Question

I’m trying to understand how delegate pattern works. Below is some code that I tried, but the problem is that the delegate methods downloadStarted and downloadFinished are never invoked.

What I miss here ?

DownloaderDelegate.h

@protocol DownloaderDelegate <NSObject>
-(void)downloadStarted;
-(void)downloadFinished;
@end

Downloader.h

#import "DownloaderDelegate.h"

@interface Downloader : NSObject 

@property (nonatomic, retain) id<DownloaderDelegate>delegate;
-(void)fileIsDownloaded;
-(void)downloadFile;
@end

Downloader.m

@implementation Downloader
@synthesize delegate = _delegate;


-(void)downloadFile
{
    [[self delegate] downloadStarted];

    [NSTimer timerWithTimeInterval:5 
                            target:self 
                            selector:@selector(fileIsDownloaded) 
                            userInfo:nil 
                            repeats:NO];
}


-(void)fileIsDownloaded
{
    [[self delegate]downloadFinished];
}
@end

AppDelegate.m

    - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
    {
        ....
        Downloader *d = [[Downloader alloc]init];
        [d downloadFile];
        [d release];
        ....
    }

-(void)downloadStarted
{
    NSLog(@"Started");
}

-(void)downloadFinished
{
    NSLog(@"Finished");
}

Answer 1

Your AppDelegate needs to implement the DownloaderDelegate protocol:

@interface AppDelegate : NSObject <UIApplicationDelegate,DownloaderDelegate>

and then, when you instantiate the downloader, make the AppDelegate its delegate.

d.delegate = self;