ASync JSON REST call problem with MVVM

I am trying to implement the MVVM patter for my WP7 Silverlight app and I am running into a problem with the async JSON Rest call. I moved into my ViewModel class the following two methods that were on my WP7 app Page.

pu开发者_开发百科blic void FetchGames()
{            

    ObservableCollection<Game> G = new ObservableCollection<Game>();
    //REST call in here
    var webClient = new WebClient();
    Uri uri = new Uri("http://www.somewebsite.com/get/games/league/" + league);
    webClient.OpenReadCompleted += new OpenReadCompletedEventHandler(OpenReadCompletedGames);   
    webClient.OpenReadAsync(uri);
}

private void OpenReadCompletedGames(object sender, OpenReadCompletedEventArgs e)
{
    DataContractJsonSerializer ser = null;
    ser = new DataContractJsonSerializer(typeof(ObservableCollection<Game>));
    Games = ser.ReadObject(e.Result) as ObservableCollection<Game>;
    this.IsDataLoaded = true;

}

Now the problem is that because it is an async call the following code does not work. The following code is on my app Page.

protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
{
    base.OnNavigatedTo(e);
    if (NavigationContext.QueryString.TryGetValue("league", out league))
    {              

 try
 {
     App.gViewModel.league = league;
     App.gViewModel.FetchGames();
     if(App.gViewModel.IsDataLoaded)
     {
  lbTeams.ItemsSource = App.gViewModel.Games;
     }     
 }
 catch ()
 {                
     //error logging in here              
 }
    }

}

Stepping thru the code shows that FetchGames is called then hits the next line ( if(App.gViewModel.IsDataLoaded) ) before the async call is finished. So IsDataLoaded is always false and I cant bind the listbox on the page.

Doing a lot of googleing I have some possible solutions but I am unable convert them to my particular problem. One is like this and it has to do with continuation passing style'. I couldn't get it to work tho and would greatly appreciate some help.

Thanks!

void DoSomethingAsync( Action<string> callback ) { 
    HttpWebRequest req; // TODO: build your request 

    req.BeginGetResponse( result => { 
        // This anonymous function is a closure and has access  
        // to the containing (or enclosing) function. 
        var response = req.EndGetResponse( result ); 

        // Get the result string and call the callback 
        string resultString = null; // TODO: read from the stream 

        callback(resultString); 
    }, null ); 
} 

---

This can be resolved by moving

lbTeams.ItemsSource = App.gViewModel.Games;

to the end of the OpenReadCompletedGames method. You'll need to use the Dispatcher to update the UI from here.

Dispatcher.BeginInvoke( () => { lbTeams.ItemsSource = App.gViewModel.Games; } );