Passing Data between View Controllers

Question or problem with Swift language programming:

I’m new to iOS and Objective-C and the whole MVC paradigm and I’m stuck with the following:

I have a view that acts as a data entry form and I want to give the user the option to select multiple products. The products are listed on another view with a UITableViewController and I have enabled multiple selections.

My question is, how do I transfer the data from one view to another? I will be holding the selections on the UITableView in an array, but how do I then pass that back to the previous data entry form view so it can be saved along with the other data to Core Data on submission of the form?

I have surfed around and seen some people declare an array in the app delegate. I read something about Singletons but don’t understand what these are and I read something about creating a data model.

What would be the correct way of performing this and how would I go about it?

How to solve the problem:

Solution 1:

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

I hope this answer is clear enough for people to understand and that I have not missed anything.

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.

1. in ViewControllerB.h create a property for the BOOL

   @property (nonatomic, assign) BOOL isSomethingEnabled; 

2. in ViewControllerA you need to tell it about ViewControllerB so use an

   #import "ViewControllerB.h" 

   Then where you want to load the view eg. didSelectRowAtIndex or some IBAction you need to set the property in ViewControllerB before you push it onto nav 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:

1. in ViewControllerB.h create a property for the BOOL

   @property (nonatomic, assign) BOOL isSomethingEnabled; 

2. in ViewControllerA you need to tell it about ViewControllerB so use an

   #import "ViewControllerB.h" 

3. Create a the segue from ViewControllerA to ViewControllerB on the storyboard and give it an identifier, in this example we’ll call it "showDetailSegue"

4. 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.

1. 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 

2. next still in the ViewControllerB.h you need to setup a delegate property and synthesize in ViewControllerB.m

   @property (nonatomic, weak) id <ViewControllerBDelegate> delegate; 

3. 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]; 

4. 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> 

5. In ViewControllerA.m implement the following method from our protocol

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

6. 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

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

NSNotification center
It’s another way to pass data.

// add 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

Solution 2:

Swift

There are tons and tons of explanations here and around StackOverflow, but if you are a beginner just trying to get something basic to work, try watching this YouTube tutorial (It’s what helped me to finally understand how to do it).

• YouTube tutorial: How to send data through segue (swift)

Passing data forward to the next View Controller

The following is an example based on the video. The idea is to pass a string from the text field in the First View Controller to the label in the Second View Controller.

Create the storyboard layout in the Interface Builder. To make the segue, you just Control click on the button and drag over to the Second View Controller.

First View Controller

The code for the First View Controller is

import UIKit

class FirstViewController: UIViewController {

    @IBOutlet weak var textField: UITextField!
    
    // This function is called before the segue
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        
        // get a reference to the second view controller
        let secondViewController = segue.destination as! SecondViewController
        
        // set a variable in the second view controller with the String to pass
        secondViewController.receivedString = textField.text!
    }

}

Second View Controller

And the code for the Second View Controller is

import UIKit

class SecondViewController: UIViewController {

    @IBOutlet weak var label: UILabel!
    
    // This variable will hold the data being passed from the First View Controller
    var receivedString = ""
    
    override func viewDidLoad() {
        super.viewDidLoad()

        // Used the text from the First View Controller to set the label
        label.text = receivedString
    }

}

Don’t forget

• Hook up the outlets for the UITextField and the UILabel.
• Set the first and second View Controllers to the appropriate Swift files in IB.

Passing data back to the previous View Controller

To pass data back from the second view controller to the first view controller, you use a protocol and a delegate. This video is a very clear walk though of that process:

• YouTube tutorial: iOS Swift Basics Tutorial: Protocols and Delegates But also read this post to make sure you don’t get into a strong reference cycle.

The following is an example based on the video (with a few modifications).

Create the storyboard layout in the Interface Builder. Again, to make the segue, you just Control drag from the button to the Second View Controller. Set the segue identifier to showSecondViewController. Also, don’t forget to hook up the outlets and actions using the names in the following code.

First View Controller

The code for the First View Controller is

import UIKit

class FirstViewController: UIViewController, DataEnteredDelegate {

    @IBOutlet weak var label: UILabel!
    
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if segue.identifier == "showSecondViewController" {
            let secondViewController = segue.destination as! SecondViewController
            secondViewController.delegate = self
        }
    }
    
    func userDidEnterInformation(info: String) {
        label.text = info
    }
}

Note the use of our custom DataEnteredDelegate protocol.

