Passing data between tab viewed controllers in swift?

Question

What it does

1. When the first page in my tab bar controller loads, I retrieve data from a json file
2. I store it in an array (in the first view controller)
3. The data obtained will be displayed in the second view controller. The data is already loaded and stored in an array in the first view controller.

Problem:

I can't figure out a way to pass the data between the two view controllers. Can't pass data based on the segue identifier since it is a tab bar controller

Please help!

Answer 1

If you need to pass the data between view controllers then :

var secondTab = self.tabBarController?.viewControllers[1] as SecondViewController
secondTab.array = firstArray 

Answer 2

I ended up using a singleton as Woodster suggested in his answer above.

In Swift 3

Create a new swift file and create a class:

class Items {
    static let sharedInstance = Items()
    var array = [String]() 
}

In any of your view controllers you can access your array like this:

Items.sharedInstance.array.append("New String")
print(Items.sharedInstance.array)

Answer 3

H. Serdar's code example is right, that's the way to access another tab's view controller and give it data.

Keep in mind that when you pass an array in Swift, you're passing it by value, unlike Objective-C, which passes it by reference. This means that changes made by your second view controller won't be reflected in your first view controller, because your second one is using a copy of the array, not the same array. If you want both view controllers to modify the same array, put the array in a class, and pass a single instance of that class around.

Some other considerations: You could subclass the TabBarController to give it a property that'll store your data, and that would be available to all tabs using:

if let tbc = tabBarController as? YourCustomTabBarSubclass {
  println("here's my data \(tbc.array)")
 }

In that situation, you'd be accessing the same array from multiple tabs, so changes in one tab would be reflected elsewhere.

I recommend against the approach of using your App Delegate as a centralized place to store data. That's not the purpose of the application's delegate. Its purpose is to handle delegate calls for the application object.

View Controllers should have all the data, encapsulated within them, that they need to do their job. They have a connection to their model data (such as your array, or a reference to a database or a managed object context) as opposed having a view controller reach out to another object by traversing a view controller graph or going into the delegate or even using a global variable. This modular, self contained construction of View Controllers lets you restructure your app for similar but unique designs on different devices, such as presenting a view controller in a popover on one device (like an iPad) and presenting it full screen on another, such as an iPhone.

Answer 4

SWIFT 3

In your first viewcontroller, declare your variable (in your case an array) like you normally would.

In your second viewcontroller, do this:

var yourVariable: YourVariableClass {
    get {
        return (self.tabBarController!.viewControllers![0] as! FirstViewControllerClass).yourVariable
    }
    set {
        (self.tabBarController!.viewControllers![0] as! FirstViewControllerClass).yourVariable = newValue
    }
}

This works because, in a tabbarcontroller all viewcontrollers behind the tab items are initialized. By doing this in your second viewcontroller you are actually getting/setting the variable from/in the first viewcontroller.

Answer 5

For Xcode 11 & Swift 5 + Storyboard + Dependency Injection Approach

Assuming you are using a storyboard this is a method I have devised.

Step 1:

Put an identifier on your tabBarController like I did in the image below.

Step 2:

In the scenedelegate.swift file (NOT appDelegate.swift), add the following code to the appropriate func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) { method.

func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        // Use this method to optionally configure and attach the UIWindow `window` to the provided UIWindowScene `scene`.
        // If using a storyboard, the `window` property will automatically be initialized and attached to the scene.
        // This delegate does not imply the connecting scene or session are new (see `application:configurationForConnectingSceneSession` instead).

        self.window = self.window ?? UIWindow()//@JA- If this scene's self.window is nil then set a new UIWindow object to it.

