See http://blog.logichigh.com/2010/09/12/cocoa-blocks

gistfile1.m

+ (void)asyncRequest:(NSURLRequest *)request success:(void(^)(NSData *,NSURLResponse *))successBlock_ failure:(void(^)(NSData *,NSError *))failureBlock_  
{  
  [[failureBlock_ copy] autorelease];
  [[successBlock_ copy] autorelease];
  [NSThread detachNewThreadSelector:@selector(backgroundSync:) toTarget:[NSURLConnection class]  
                         withObject:[NSDictionary dictionaryWithObjectsAndKeys:  
                                     request,@"request",  
                                     successBlock_,@"success",  
                                     failureBlock_,@"failure",  
                                     nil]];  
}  


// See http://blog.logichigh.com/2010/09/12/cocoa-blocks/

  [NSURLConnection asyncRequest:theRequest  
                        success:^(NSData *data, NSURLResponse *response) {  
                          [self performSelectorOnMainThread:@selector(fadeOut) withObject:nil waitUntilDone:YES];
                        }  
                        failure:^(NSData *data, NSError *error) {  
                          NSLog(@"Error! %@",[error localizedDescription]);  
                        }];
}

- (void)fadeOut {
  NSLog(@"yo");
}