UILabel is not updating with value from another class

Question

I'm a beginner iPhone developer.

I have 2 classes. TestViewController (this is linked withe view in the storyboard) and ViewController.

I am trying to call a method([tc refresh:val];) in TestViewController with val as the parameter from ViewController class. I can see from the logs that val reaches the TestViewController, but for some reason, the label does not update and I do not get the current text of the label also which I have set in the view. It gives null value. Please see the code and log i get and advice me how can I get the label updated by calling a method from VC to TVC.

TestViewController.h

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

@interface TestViewController : UIViewController

@property (retain, nonatomic) IBOutlet UILabel *lblDisp;

- (IBAction)chngText:(id)sender;
- (void)refresh:(NSString *)val;

@end

TestViewController.m

#import "TestViewController.h"
#import "ViewController.h"

@implementation TestViewController
@synthesize lblDisp;
- (void)viewDidLoad
{
    [super viewDidLoad];
    NSLog(@"TEST VC LOADED");
    NSLog(@"TEXT CUrret VALUE SUPERVIEW %@",lblDisp.text);
}

- (IBAction)chngText:(id)sender {
    ViewController *dd=[[ViewController alloc]init];
    [dd display];
}

-(void)refresh:(NSString *)val{
    NSLog(@"Value of Val = %@",val);
    NSLog(@"TEXT CUrret VALUE %@",lblDisp.text);
    lblDisp.text=val;
}
@end

ViewController.h

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

@interface ViewController : UIViewController

-(void)display;

@end

ViewController.m

#import "ViewController.h"
#import "TestViewController.h"

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    NSLog(@"VC LOADED");
}

-(void)display{
    NSLog(@"Reached VC");
   NSString *val=@"1";
    TestViewController *tc=[[TestViewController alloc]init];
    [tc refresh:val];
}

@end

Log

2013-07-24 03:13:36.413 Simple test[38477:11303] TEST VC LOADED
2013-07-24 03:13:36.415 Simple test[38477:11303] TEXT CUrret VALUE SUPERVIEW sdfgdfgd
2013-07-24 03:13:37.909 Simple test[38477:11303] Reached VC
2013-07-24 03:13:37.910 Simple test[38477:11303] Value of Val = 1
2013-07-24 03:13:37.911 Simple test[38477:11303] TEXT CUrret VALUE (null)

lblDisp.text=val;NSLog(@"TEXT CUrret VALUE %@",lblDisp.text); edit these two lines in your code — Apple, Jul 24 '13 at 11:21
are u calling display method from TestViewController's changetxt method ? — stack2012, Jul 24 '13 at 11:23
@TapanNathvani edit it to what?It doesnt show the current value which is "sdfgdfgd" — Reuben Kurian, Jul 24 '13 at 11:25
can you tell the flow of the application?It is really confusing . `Appdelegate-> TestViewController-> ViewController`?is this the flow — Lithu T.V, Jul 24 '13 at 11:28
@TapanNathvani If u meant to edit the code by changing the order of those 2 lines i changed and i still get null as the value. — Reuben Kurian, Jul 24 '13 at 11:31
@LithuT.V TestViewController - >ViewController -> TestViewController. — Reuben Kurian, Jul 24 '13 at 11:32
you are not passing the data to the original instance which u created initially. instead u are passing the data by creating a new instance of the TestViewController. So you are getting nil... — stack2012, Jul 25 '13 at 06:01

score 4 · Accepted Answer · edited May 23 '17 at 10:32

Your problem lies in the way you pass the value back.

TestViewController *tc=[[TestViewController alloc]init];
[tc refresh:val];

The first line creates and initializes a new instance tc of your TestViewController Class. This allows you to access its methods, but that doesn't mean you're accessing the instance you created initially or the data you originally assigned. This means your label lblDisp, along with the rest of your new TestViewController instance's properties, are nil.

Basically you can't use this strategy to pass data back and forth. See this SO post:

Passing Data between View Controllers

Lithu T.V · Answer 2 · 2013-07-24T11:50:39.873

The problem is pretty simple

  -(void)refresh:(NSString *)val{
        NSLog(@"Value of Val = %@",val);
        NSLog(@"TEXT CUrret VALUE %@",lblDisp.text);
        lblDisp.text=val;
    }

here after the log is printed, value is updated so change the code like this order in which label is updated first and then log the value

So use

  -(void)refresh:(NSString *)val{
        lblDisp.text=val;
        NSLog(@"Value of Val = %@",val);
        NSLog(@"TEXT CUrret VALUE %@",lblDisp.text);
    }

EDIT

Well the reason may be the lblDisp is loaded from nib and it is not a valid memory till the view is presented/pushed .And then only the method will have valid label instance and then only you can update it

UILabel is not updating with value from another class

TestViewController.h

TestViewController.m

ViewController.h

ViewController.m

Log

2 Answers2