delegate for a singleton object

Question

I have a NSObject which is a singleton. Is there any issue of having a delegate for this singleton class? I am worried that it would fail for a singleton type.

Here's my scenario. I have a function (inside this singleton class) that does a async request to pull out a NSDictionary from an API. Basically when this request is done I want to notify a class that the request has finished.

Answer 1

No, a delegate wouldn't fail, but consider using NSNotificationCenter instead:

static NSString *const kMyClassNotificationName = @"myClassNotificationName";

// where you would call a delegate method (e.g. [self.delegate doSomething])
[[NSNotificationCenter defaultCenter] postNotificationName:kMyClassNotificationName object:self userInfo: /* dictionary containing variables to pass to the delegate */];

// where you would set up a delegate (e.g. [Singleton instance].delegate = self)
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(doSomething) name:kMyClassNotificationName object:[Singleton instance]];

Answer 2

You have basically three options:

• Use a delegate. A singelton is a objetct, so of couse it can have a delegate. If several objects whants to use it and needs to set themselves as delegates, you can reset them each time, but that might get hairy.

• Use notifications, as shown by Richard J. Ross III., but seriously: It seems to be strange to me, if you have a singleton, that needs to inform one delegate, but you'd use a broadcasting technology.

• use completion blocks, where the calling objects passes a block to the singleton, that gets executed, once the singleton fulfilled a task. See [NSURLConnection sendAsynchronousRequest:queue:completionHandler:] (ok, this is not a singleton, but a class method. The principle is the same),that uses one completion block, or the great AFNetworking, that uses a success and a failure block.
  From it's example codes:

  [[AFGowallaAPIClient sharedClient] getPath:urlString 
                                  parameters:mutableParameters 
                                     success:^(__unused AFHTTPRequestOperation 
                                               *operation, 
                                               id JSON) 
      {
          NSMutableArray *mutableRecords = [NSMutableArray array];
          for (NSDictionary *attributes in [JSON valueForKeyPath:@"spots"]) {
              Spot *spot = [[[Spot alloc] initWithAttributes:attributes] autorelease];
              [mutableRecords addObject:spot];
          }
  
          if (block) {
              block([NSArray arrayWithArray:mutableRecords]);
          }
      } failure:^(__unused AFHTTPRequestOperation *operation, NSError *error) {
          if (block) {
              block([NSArray array]);
          }
  }];