Second View Controller and Protocol

The code for the second view controller is

import UIKit

// protocol used for sending data back
protocol DataEnteredDelegate: AnyObject {
    func userDidEnterInformation(info: String)
}

class SecondViewController: UIViewController {

    // making this a weak variable so that it won't create a strong reference cycle
    weak var delegate: DataEnteredDelegate? = nil
    
    @IBOutlet weak var textField: UITextField!

    @IBAction func sendTextBackButton(sender: AnyObject) {
        
        // call this method on whichever class implements our delegate protocol
        delegate?.userDidEnterInformation(info: textField.text!)
    
        // go back to the previous view controller
        _ = self.navigationController?.popViewController(animated: true)
    }
}

Note that the protocol is outside of the View Controller class.

That’s it. Running the app now you should be able to send data back from the second view controller to the first.

Solution 3:

The M in MVC is for “Model” and in the MVC paradigm the role of model classes is to manage a program’s data. A model is the opposite of a view — a view knows how to display data, but it knows nothing about what to do with data, whereas a model knows everything about how to work with data, but nothing about how to display it. Models can be complicated, but they don’t have to be — the model for your app might be as simple as an array of strings or dictionaries.

The role of a controller is to mediate between view and model. Therefore, they need a reference to one or more view objects and one or more model objects. Let’s say that your model is an array of dictionaries, with each dictionary representing one row in your table. The root view for your app displays that table, and it might be responsible for loading the array from a file. When the user decides to add a new row to the table, they tap some button and your controller creates a new (mutable) dictionary and adds it to the array. In order to fill in the row, the controller creates a detail view controller and gives it the new dictionary. The detail view controller fills in the dictionary and returns. The dictionary is already part of the model, so nothing else needs to happen.

Solution 4:

There are various ways by which a data can be received to a different class in iOS. For example –

1. Direct initialization after the allocation of another class.
2. Delegation – for passing data back
3. Notification – for broadcasting data to multiple classes at a single time
4. Saving in NSUserDefaults – for accessing it later
5. Singleton classes
6. Databases and other storage mechanisms like plist, etc.

But for the simple scenario of passing a value to a different class whose allocation is done in the current class, the most common and preferred method would be the direct setting of values after allocation. This is done as follows:-

We can understand it using two controllers – Controller1 and Controller2

Suppose in Controller1 class you want to create the Controller2 object and push it with a String value being passed. This can be done as this:-

- (void)pushToController2 {

    Controller2 *obj = [[Controller2 alloc] initWithNib:@"Controller2" bundle:nil];
    [obj passValue:@"String"];
    [self pushViewController:obj animated:YES];
}

In the implementation of the Controller2 class there will be this function as-

@interface Controller2  : NSObject

@property (nonatomic , strong) NSString* stringPassed;

@end

@implementation Controller2

@synthesize stringPassed = _stringPassed;

- (void) passValue:(NSString *)value {

    _stringPassed = value; //or self.stringPassed = value
}

@end

You can also directly set the properties of the Controller2 class in the similar way as this:

- (void)pushToController2 {

    Controller2 *obj = [[Controller2 alloc] initWithNib:@"Controller2" bundle:nil];
    [obj setStringPassed:@"String"];  
    [self pushViewController:obj animated:YES];
}

To pass multiple values you can use the multiple parameters like :-

Controller2 *obj = [[Controller2 alloc] initWithNib:@"Controller2" bundle:nil];
[obj passValue:@“String1” andValues:objArray withDate:date]; 

Or if you need to pass more than 3 parameters which are related to a common feature you can store the values to a Model class and pass that modelObject to the next class

ModelClass *modelObject = [[ModelClass alloc] init]; 
modelObject.property1 = _property1;
modelObject.property2 = _property2;
modelObject.property3 = _property3;

Controller2 *obj = [[Controller2 alloc] initWithNib:@"Controller2" bundle:nil];
[obj passmodel: modelObject];

So in-short if you want to –

1) set the private variables of the second class initialise the values by calling a custom function and passing the values.
2) setProperties do it by directlyInitialising it using the setter method.
3) pass more that 3-4 values related to each other in some manner , then create a model class and set values to its object and pass the object using any of the above process.

Hope this helps

Solution 5:

After more research it seemed that Protocols and Delegates is the correct/Apple prefered way of doing this.

I ended up using this example

Sharing data between view controllers and other objects @ iPhone Dev SDK

Worked fine and allowed me to pass a string and an array forward and back between my views.

Thanks for all your help

Hope this helps!