                    if let second = controller as? SecondViewController {
                        //this will be executed only when a controller is SeconfViewController

                        //set the delegate - who needs the data
                        delegate = second
                        //call the delegate function which will commmunicate with the delegate
                        delegate?.changeLabelWithText(textField.text!)
                        //don't know why you need this
                        tabBarController.selectedIndex = 0
                    }
                }

            }
        }
    }
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

    }

    override func viewDidLoad() {
         super.viewDidLoad()
         // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
         super.didReceiveMemoryWarning()
         // Dispose of any resources that can be recreated.
    }


}

SecondViewController:

import Foundation
import UIKit

class SecondViewController: UIViewController, LabelChangeDelegate {

    @IBOutlet weak var label: UILabel!

    //lazy init
    lazy var myString = String()

    override func viewDidLoad() {
        super.viewDidLoad()

        //set label when the view loads, not in the first controller
        label.text = myString

        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
         super.didReceiveMemoryWarning()
         // Dispose of any resources that can be recreated.
    }

    //delegate function
    func changeLabelWithText(_ text: String?) {
        guard let sentText = text else {

            //no text sent
            return
        }

        myString = sentText
    }

}

Answer 3

I'm not recommending that you do it this way. This is an explanation for understanding.

1. The viewControllers in a tabBarController know their tabBarController . You can access it with self.tabBarController .
2. The secondViewController is the second one in the list of viewControllers so let second = tabBarController.viewControllers?[1] as? SecondViewController let second = tabBarController.viewControllers?[1] as? SecondViewController .
3. If you haven't visited the secondViewController yet, its view will not have loaded, so the outlets will still be nil . You can force the view to load with _ = second.view .
4. If you want to switch to the second tab, then you need to use tabBarController.selectedIndex = 1 .

---

@IBAction func enter(_ sender: Any) {
    if textField.text != "" {
        if let tabBarController = self.tabBarController as? UITabBarController, let second = tabBarController.viewControllers?[1] as? SecondViewController {
            // make sure view has loaded
            _ = second.view

            second.label.text = textField.text

            // change to second tab
            tabBarController.selectedIndex = 1
        }
    }
}

---

A better way...

Instead of setting the outlet directly, you should instead pass the string to a property of the SecondViewController :

second.myString = textField.text ?? ""

and then assign that string to the label in an override of viewWillAppear .

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    label.text = myString
}

The reason to set it in viewWillAppear is that viewWillAppear will run every time before the view is displayed. viewDidLoad will only run once when the view is first loaded. Since you want the functionality to work multiple times, viewWillAppear is the correct override.