Question

I am a newbie to iPhone development and have some basic questions to ask about protocols and delegates. I have two view controllers: view controller and viewcontroller2nd. I have UITextField in one of them and would like to type something (like a name) in it and in the viewcontroller2nd, I have a UILabel and i would like it to appear Hello, name when the UITextField is changed.

I am following this video: http://www.youtube.com/watch?v=odk-rr_mzUo to get the basic delegate to work in a single view controller.

I am using protocols to implement this:

SampleDelegate.h

#import <Foundation/Foundation.h>

@protocol ProcessDelegate <UITextFieldDelegate>
@optional
- (BOOL)textFieldShouldReturn:(UITextField *)textField;
@end

@interface SampleDelegate : NSObject
{
    id <ProcessDelegate> delegate;
}

@property (retain) id delegate;

@end

SampleDelegate.m

#import "SampleDelegate.h"

@implementation SampleDelegate

@synthesize delegate;

- (BOOL)textFieldShouldReturn:(UITextField *)textField{

    lbl.text = [NSString stringWithFormat:@"Hello, %@",txtField.text];
    [txtField resignFirstResponder];

}

@end

ViewController.h

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

@interface ViewController : UIViewController <ProcessDelegate>
{
    IBOutlet UITextField *txtField;
}

@end

Viewcontroller.m

#import "ViewController.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.
}

@end

ViewController2nd.h

#import <UIKit/UIKit.h>

@interface ViewController2nd : UIViewController <ProcessDelegate> {

    IBOutlet UILabel *lbl;
}



@end

and ViewController2nd.m is standard code from Xcode.

My question is how do i link my delegate function to my viewcontroller and viewcontroller2nd to get it working?

Pardon me if the question is stupid.. Need some guidance. Do point me any other mistakes that i am doing as well.. Thanks..

Answer 1

Your delegation is a bit… Off.

Firstly: Don’t override UIKit delegate methods through protocol inheritance. It’s pointless. Why not just make your class conform to the specified delegate in the first place?

@protocol ProcessDelegate //No more protocol inheritance!
 //...
@end

Secondly: When an object has defined a protocol, a valid instance of that object must be in use by its delegate (or at least passed to it). So, anything that wants to be the delegate of SampleDelegate (really a bad name for a class, by the way) would initialize a valid SampleDelegate object, and call -setDelegate: as though it were any other property.

//#import "SampleDelegate"
@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    //make this a property, so it isn't crushed when the function exits.
    SampleDelegate *myDelegateObject = [[SampleDelegate alloc]init];
    [myDelegateObject setDelegate:self];  //conform to the delegate
}

Thirdly: You don’t actually define any delegate methods! What’s the point of delegation if there’s nothing to delegate!l

@protocol ProcessDelegate 
 -(void)someMethod;
@end

Fourth, and most important: Never, ever, ever, ever use the retain, or strong storage specifiers with a delegate! Delegate objects are supposed to be weak or assign to prevent nasty retain cycles.

@property (assign, nomatomic) id delegate;