Question

I want to pass data from second tab to first tab using delegates.But delegate method not triggered.here is the code (I m using storyboard)

in SecondViewController.h

#import <UIKit/UIKit.h>

 @class SecondViewController;
 @protocol SecondViewControllerDelegate <NSObject>

 -(void) sliderValueDidChanged: (SecondViewController *)controller data:(float)      sliderValue;

 @end

 @interface SecondViewController : UIViewController{
__weak id<SecondViewControllerDelegate> delegate;   
 }
 - (IBAction)sliderPressed:(id)sender;

 @property (weak, nonatomic) IBOutlet UISlider *mySlider;
 @property(weak,nonatomic) id<SecondViewControllerDelegate> delegate;

 @end

in SecondViewController.m

#import "SecondViewController.h"


@interface SecondViewController ()

@end

 @implementation SecondViewController
 @synthesize mySlider;
 @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.
 }

 - (void)viewDidUnload
 {
[self setMySlider:nil];
[super viewDidUnload];
// Release any retained subviews of the main view.
 }

 - (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
 {
return (interfaceOrientation == UIInterfaceOrientationPortrait);
 }

- (IBAction)sliderPressed:(id)sender {
float value = mySlider.value;

[self.delegate sliderValueDidChanged:self data:value];
NSLog(@"%@",self.delegate);
}
@end

FirstViewController.h

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

@interface FirstViewController : UIViewController<SecondViewControllerDelegate>
@property (weak, nonatomic) IBOutlet UILabel *myLabel;

@end

in FirstViewController.h

#import "FirstViewController.h"


@interface FirstViewController ()

@end

@implementation FirstViewController
@synthesize myLabel;

- (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.
   SecondViewController *svc = [[SecondViewController alloc]init];
   svc.delegate = self;




}

- (void)viewDidUnload
{
    [self setMyLabel:nil];
    [super viewDidUnload];
    // Release any retained subviews of the main view.
}

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
    return (interfaceOrientation == UIInterfaceOrientationPortrait);
}
-(void) sliderValueDidChanged: (SecondViewController *)controller data:(float) sliderValue{
    myLabel.text = [[NSString alloc]initWithFormat:@"%f",sliderValue];
    NSLog(@"delegate called");
}

@end

I am trying to set a slider in second tab and send the sider value to first tab which has a label. self.delegate printing null and delegate method never called.

Answer 1

Simply enough, as you already have the delegation code, only change the line where you create a SecondViewController object. Rather than creating a new one, you should have a reference to the one that the tab bar will show.

In the viewDidLoad of FirstViewController,
Change

SecondViewController *svc = [[SecondViewController alloc]init];

to

//assuming you have 2 view controllers in the tab bar controller, first being FirstViewController
SecondViewController *svc = [self.tabBarController.viewControllers objectAtIndex:1]; 

and that should do it