Passing Variables Between View Controllers

Passing data between view controllers

This question seems to be very popular here on Stack Overflow so I thought I would try and give a better answer to help out people starting in the world of iOS like me.

Passing Data Forward

Passing data forward to a view controller from another view controller. You would use this method if you wanted to pass an object/value from one view controller to another view controller that you may be pushing on to a navigation stack.

For this example, we will have ViewControllerA and ViewControllerB

To pass a BOOL value from ViewControllerA to ViewControllerB we would do the following.

in ViewControllerB.h create a property for the BOOL

 @property (nonatomic, assign) BOOL isSomethingEnabled;

in ViewControllerA you need to tell it about ViewControllerB so use an
```
 #import "ViewControllerB.h"
```

Then where you want to load the view, for example, didSelectRowAtIndex or some IBAction, you need to set the property in ViewControllerB before you push it onto the navigation stack.

    ViewControllerB *viewControllerB = [[ViewControllerB alloc] initWithNib:@"ViewControllerB" bundle:nil];
    viewControllerB.isSomethingEnabled = YES;
    [self pushViewController:viewControllerB animated:YES];

This will set isSomethingEnabled in ViewControllerB to BOOL value YES.

Passing Data Forward using Segues

If you are using Storyboards you are most likely using segues and will need this procedure to pass data forward. This is similar to the above but instead of passing the data before you push the view controller, you use a method called

-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender

So to pass a BOOL from ViewControllerA to ViewControllerB we would do the following:

in ViewControllerB.h create a property for the BOOL

 @property (nonatomic, assign) BOOL isSomethingEnabled;

in ViewControllerA you need to tell it about ViewControllerB, so use an
```
 #import "ViewControllerB.h"
```
Create the segue from ViewControllerA to ViewControllerB on the storyboard and give it an identifier. In this example we'll call it "showDetailSegue"

Next, we need to add the method to ViewControllerA that is called when any segue is performed. Because of this we need to detect which segue was called and then do something. In our example, we will check for "showDetailSegue" and if that's performed, we will pass our BOOL value to ViewControllerB

 -(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender{
     if([segue.identifier isEqualToString:@"showDetailSegue"]){
         ViewControllerB *controller = (ViewControllerB *)segue.destinationViewController;
         controller.isSomethingEnabled = YES;
     }
 }

If you have your views embedded in a navigation controller, you need to change the method above slightly to the following

    -(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender{
        if([segue.identifier isEqualToString:@"showDetailSegue"]){
            UINavigationController *navController = (UINavigationController *)segue.destinationViewController;
            ViewControllerB *controller = (ViewControllerB *)navController.topViewController;
            controller.isSomethingEnabled = YES;
        }
    }

This will set isSomethingEnabled in ViewControllerB to BOOL value YES.

Passing Data Back

To pass data back from ViewControllerB to ViewControllerA you need to use Protocols and Delegates or Blocks, the latter can be used as a loosely coupled mechanism for callbacks.

To do this we will make ViewControllerA a delegate of ViewControllerB. This allows ViewControllerB to send a message back to ViewControllerA enabling us to send data back.

For ViewControllerA to be a delegate of ViewControllerB it must conform to ViewControllerB's protocol which we have to specify. This tells ViewControllerA which methods it must implement.

In ViewControllerB.h, below the #import, but above @interface you specify the protocol.

 @class ViewControllerB;

 @protocol ViewControllerBDelegate <NSObject>
 - (void)addItemViewController:(ViewControllerB *)controller didFinishEnteringItem:(NSString *)item;
 @end

Next still in the ViewControllerB.h, you need to set up a delegate property and synthesize in ViewControllerB.m
```
 @property (nonatomic, weak) id <ViewControllerBDelegate> delegate;
```

In ViewControllerB we call a message on the delegate when we pop the view controller.

