How to wait for async calls to finish

Question

I am running three API calls using the Spotify API, in order to get the following data and store them into my Artist object:

1. The artist.
2. That artist's albums.
3. Tracks of each albums of the artist.

Once they are done, I would like to pass the object to a different view controller. It looks something like this: (If could post all the NSURLSession code in there if necessary)

func getArtist(artistName) {
 getAlbums(artistIdString)}

func getAlbums(artistIdString) {
 a = Album
 Artist.albumsArray.append(a)
 for album in Artist.albumsArray {
 getTracks(albumIdString) 
 }
}

func getTracks (albumIdString) {
 t = Track
 for album in Artist.albumsArray {
  if idString == albumIdString {
  album.append(t)
  }
 }
}

func passArtist {  
 DataStore.sharedInstance().Artist = Artist
}

Answer 1

In the callback function/block (or completion handler in Apple's term) of NSURLSession (or NSURLSessionDataTask), run a check function against the current Artist object and see if all the 3 required elements were there (and if so, make a call to passArtist).

No matter how the finishing order is, it will eventually call the passArtist once it's done fetching 3 elements if you did so.

Here's a simple example to demonstrate the idea in Objective-C (as OP requested):

- (void)getArtistDataWithRequest:(NSURLRequest *)request
{
    NSURLSession         *session = [NSURLSession sharedSession];
    NSURLSessionDataTask *task    = [session dataTaskWithRequest:request
                                               completionHandler:^(NSData        *data,
                                                                   NSURLResponse *response,
                                                                   NSError       *error)
                                     {
                                         if (error)
                                         {
                                             // Something went wrong, fallback.
                                             NSLog(@"An error occurred: %@", error.localizedDescription);

                                             // You could use some error handler e.g.:
                                             // [self connectionDidFailWithError:error];

                                             return;
                                         }
                                         else if (response)
                                         {
                                             // Seems working, moving forward.
                                             [self connectionDidReceiveSpotifyServerResponse:response
                                                                                     andData:data];
                                         }

                                     }];
    [task resume];
}

- (void)connectionDidReceiveSpotifyServerResponse:(NSURLResponse *)aResponse
                                          andData:(NSData *)aResponseData
{
    // Do what you want here with the data you get, either it's album, track,
    // or whatever.

    // Assuming your "Artist" property is named "myArtist", and the "Artist"
    // class has a method to process the data you get from Spotify API:
    [_myArtist handleArtistData:aResponseData];

    // Now check your Artist object's element with another method. e.g.:
    if ([_myArtist isLoadingComplete])
    {
        // Done loading the object, pass it to view controller.
    }

    // Not yet done, do nothing for now.
}

This is just one solution. There are many ways to achieve what you wanted once you get the idea.