Unit Testing with NSURLSession for OCMock

I have a networking class called: ITunesAlbumDataDownloader

@implementation AlbumDataDownloader

- (void)downloadDataWithURLString:(NSString *)urlString
                completionHandler:(void (^)(NSArray *, NSError *))completionHandler
{        
    NSURLSession *session = [NSURLSession sharedSession];

    NSURLSessionDataTask *dataTask = [session dataTaskWithURL:[NSURL URLWithString:urlString]
                                            completionHandler:^(NSData *data, NSURLResponse *response, NSError *error)
    {        
        NSArray *albumsArray = [self parseJSONData:data];

        completionHandler(albumsArray, error);
    }];

    [dataTask resume];
}


    - (NSArray *)parseJSONData:(NSData *)data {

        NSMutableArray *albums = [[NSMutableArray alloc] init];

...
...

        // Return the array
        return [NSArray arrayWithArray:albums];
    }

    @end

and i need to create a Unit Test for this which does the following:

• The NSURLSession dataTaskWithRequest:completionHandler: response is mocked to contain the fake JSON data i have:

// Expected JSON response

NSData *jsonResponse = [self sampleJSONData];

• The returned array from the public method downloadDataWithURLString:completionHandler: response should contain all the albums and nil error.

Other points to bare in mind is that i need to mock NSURLSession with the fake JSON data "jsonResponse" to the downloadDataWithURLString:completionHandler: method WITHOUT invoking an actual network request.

I have tried various different things but i just can not work it out, i think its the combination of faking the request and the blocks which is really confusing me.

Here is two examples of my test method that i tried (i actually tried a lot of other ways also but this is what i have remaining right now):

- (void)testValidJSONResponseGivesAlbumsAndNilError {

    // Given a valid JSON response containing albums and an AlbumDataDownloaderTests instance

    // Expected JSON response
    NSData *jsonResponse = [self sampleJSONDataWithAlbums];

    id myMock = [OCMockObject mockForClass:[NSURLSession class]];

    [[myMock expect] dataTaskWithRequest:OCMOCK_ANY
                       completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error)
    {

    }];

    [myMock verify];
}

and

- (void)testValidJSONResponseGivesAlbumsAndNilError {

                // Given a valid JSON response containing albums and an AlbumDataDownloaderTests instance

                // Expected JSON response
                NSData *jsonResponse = [self sampleJSONDataWithAlbums];

            id myMock = [OCMockObject mockForClass:[AlbumDataDownloader class]];

            [[[myMock stub] andReturn:jsonResponse] downloadDataWithURLString:OCMOCK_ANY
                                                            completionHandler:^(NSArray *response, NSError *error)
            {

            }];

            [myMock verify];
            }
        }

I have a feeling that in both instances I'm probably way off the mark :(

I would really appreciate some help with this.

Thanks.