 NSString *itemToPassBack = @"Pass this value back to ViewControllerA";
 [self.delegate addItemViewController:self didFinishEnteringItem:itemToPassBack];

That's it for ViewControllerB. Now in ViewControllerA.h, tell ViewControllerA to import ViewControllerB and conform to its protocol.
```
 #import "ViewControllerB.h"

 @interface ViewControllerA : UIViewController <ViewControllerBDelegate>
```

In ViewControllerA.m implement the following method from our protocol

 - (void)addItemViewController:(ViewControllerB *)controller didFinishEnteringItem:(NSString *)item
 {
     NSLog(@"This was returned from ViewControllerB %@", item);
 }

Before pushing viewControllerB to navigation stack we need to tell ViewControllerB that ViewControllerA is its delegate, otherwise we will get an error.

 ViewControllerB *viewControllerB = [[ViewControllerB alloc] initWithNib:@"ViewControllerB" bundle:nil];
 viewControllerB.delegate = self
 [[self navigationController] pushViewController:viewControllerB animated:YES];

References

Using Delegation to Communicate With Other View Controllers in the View Controller Programming Guide
Delegate Pattern

NSNotification center

It's another way to pass data.

// Add an observer in controller(s) where you want to receive data
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(handleDeepLinking:) name:@"handleDeepLinking" object:nil];

-(void) handleDeepLinking:(NSNotification *) notification {
    id someObject = notification.object // Some custom object that was passed with notification fire.
}

// Post notification
id someObject;
[NSNotificationCenter.defaultCenter postNotificationName:@"handleDeepLinking" object:someObject];

Passing Data back from one class to another (A class can be any controller, Network/session manager, UIView subclass or any other class)

Blocks are anonymous functions.

This example passes data from Controller B to Controller A

Define a block

@property void(^selectedVoucherBlock)(NSString *); // in ContollerA.h

Add block handler (listener)

Where you need a value (for example, you need your API response in ControllerA or you need ContorllerB data on A)

// In ContollerA.m

- (void)viewDidLoad {
    [super viewDidLoad];
    __unsafe_unretained typeof(self) weakSelf = self;
    self.selectedVoucherBlock = ^(NSString *voucher) {
        weakSelf->someLabel.text = voucher;
    };
}

Go to Controller B

UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"Main" bundle:nil];
ControllerB *vc = [storyboard instantiateViewControllerWithIdentifier:@"ControllerB"];
vc.sourceVC = self;
    [self.navigationController pushViewController:vc animated:NO];

Fire block

-(void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:
(NSIndexPath *)indexPath {
    NSString *voucher = vouchersArray[indexPath.row];
    if (sourceVC.selectVoucherBlock) {
        sourceVC.selectVoucherBlock(voucher);
    }
    [self.navigationController popToViewController:sourceVC animated:YES];
}

Another Working Example for Blocks

How to pass data between view controllers - not working

This is an inefficient way to do this as you are passing data through 3 different objects. However, going on with this methodology, the problem is the label's are not created yet inside

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if let VC = segue.destination as? ThirdViewController {

        VC.player1ScoreLabel.text = String(player1CurrentScore)
        VC.player2ScoreLabel.text = String(player2CurrentScore)
        VC.player3ScoreLabel.text = String(player3CurrentScore)
        VC.player4ScoreLabel.text = String(player4CurrentScore)

    }
}

See, the label isn't created yet. So, you are setting text on aUILabel that isn't initialized. Therefore, you need to create variables for the labels inside ThirdViewController.

Third View Controller

class ThirdViewController: UIViewController {

    @IBOutlet var player1ScoreLabel: UILabel!
    @IBOutlet var player2ScoreLabel: UILabel!
    @IBOutlet var player3ScoreLabel: UILabel!
    @IBOutlet var player4ScoreLabel: UILabel!

    var score0:Int!
    var score1:Int!
    var score2:Int!
    var score3:Int!

    override func viewDidLoad() {
        super.viewDidLoad()
        self.player1ScoreLabel.text = String(score0)
        self.player2ScoreLabel.text = String(score1)
        self.player3ScoreLabel.text = String(score2)
        self.player4ScoreLabel.text = String(score3)
    }
}

and change the segue in SecondViewController

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if let VC = segue.destination as? ThirdViewController {

        VC.score0 = player1CurrentScore
        VC.score1 = player2CurrentScore
        VC.score2 = player3CurrentScore
        VC.score3 = player4CurrentScore

    }
}

Another Way:

Let's create a singleton called Game. Within this (assuming you only have 4 players) we can create 4 players that will never change. This allows us to create instances of players in one location and call upon them as necessary.

