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?

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.

@property (nonatomic, assign) BOOL isSomethingEnabled;

#import "ViewControllerB.h"

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

@property (nonatomic, assign) BOOL isSomethingEnabled;

#import "ViewControllerB.h"

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

-(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;
    }
}

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

@class ViewControllerB;

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

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

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

#import "ViewControllerB.h"

@interface ViewControllerA : UIViewController <ViewControllerBDelegate>

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

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

References

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).

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

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:

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: class {
    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.

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.

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