Question

I have two view controllers: view controller and viewcontroller2nd. I have UILabel in one of them and would like to change it when the button( named Go) in the viewcontroller2nd is clicked. I am using delegates and protocols to do it.

The code looks like this:

ViewController.h

#import <UIKit/UIKit.h>
#import "ViewController2nd.h"

@interface ViewController : UIViewController <SecondViewControllerDelegate>
{
    IBOutlet UILabel *lbl;
    ViewController2nd *secondview;

}
-(IBAction)passdata:(id)sender;

@end

ViewController.m

#import "ViewController.h"
#import "ViewController2nd.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}
-(void) changeLabel:(NSString*)str{
    lbl.text = str;
}

-(IBAction)passdata:(id)sender{
    ViewController2nd *second = [[ViewController2nd alloc] initWithNibName:nil bundle:nil];
    [self presentViewController:second animated:YES completion:NULL];
}

@end

Viewcontroller2nd.h

#import <UIKit/UIKit.h>


@protocol SecondViewControllerDelegate <NSObject>
@optional
-(void) changeLabel:(NSString*)str;
@end

@interface ViewController2nd : UIViewController{

    IBOutlet UIButton *bttn;
    id <SecondViewControllerDelegate> delegate;

}

@property (retain) id delegate;

-(IBAction)bttnclicked;
-(IBAction)back:(id)sender;
@end

ViewController2nd.m

#import "ViewController2nd.h"
#import "ViewController.h"

@interface ViewController2nd ()

@end

@implementation ViewController2nd

@synthesize delegate;

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view from its nib.
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

-(IBAction)bttnclicked{
   [[self delegate] changeLabel:@"Hello"];
}

-(IBAction)back:(id)sender{
    [self dismissViewControllerAnimated:YES completion:NULL];
}

@end

The passing of the control between the two views is working correctly. However, when i click the go button in the viewcontroller2nd, it doesn’t change the value of the label to Hello. What is wrong with the code? Need some guidance.

Answer 1

It seems you never set the delegate.
You can do it in your passData method :

-(IBAction)passdata:(id)sender{
    ViewController2nd *second = [[ViewController2nd alloc] initWithNibName:nil bundle:nil];
    second.delegate = self;
    [self presentViewController:second animated:YES completion:NULL];
}