How do I configure the MainViewModel

See more:

C#

Windows

XAML

MVVM

Win8

I'm developing a Windows 8 app and I'm fairly new to the MVVM and I'm stuck at setting up the MainViewModel. Here's my code so far:

My DataItem model class

public class DataItem : ObservableObject
{
  public const string AssetIDPropertyName = "AssetID";      
  public const string TitlePropertyName = "Title";
  public const string DescriptionPropertyName = "Description";
  public const string ImagePropertyName = "Image";
  private string assetID;
  private string title;
  private string description;
  private string image;

  public string AssetID
  {
    get { return assetID; }
    set { Set(AssetIDPropertyName, ref assetID, value); }
  }

  public string Title
  {
    get { return title; }
    set { Set(TitlePropertyName, ref title, value); }
  }      

  public string Description
  {
    get { return description; }
    set { Set(DescriptionPropertyName, ref description, value); }
  }

  public string Image
  {
    get { return image; }
    set { Set(ImagePropertyName, ref image, value); }
  }
}

My DataGroup model class

public class DataGroup : ObservableObject
{
  public const string TitlePropertyName = "Title";
  public const string ImagePropertyName = "Image";
  public const string ItemsPropertyName = "Items";

  private string title;
  private string image;
  private ObservableCollection<DataItem> items;

  public string Title
  {
    get { return title; }
    set { Set(TitlePropertyName, ref title, value); }
  }

  public string Image
  {
    get { return image; }
    set { Set(ImagePropertyName, ref image, value); }
  }

  public ObservableCollection<DataItem> Items
  {
    get { return items; }
    set { Set(ItemsPropertyName, ref items, value); }
  }

  public IEnumerable<DataItem> TopItems
  {
    get { return items.Take(6); }
  }

  public int VideoCount
  {
    get { return Items.Count; }
  }
}

My DataService class

public class DataSource : IDataSource
{
  private string baseUri = "http://www.mywebservice.com/W8/";
  private ObservableCollection<DataGroup> groups;

  public ObservableCollection<DataGroup> Groups
  {
    get { return groups; }
  }

  public async Task GetVideosAsync()
  {
    Task<DataGroup> featured = GetVideoAsync("featured");
    Task<DataGroup> popular = GetVideoAsync("popular");
    Task<DataGroup> recent = GetVideoAsync("recent");

    Groups.Add(await featured);
    Groups.Add(await popular);
    Groups.Add(await recent);
  }

  private async Task<DataGroup> GetVideoAsync(string groupType)
  {
    string serviceUri = string.Format("{0}/viewservice.php?listtype={1}", baseUri, groupType);
    HttpClient client = new HttpClient();

    // Read up to 1MB of data
    client.MaxResponseContentBufferSize = 1024 * 1024;

    HttpResponseMessage response = await client.GetAsync(serviceUri);

    if (response.IsSuccessStatusCode)
    {
      DataGroup group = new DataGroup();
      
      switch (groupType)
      {
        case "featured":
          group.Title = "Featured Content";
          group.Image = "Assets/Images/Kratos.jpg";
          break;
        case "popular":
          group.Title = "Most Popular";
          group.Image = "Assets/Images/Ratchet.jpg";
          break;
        case "recent":
          group.Title = "Most Recent";
          group.Image = "Assets/Images/Assassins.jpg";
          break;
      }

      var result = await response.Content.ReadAsStringAsync();
      var items = JsonArray.Parse(result);

      foreach (var item in items)
      {
        DataItem dataItem = new DataItem();

        dataItem.AssetID = item.GetObject()["assetid"] != null ? item.GetObject()["assetid"].GetString() : string.Empty;
        dataItem.Title = item.GetObject()["title"] != null ? item.GetObject()["title"].GetString() : string.Empty;
        dataItem.Description = item.GetObject()["description"] != null ? item.GetObject()["description"].GetString() : string.Empty;
        dataItem.Image = string.Format("Assets/thumbnails/large/{0}.png", item.GetObject()["assetid"].GetString());          

        group.Items.Add(dataItem);
      }

      return group;
    }

    return null;
  }
}

Right now I'm not sure how to proceed so that I can bind the data to a CollectionViewSource. This is what I've done so far but I get NullReferenceException error. Can someone point me to the right direction?

public class MainViewModel : ViewModelBase
{
  private ObservableCollection<DataGroup> videos;
  private IDataSource dataSource;

  public MainViewModel(IDataSource dataSource)
  {
    this.dataSource = dataSource;

    GetVideos(dataSource);
  }

  public ObservableCollection<DataGroup> Videos 
  {
    get { return videos; }
    set { Set("Videos", ref videos, value); } 
  }

  private async void GetVideos(IDataSource dataSource)
  {
    await dataSource.GetVideosAsync(); //NullReferenceException error here
  }
}