在iPhone中的两个ViewController之间使用委托

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];
}

Answer 2

Hm, try the following in your changeLabel delegate method:

-(void) changeLabel:(NSString*)str{
    lbl.text = str;
    [lbl setNeedsDisplay];
}

EDIT:

Other than that: Your label is a IBOutlet, right? Did you connect the label in your xib with the IBOutlet property (ie lbl) correctly in interface builder with the files owner

Answer 3

If you want to do this by using delegates then change passdata like:

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

You can do this without delegates. Just create a object of ViewController in secondView and synthesize it like:

#import "ViewController.h"
@interface ViewController2nd : UIViewController
{

    IBOutlet UIButton *bttn;
    ViewController  *parent;

}

@property (nonatomic, assign) ViewController  *parent;

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

and in the passdata change it like

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

and change the bttnclicked like:

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

Answer 4

I wrote following for you!

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController

@property (strong, nonatomic) IBOutlet UILabel *label;

@end

#import "ViewController.h"
#import "ViewController2.h"

@interface ViewController () <ViewController2Delegate>

@end

@implementation ViewController

-(void)passdata:(NSString *)data
{
    self.label.text = data;
}

- (IBAction)buttonClicked:(id)sender {
    ViewController2 *v = [[ViewController2 alloc] initWithNibName:@"ViewController2" bundle:nil];
    v.delegate = self;
    [self presentViewController:v animated:YES completion:nil];
}

@end

#import <UIKit/UIKit.h>

@protocol ViewController2Delegate <NSObject>

- (void)passdata:(NSString*)data;

@end

@interface ViewController2 : UIViewController

@property(assign, nonatomic) id<ViewController2Delegate> delegate;

@end

#import "ViewController2.h"

@interface ViewController2 ()

@end

@implementation ViewController2

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

- (IBAction)buttonClicked:(id)sender {
    [self.delegate passdata:@"hello"];
}

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

@end

Answer 5

• You don't need to import the header "ViewController2nd.h" in the both ViewController.h and ViewController.m
• You declare the UILabel without property like, @property (strong, nonatomic).
• May be, You didn't properly map the UILabel to your .storyboard or .xib file.

• You forget to assign the second view controller delegate while calling the controller

   ViewController2nd *second = [[ViewController2nd alloc] initWithNibName:nil bundle:nil]; second.delegate = self; [self presentViewController:second animated:YES completion:NULL]; 

If anyone is using storyboard for viewcontrollers navigation.

    - (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
    {
        if ([segue.identifier hasPrefix:@"view_to_capture"]) {
            ViewController2nd *destination = segue.destinationViewController;
            destination.delegate = self;
        }
    }