Question

I’m having a hard time getting my head around delegates. I tried putting together a simple example to teach myself, but it’s just not working.

My simple test has two classes. ClassA and ClassB. ClassA has the delegate definition. As well as a button that calls a method (-(void)buttonHasBeenPressed:(id)sender) that contains the delegate method call (-(void)buttonPressed). ClassB is set (I think) to listen to the ClassA delegate and when the -(void)buttonHasBeenPressed:(id)sender is called it writes out to the console.

Here’s the AppDelegate:

.h:

#import <UIKit/UIKit.h>

@interface DelegateTestAppDelegate : NSObject <UIApplicationDelegate> {

}

@property (nonatomic, retain) IBOutlet UIWindow *window;

@end

.m:

#import "DelegateTestAppDelegate.h"
#import "ClassA.h"

@implementation DelegateTestAppDelegate


@synthesize window=_window;

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    ClassA *classA = [[ClassA alloc] init];
    [self.window addSubview:[classA view]];

    // Override point for customization after application launch.
    [self.window makeKeyAndVisible];
    return YES;
}


- (void)dealloc
{
    [_window release];
    [super dealloc];
}

@end

Here’s ClassA:

.h:

#import <UIKit/UIKit.h>

@protocol ClassADelegate <NSObject>;
-(void)buttonPressed;
@end

@interface ClassA : UIViewController {

    id <ClassADelegate> delegate;
}

@property (retain) id <ClassADelegate> delegate;

-(void)buttonHasBeenPressed:(id)sender;

@end

.m:

#import "ClassA.h"

@implementation ClassA

@synthesize delegate;

-(id)init {

    return self;
}

- (id)initWithNibName:(NSString *)nibNameOrNil
               bundle:(NSBundle *)nibBundleOrNil {
    return [self init];
}

-(void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    UIButton *button = [UIButton buttonWithType:UIButtonTypeRoundedRect];
    [button addTarget:self action:@selector(buttonHasBeenPressed:) forControlEvents:UIControlEventTouchDown];
    [button setTitle:@"hola" forState:UIControlStateNormal];
    button.frame =CGRectMake(50, 200, 100, 20);
    [self.view addSubview:button];
}


-(void)buttonHasBeenPressed:(id)sender {
    [self.delegate buttonPressed];
    NSLog(@"button has been pressed");
}


@end

Here’s ClassB:

.h:

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


@interface ClassB : UIViewController <ClassADelegate> {

}

@end

.m:

#import "ClassB.h"

@implementation ClassB

-(id)init {


    return self;
}

- (id)initWithNibName:(NSString *)nibNameOrNil
               bundle:(NSBundle *)nibBundleOrNil {
    return [self init];
}


-(void)buttonPressed {
    NSLog(@"delegating!");
}

@end

The expected result is to have both “button has been pressed” and “delegating!” written to the console when clicking on the “hola” button.

Thanks for your help…
Shawn

Answer 1

Where is your ClassB’s instance? I can’t find it in your posted code.
Also, where do you set the instance into ClassA as its delegate at? I can’t find it neither.

If you are not doing this two things, your delegate will never be invoked.