IOS application - define protocol in separate file

Question

I have defined a protocol in a separate file (myProtocol.h). Here is the code for it:

#import <Foundation/Foundation.h>

@protocol myProtocol <NSObject>
    -(void) loadDataComplete;
@end

Now I want to call this method so I have done the following code:

firstViewController.h:

#import "myProtocol.h"

@interface firstViewController : UIViewController{
    id <myProtocol> delegate;
}
@property (retain) id delegate;
-(void) mymethod;

firstViewController.m

@implementation firstViewController
@synthesize delegate;

- (void)viewDidLoad {
    [self mymethod];
}

-(void) mymethod {
    //some code here...
    [delegate loadDataComplete];
}

I have another file where the protocol is also utilized:

secondViewController.h:

#import "myProtocol.h"
@interface secondViewController : UIViewController<myProtocol>{
}

secondViewController.m:

-(void) loadDataComplete{
    NSLog(@"loadDataComplete called");
}

but my secondViewController is not calling the protocol methad. Why is it so? Any suggestion will be appreciated.

Are you actually creating an object and assigning it as a delegate? Also - having a strong reference to a delegate is not usual. Finally, class names should start with a capital letter. — Abizern
– Abizern, Commented Jul 14, 2012 at 10:33
do you set your delegate , what's the value of self.delegate into mymethod message ? i have the habit to use "self." for property access — divol
– divol, Commented Jul 14, 2012 at 10:45

Lorenzo B · Accepted Answer · 2012-07-14 13:25:50Z

First, as @Abizern suggested, try to reformat your code a little bit. Use capital letter for classes. Said this here the solution for your answer.

This is the protocol. I would name it like FirstViewControllerDelegate since the class that implements the object is a delegate for FirstViewController.

#import <Foundation/Foundation.h>

@protocol MyProtocol <NSObject>

- (void)doSomething;

@end

This is SecondViewController.

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

@interface SecondViewController : UIViewController <MyProtocol>

@end

@implementation SecondViewController

// other code here...

- (void)doSomething
{
    NSLog(@"Hello FirstViewController");
}

@end

This is FirstViewController.

#import <UIKit/UIKit.h>

@interface FirstViewController : UIViewController

// it coud be better to declare these properties within a class extension but for the sake of simplicity you could leave here
// the important thing is to not declare the delegate prop with a strong/retain property but with a weak/assign one, otherwise you can create cycle
@property (nonatomic, strong) SecondViewController* childController;
@property (nonatomic, weak) id<MyProtocol> delegate;

@end

@implementation FirstViewController

// other code here...

- (void)viewDidLoad
{
    [super viewDidLoad];

    self.childController = [[SecondViewController alloc] init];
    self.delegate = self.childController; // here the central point

    // be sure your delegate (SecondViewController) responds to doSomething method
    if(![self.delegate respondsToSelector:@selector(doSomething)]) {

        NSLog(@"delegate cannot respond");
    } else {

        NSLog(@"delegate can respond");
        [self.delegate doSomething];
    }    
}

@end

For the sake of completeness, be sure to understand the delegate pattern means. Apple doc is your friend. You could take a look at the-basics-of-protocols-and-delegates to have a basic intro on the argument. Furthermore, SO search allows you to find a lot of answers on the topic.

Hope that helps.

Obviously, the if-else pattern can be moved in myMethod if you want.
Thank you Flex_Addicted. Amazing answer. little bit correction in code self.delegate = self.childController; instead of self.delegate = child; thanks again.
@ViralNarshana You're welcome! Sorry for the mistake. I fixed it :)
I am confused. Shouldn't the delegate be on the SecondViewController instead of the FirstViewController?

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