Question

On iOS, how do I create a delegate (user defined)?

Answer 1

First define a declare a delegate like this –

@protocol IconDownloaderDelegate;

Then create a delegate object like this –

@interface IconDownloader : NSObject
{
    NSIndexPath *indexPathInTableView;
    id <IconDownloaderDelegate> delegate;
    NSMutableData *activeDownload;
    NSURLConnection *imageConnection;
}

Declare a property for it –

@property (nonatomic, assign) id <IconDownloaderDelegate> delegate;

Define it –

@protocol IconDownloaderDelegate 

- (void)appImageDidLoad:(NSIndexPath *)indexPath;

@end

Then you can call methods on this delegate –

[delegate appImageDidLoad:self.indexPathInTableView];

Here is the complete source code of the image downloader class –

.h file –

@class AppRecord;
@class RootViewController;

@protocol IconDownloaderDelegate;

@interface IconDownloader : NSObject
{
    AppRecord *appRecord;
    NSIndexPath *indexPathInTableView;
    id <IconDownloaderDelegate> delegate;

    NSMutableData *activeDownload;
    NSURLConnection *imageConnection;
}

@property (nonatomic, retain) AppRecord *appRecord;
@property (nonatomic, retain) NSIndexPath *indexPathInTableView;
@property (nonatomic, assign) id <IconDownloaderDelegate> delegate;

@property (nonatomic, retain) NSMutableData *activeDownload;
@property (nonatomic, retain) NSURLConnection *imageConnection;

- (void)startDownload;
- (void)cancelDownload;

@end

@protocol IconDownloaderDelegate 

- (void)appImageDidLoad:(NSIndexPath *)indexPath;

@end

.m file –

#import "IconDownloader.h"
#import "MixtapeInfo.h"

#define kAppIconHeight 48
#define TMP NSTemporaryDirectory()

@implementation IconDownloader

@synthesize appRecord;
@synthesize indexPathInTableView;
@synthesize delegate;
@synthesize activeDownload;
@synthesize imageConnection;

#pragma mark

- (void)dealloc
{
    [appRecord release];
    [indexPathInTableView release];

    [activeDownload release];

    [imageConnection cancel];
    [imageConnection release];

    [super dealloc];
}

- (void)startDownload
{
    self.activeDownload = [NSMutableData data];

    // alloc+init and start an NSURLConnection; release on completion/failure
    NSURLConnection *conn = [[NSURLConnection alloc] initWithRequest:
                             [NSURLRequest requestWithURL:
                              [NSURL URLWithString:appRecord.mixtape_image]] delegate:self];
    self.imageConnection = conn;
    [conn release];

}

- (void)cancelDownload
{
    [self.imageConnection cancel];
    self.imageConnection = nil;
    self.activeDownload = nil;
}


#pragma mark -
#pragma mark Download support (NSURLConnectionDelegate)

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
    [self.activeDownload appendData:data];
}

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
    // Clear the activeDownload property to allow later attempts
    self.activeDownload = nil;

    // Release the connection now that it's finished
    self.imageConnection = nil;
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{   
    // Set appIcon and clear temporary data/image
    UIImage *image = [[UIImage alloc] initWithData:self.activeDownload];
    self.appRecord.mixtape_image_obj = image;

    self.activeDownload = nil;
    [image release];

    // Release the connection now that it's finished
    self.imageConnection = nil;

    // call our delegate and tell it that our icon is ready for display
    [delegate appImageDidLoad:self.indexPathInTableView];
}

@end

and here is how we use it –

#import "IconDownloader.h"

@interface RootViewController : UITableViewController <UIScrollViewDelegate, IconDownloaderDelegate>
{
    NSArray *entries;   // the main data model for our UITableView
    NSMutableDictionary *imageDownloadsInProgress;  // the set of IconDownloader objects for each app
}

in .m file –

- (void)startIconDownload:(AppRecord *)appRecord forIndexPath:(NSIndexPath *)indexPath
{
    IconDownloader *iconDownloader = [imageDownloadsInProgress objectForKey:indexPath];
    if (iconDownloader == nil) 
    {
        iconDownloader = [[IconDownloader alloc] init];
        iconDownloader.appRecord = appRecord;
        iconDownloader.indexPathInTableView = indexPath;
        iconDownloader.delegate = self;
        [imageDownloadsInProgress setObject:iconDownloader forKey:indexPath];
        [iconDownloader startDownload];
        [iconDownloader release];   
    }
}

here is delegate gets called automatically –

// called by our ImageDownloader when an icon is ready to be displayed
- (void)appImageDidLoad:(NSIndexPath *)indexPath
{
    IconDownloader *iconDownloader = [imageDownloadsInProgress objectForKey:indexPath];
    if (iconDownloader != nil)
    {
        UITableViewCell *cell = [self.tableView cellForRowAtIndexPath:iconDownloader.indexPathInTableView];

        // Display the newly loaded image
        cell.imageView.image = iconDownloader.appRecord.appIcon;
    }
}