Retreiving data from second screen in push segue

You should go for delegation. It's not easy at first, but when you get the hang of it, it's really nice to work with.

Say you have two view controllers - ParentViewController and ChildViewController. You want to send data from the ChildViewController to the ParentViewController.

To summarize, this is what we're gonna do:

1. Declare a protocol in the ChildViewController, as well as declaring a delegate method and a delegate property.
2. Call the delegate method in ChildViewController.
3. Make ParentViewController conform to the delegate declared in ChildViewController
4. Set the delegate property in ParentViewController
5. Implement the delegate method in ParentViewController

STEP 1 - Declare a protocol in the ChildViewController, as well as declaring a delegate method and a delegate property

// ChildViewController.h

@class ChildViewController;

@protocol ChildViewControllerDelegate <NSObject>
- (void)viewDidLoadInViewController:(ChildViewController *)viewController;
@end

@interface ChildViewController : UIViewController
@property (strong, nonatomic) id <ChildViewControllerDelegate> delegate;
@end

This is av very simple protocol as it only has one method. You have to put the protocol in the ChildViewController, because it's that view controller which will decide when the method will be run (as I like to think about it). You also have to declare a delegate property. You will send the delegate method to this object in the ChildViewController.

STEP 2 - Call the delegate method in ChildViewController

Here you will decide when the delegate method will be sent to the delegate object. I will put it in the viewDidLoad. Of course, you can put it anywhere you want.

// ChildViewController.m

- (void)viewDidLoad
{
    [self.delegate viewDidLoadInViewController:self];
}

It's nice to include the ChildViewController in the delegate method, so that you can access properties (as an example).

STEP 3 - Make ParentViewController conform to the delegate declared in ChildViewController

This is of course done in the ParentViewController like this (remember to import the ChildViewController.h file):

// ChildViewController.m

@interface ParentViewController () <ChildViewControllerDelegate>

You can do this in both the .h and the .m file. Xcode will now tell you that you need to implement certain delegate methods, which we will do in the last step.

STEP 4 - Set the delegate property in ParentViewController

Since we're using a segue, we can set the delegate property there, like this:

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender 
{
    if ([segue.identifier isEqualToString:@"segueIdentifier"])
    {
        ChildViewController *childViewController = (ChildViewController *)segue.destinationViewController;
        childViewController.delegate = self;
    }
}

Now the ChildViewController knows which object to send the delegate method to, specifically the ParentViewController.

STEP 5 - Implement the delegate method in ParentViewController

- (void)viewDidLoadInViewController:(ChildViewController *)viewController
{
    NSLog(@"Hello, world");
}

When you run the application and segue to the ChildViewController, ChidViewController will send the delegate method to the delegate object - the ParentViewController - which will write "Hello, world" to the console.

Don't hesitate to ask if there is something you don't understand or something I've done wrong.