Question

I am trying to send a simple string from One viewcontroller to another viewcontroller using delegate but my delegate is not working. I am using storyboard and dont want to use segue to pass data. Here is my simple code

ViewControllerA.h

#import <UIKit/UIKit.h>

@class ViewControllerA;

@protocol viewControllerADelegate <NSObject>

-(void) addItem: (ViewControllerA *)controller data:(NSString *)item;

@end

@interface ViewControllerA : UIViewController{
 __weak id <viewControllerADelegate> delegate;
}
- (IBAction)buttonPressed:(id)sender;

@property (weak) id<viewControllerADelegate> delegate;
@end

ViewControllerA.m

#import "ViewControllerA.h"

@interface ViewControllerA ()

@end

@implementation ViewControllerA
@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
{
[super viewDidUnload];
// Release any retained subviews of the main view.
}

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

 - (IBAction)buttonPressed:(id)sender {
[self.delegate addItem:self data:@"this is data"];

 }
 @end

ViewControllerB.h

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

@interface ViewControllerB : UIViewController<viewControllerADelegate>

@end

ViewControllerB.m

#import "ViewControllerB.h"
#import "ViewControllerA.h"

@interface ViewControllerB ()

@end

@implementation ViewControllerB

- (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.
UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"MainStoryboard" bundle:nil];

ViewControllerA *vca = [storyboard instantiateViewControllerWithIdentifier:@"A"];
[vca setDelegate:self];

 }

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

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
 {
return (interfaceOrientation == UIInterfaceOrientationPortrait);
 }
-(void) addItem: (ViewControllerA *)controller data:(NSString *)item{
NSLog(@"delegate function called");
 }

 @end

Am I missing something silly ? The delegate method -(void) addItem: (ViewControllerA *)controller data:(NSString *)item is not triggered.

Answer 1

The flaw in your design is to try sending data with delegates to an object you create yourself. You certainly don’t need delegates for that as you can do it with simple properties. myViewControllerBObject.dataProperty = @"some data"]; is simply enough. What delegation is usually used for is send data back to the controller that created the current object or to any other controller somewhere in your app.

For example, move the delegate code in ViewControllerA to ViewControllerB and add a UIButton and an IBAction to ViewControllerB that will send the data back to ViewControllerA. Here it goes:

ViewControllerB.h

#import <UIKit/UIKit.h>

@class ViewControllerB;

@protocol viewControllerBDelegate <NSObject>

-(void) sendDataBack: (ViewControllerB *)controller data:(NSString *)item;

@end

@interface ViewControllerB : UIViewController
{
   __unsafe_unretained id <viewControllerBDelegate> delegate;
}

@property (nonatomic, assign) id<viewControllerBDelegate> delegate;
- (IBAction)buttonTouch:(id)sender;

@end

ViewControllerB.m

#import "ViewControllerB.h"

@interface ViewControllerB ()

@end

@implementation ViewControllerB
@synthesize delegate;
....
- (IBAction)buttonTouch:(id)sender {
    [self.delegate sendDataBack:self data:@"my data"];
}
@end

And in ViewControllerA, you first need to get a hold of that pushed ViewControllerB object via segues. Then implement the delegate method.

ViewControllerA.h

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

@interface ViewControllerA : UIViewController <viewControllerBDelegate>
- (IBAction)buttonPressed:(id)sender;

@end

ViewControllerA.m

#import "ViewControllerA.h"

@interface ViewControllerA ()

@end

@implementation ViewControllerA
......
- (IBAction)buttonPressed:(id)sender {

}
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    NSLog(@"id %@", [segue destinationViewController]);
    ViewControllerB *vcb = (ViewControllerB *)[segue destinationViewController];
    vcb.delegate = self;  //key point
}
- (void)sendDataBack:(ViewControllerB *)controller data:(NSString *)item
{
    NSLog(@"Here is the data %@ and the controller it comes from %@", item, controller);
}
@end

Hope this will help you get a better understanding of communication with delegates.