ObjC delegate methods never gets called

Question

I am creating instances of a class FlickrImage parsing a Flickr API photos response. The class has a method getLocation that does another API call to get the geolocation:

NSLog(@"getting location for %i",self.ID);
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
OFFlickrAPIRequest *flickrAPIRequest = [[OFFlickrAPIRequest alloc] initWithAPIContext[appDelegate sharedDelegate].flickrAPIContext];
[flickrAPIRequest setDelegate:self];

NSString *flickrAPIMethodToCall = @"flickr.photos.geo.getLocation";
NSDictionary *requestArguments = [[NSDictionary alloc] initWithObjectsAndKeys:FLICKR_API_KEY,@"api_key",self.ID,@"photo_id",nil];

[flickrAPIRequest callAPIMethodWithGET:flickrAPIMethodToCall arguments:requestArguments];
[pool release];

I have implemented the callback method that would catch the response from the API and update the FlickrImage instance with the geolocation data - but it never gets called. Here's where the instances get created:

NSDictionary *photosDictionary = [inResponseDictionary valueForKeyPath:@"photos.photo"];
NSDictionary *photoDictionary;
FlickrImage *flickrImage;
for (photoDictionary in photosDictionary) {
    flickrImage = [[FlickrImage alloc] init];
    flickrImage.thumbnailURL = [[appDelegate sharedDelegate].flickrAPIContext photoSourceURLFromDictionary:photoDictionary size:OFFlickrThumbnailSize];
    flickrImage.hasLocation = TRUE; // TODO this is actually to be determined...
    flickrImage.ID = [NSString stringWithFormat:@"%@",[photoDictionary valueForKeyPath:@"id"]];
    flickrImage.owner = [photoDictionary valueForKeyPath:@"owner"];
    flickrImage.title = [photoDictionary valueForKeyPath:@"title"];
    [self.flickrImages addObject:[flickrImage retain]];
    [flickrImage release];
    [photoDictionary release];
}

The retain is there because I thought it might help solve this but it doesn't - and doesn't the NSMutableArray (flickrImages is a NSMutableArray) retain its members anyway?

EDIT I should add that the getLocation method (first code snippet) is launched in a thread: [NSThread detachNewThreadSelector:@selector(getLocation) toTarget:self withObject:nil];

Answer 1

Your delegate method is never being called because the request is never being made. When you call callAPIMethodWithGET:, it sets up communications to run asynchronously on the current thread's run loop, then returns immediately. That way you can safely call it on the main thread without blocking.

Because you are calling the method from a thread you created yourself, it does not see the main run loop, but the run loop for your new thread. However, because you never execute the run loop, the messages are never sent, a response is never received, and your delegate is never called.

You could fix this by calling [[NSRunLoop currentRunLoop] run] in your new thread. That will let the work happen. But in this case would be easier to never detach a new thread in the first place. Your program won't block, and you won't have to worry about your delegate method needing to be reentrant.