NOTE: A singleton can be misused EASILY.

https://cocoacasts.com/what-is-a-singleton-and-how-to-create-one-in-swift
https://cocoacasts.com/are-singletons-bad/

class Game {

    static var score0:Int = 0
    static var score1:Int = 0
    static var score2:Int = 0
    static var score2:Int = 0

}

Then, anywhere in your code you can access Game.score0, Game.score1.

CAUTION:

I would caution you to very carefully use singletons. You don't want everything with public access. You need to determine if this is good for you or not. Cheers.

Passing Data Between View Controllers in Swift 4

Your problem is that your prepare methods never run because you never call them.

Look, when you call performSegue, then prepare(for segue: sender:) is called too, so you can override this method in your ViewController and because you're passing identifier as parameter of performSegue method you can determine what should happen if segue has this or this identifier

So, delete prepare for segue methods from IBActions

@IBAction func backButton(_ sender: Any) {
    if installer == "verified"{
        performSegue(withIdentifier: "main/login", sender: self)
    }
}

@IBAction func button1(_ sender: Any) {
    if installer == "verified"{
        performSegue(withIdentifier: "Button1", sender: self)
    }
}

instead override prepare(for segue: sender:) method of ViewController and inside specify what should happen if segue has "main/login" indentifier or "Button1":

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == "main/login" {
        let mainController = segue.destination as! ViewController
        mainController.myvar = installer
    } else if segue.identifier == "Button1"
        let IPController = segue.destination as! IP_ModuleQuickStartViewController
        IPController.verified = installer
    }
}

How to pass a (changing) variable between two view controllers?

Use a delegate pattern.

In VC2:

protocol VC2Delegate: AnyObject {
  var id2: Int { get }
}

class VC2 {
  weak var delegate: VC2Delegate?

  override func viewDidLoad() {
    super.viewDidLoad()
    print(delegate?.id2)
  }
}

In VC1:

class VC1: UIViewController, VC2Delegate {
...
  var id2: Int = 0 

  override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    guard let vc2 = segue.destination as? navigationScreenVC else { return }
    vc2.delegate = self
  }
...
}

Pass data between view controllers constantly without a segue? USING SWIFT

You could make a global variable in it's own file and call the variable in both view controllers:

struct Variables {
    static var coinsVariable = 0
}

//call
Variables.coinsVariable = 10

Also you could make a protocol where you set a function to update the coinValue. Then you can use it inside your ViewControllers as delegate and get the value from the method in the delegate. Check the Apple Documentation how to use protocols.

How to pass data between view controller in objective-c?

Two points to note:

1: This is not the way to initiate a view controller to show/present in your app.

2: You should declare your NSString *data in your SecondVC's .h file.

Now point 1 solutions is to change your code with below in your didSelectItemAtIndexPath: function

switch (indexPath.row)
{
    case 0:
    {
        // By Default storyboard name is "Main". Change it if you you have different name in your project
        UIStoryboard *mainStoryboard = [UIStoryboard storyboardWithName:@"Main" bundle: nil];

        // Now instantiate your view controller with its identifier on this storyboard object. 
        // Note: don't forget to set viewControllers' identifier
        WebViewController *vc = [mainStoryboard instantiateViewControllerWithIdentifier: @"WebViewController"];

        // Now Pass your value 
        WebViewController.data = arraySliderProducts[indexPath.row][@"title"]; 

        //Finally push this object on your navigation stack
        [self.navigationController pushViewController: WebViewController animated:YES];
    }... 
}