        //@Grab the storyboard and ensure that the tab bar controller is reinstantiated with the details below.
        let storyboard = UIStoryboard(name: "Main", bundle: nil)
        let tabBarController = storyboard.instantiateViewController(withIdentifier: "tabBarController") as! UITabBarController

        for child in tabBarController.viewControllers ?? [] {
            if let top = child as? StateControllerProtocol {
                print("State Controller Passed To:")
                print(child.title!)
                top.setState(state: stateController)
            }
        }

        self.window!.rootViewController = tabBarController //Set the rootViewController to our modified version with the StateController instances
        self.window!.makeKeyAndVisible()

        print("Finished scene setting code")
        guard let _ = (scene as? UIWindowScene) else { return }
    }

You will notice that the scenedelgate.swift file has a member variable; var window: UIWindow?. This used to be part of appDelegate but was changed in xCode 11 and Swift 5 so a lot of similar answers and tutorials will be out of date.

The part in the code that says storyboard.instantiateViewController(withIdentifier: you will want to add the name you used for the parameter. In my screenshot you will see I called it tabBarController.

To make this function work on any type of viewController without having to instantiate each one separately on an index, I've used a protocol strategy called StateControllerProtocol. We will be creating this next along with the StateController which will hold the global variables.

Step 3:

In stateController.swift or whatever you want to name this file, add the following code removing aspects that do not apply to your project.

import Foundation

struct tdfvars{
    var lateBED:Double = 0.0
    var acuteBED:Double = 0.0
    var rbe:Double = 1.4
    var t1half:Double = 1.5
    var alphaBetaLate:Double = 3.0
    var alphaBetaAcute:Double = 10.0
    var totalDose:Double = 6000.00
    var dosePerFraction:Double = 200.0
    var numOfFractions:Double = 30
    var totalTime:Double = 168
    var ldrDose:Double = 8500.0
}

//@JA - Protocol that view controllers should have that defines that it should have a function to setState
protocol StateControllerProtocol {
  func setState(state: StateController)
}

class StateController {
    var tdfvariables:tdfvars = tdfvars()
}

The variables you want to share between views I recommend adding to the struct. I named mine tdfvariables but you will want to name this something relevant to your project. Note the protocol defined here as well. This is a protocol that will be added to each viewController as an extension that defines that there should be a function to set its stateController member variable (which we have not defined yet, but will in a later step).

Step 4:

In my case I have 2 views controlled by the tabBarController. StandardRegimenViewController and settingsViewController. This is the code you will want to add for your viewControllers.

import UIKit

class SettingsViewController: UIViewController {

    var stateController: StateController?

    override func viewDidLoad() {
        super.viewDidLoad()

    }


}

//@JA - This adds the stateController variable to the viewController
extension SettingsViewController: StateControllerProtocol {
  func setState(state: StateController) {
    self.stateController = state
  }
}

The extension here adds the protocol to your class and adds the function as required by it that we defined earlier in the stateController.swift file. This is what will eventually get the stateController and it's struct values into your viewController.

Step 5:

Use the stateController to get access to your variables! You are done!

Here is some examples of how I did this in one of my controllers.

stateController?.tdfvariables.lateBED = 100

You can read the variables the same way! The advantage of this approach is you are NOT using Singletons and instead Dependency Injection for your viewControllers and anything else that may need access to your variables. Read more about dependency injection to see the benefits vs singletons to learn more.

Answer 6

I have a tabbed view controller in my application and I use the same array for multiple tab views. I accomplish this by declaring the array outside of any classes (in the lines between import UIKit and the class declaration) so that it is essentially a global variable that every view can access. Have you tried this?

Answer 7

You can override the tabBar(didSelect:) method and then index the array of ViewControllers on the UITabViewController, and cast the ViewController to the desired Custom ViewController. No need for shared mutable state and all the problems that come with it.

class SecondViewController: UIViewController {
    
    var array: [Int] = []
    override func viewDidLoad() {
        super.viewDidLoad()
    }
}


class TabViewController: UITabBarController {

   override func tabBar(
        _ tabBar: UITabBar, 
        didSelect item: UITabBarItem
    ) {
        super.tabBar(tabBar, didSelect: item)
        var secondTab = viewControllers?[1] as? SecondViewController
        secondTab?.array = [1, 2, 3]
    }
}