problem with delegate of object which is delegate of NSURLConnection - iphone

Question

I've a class PictureDownloader for the purpose of asynchronously loading images from a server. It assigns itself as a delegate of NSURLConnection and as such, is retained by NSURLConnection. I create several of those PictureDownloader in a DetailViewController to fetch the corresponding images, so the DetailViewController is a delegate of each PictureDownloader.
When the user leaves the DetailViewController, all remaining downloads are cancelled, however sometimes it seems to be the case, that a PictureDownloader has finished loading an image (connectionDidFinishedLoading called) before the connection was cancelled, but the DetailViewController doesn't exist anymore (but the PictureDownloader does, because it's retained by NSURLConnection), so the call
[self.delegate didLoadPictureWithID:self.ID];
inside PictureDownloader will give an EXC_BAD_ACCESS or sometimes a "unrecognized selector sent to instance".
Here are the relevant parts of the source code:
creation of the PictureDownloader inside the DetailViewController
- (void)startPictureDownload:(Picture *)pic withPictureId:(NSString *)pId forID:(int)ID
{
PictureDownloader *downloader = [self.downloadsInProgress objectForKey:[NSNumber numberWithInt:ID]];
if(!downloader)
{
downloader = [[PictureDownloader alloc] init];
downloader.picture = pic;
downloader.pictureId = pId;
downloader.ID = ID;
downloader.delegate = self;
[self.downloadsInProgress setObject:downloader forKey:[NSNumber numberWithInt:ID]];
[downloader startDownload];
[downloader release];
}
}
canceling the downloads (called when the DetailViewController returns to the overview)
- (void)cancelAllDownloads
{
[self.downloadsInProgress enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop){
[obj cancelDownload];
}];
}
delegate method which is called when PictureDownloader finished loading
- (void)didLoadPictureWithID:(int)dID;
{
PictureDownloader *downloader = [self.downloadsInProgress objectForKey:[NSNumber numberWithInt:dID]];
if(downloader)
{
UIImageView *imageView = (UIImageView *)[self.view viewWithTag:dID];
imageView.image = [UIImage imageWithData:downloader.imageData];
[self.downloadsInProgress removeObjectForKey:[NSNumber numberWithInt:dID]];
}
}
cancelDownload method inside PictureDownloader
- (void)cancelDownload
{
[self.imageConnection cancel];
self.imageConnection = nil;
self.imageData = nil;
}
connectionDidFinishedLoading inside PictureDownloader
- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
if(self.picture)
{
self.picture.data = self.imageData;
NSError *error = nil;
[self.picture.managedObjectContext save:&error];
}
if(self.delegate != nil && [self.delegate respondsToSelector:#selector(didLoadPictureWithID:)] ) //place of failure
[self.delegate didLoadPictureWithID:self.ID];
self.imageData = nil;
self.imageConnection = nil;
}
Can someone give me a hint, how I can deal with this problem?
Help is much appreciated.

Answer 1

To avoid situations like this, I usually add a check like this at the top of connectionDidFinishLoading: and other NSURLConnection delegate methods:
if (connection != self.imageConnection) return;
As another option, you could set the delegate on each PictureDownloader to nil as you cancel it in cancelAllDownloads. Or you could set self.delegate = nil in cancelDownload.

Answer 2

You should check for the existence of the delegate object (and ideally the method/selector) before you attempt to make the call.
For example:
if(self.delegate && [[self.delegate] respondsToSelector:#selector(didLoadPictureWithID:)]) {
...
}
By doing this, you'll ensure that your not attempting to call a delegate that's no longer there. For more information on the respondsToSelector method, see the NSObject Protocol Reference.

Answer 3

When your DetailViewController goes out of scope - dealloc -, set the PictureDownloader's delegate property to nil as well.
Your issue is interesting in that the NSUrlConnection's delegate can't be set to nil in the same way. e.g. When you're PictureDownloader is de-alloced. All you can do is cancel the NSUrlConnection.
The NSURLConnection docs say the following:
Unless a NSURLConnection receives a cancel message, the delegate will receive one and only one of connectionDidFinishLoading:, or connection:didFailWithError: message, but never both. In addition, once either of messages are sent, the delegate will receive no further messages for the given NSURLConnection.
Indicating that the you can verify that the delegate will not be called back after the above messages are received.