asynchronous UI update from ViewModel in WPF

Question

I am having a problem with getting data from db and showing in UI asynchronously. I am using MVVM light, when I click the button, action is triggered in ViewModel:

    private void SearchQuery(string query)
    {
        _redisModel.GetFriendsListAsync(query);
    } 

At some point GetFriendsListCompleted is called by background thread notifing viewmodel that job is done. At this point I need to update ListBox ItemSource. But when I try to update is I get “The calling thread cannot access this object because a different thread owns it” I have tried Dispatcher.CurrentDispatcher.Invoke(),App.Current.Dispatcher.Invoke() and different magic, but it still doesn’t work.

I tried to give UI dispatcher to ViewModel and then call it from there - didn't work.

private string filterText = string.Empty;
    public string FilterText
    {
        get { return filterText; }
        set
        {
            filterText = value;
            this.RaisePropertyChanged(() => this.FilterText);

            this.FriendsList.View.Refresh(); // Here where exception is happening.
        }
    }

I tried to change this line to

Dispatcher.Invoke(DispatcherPriority.Normal, new Action( () =>this.FriendsList.View.Refresh())); - still the same.

I am using Telerik ListBox to display items. FriendList is CollectionViewSource(http://www.telerik.com/help/wpf/radlistbox-overview.html). It works when I use Telerik example from WPF Control Examples. Problems start to occur when I use my async methods. Type of view is System.ComponentModel.ICollectionView it is used for Filtering and Grouping.

I have also tried to just assign ObservableCollection to Items property of the ListBox and it doesn't work either.

A bit more details on how _redisModel.GetFriendsListAsync works: In the end(after all chain of calls) it ends up here:

public GetAsyncResult(Func<T> workToBeDone, Action<IAsyncResult> cbMethod, Object state)
{
   _cbMethod = cbMethod;
   _state = state;
   QueueWorkOnThreadPool(workToBeDone);
}

ThreadPool.QueueUserWorkItem(state =>
{
  try
  {
     _result = workToBeDone();
  }
  catch (Exception ex)
  {
       _exception = ex;
  }
  finally
  {
     UpdateStatusToComplete(); //1 and 2 
     NotifyCallbackWhenAvailable(); //3 callback invocation 
  }
 });

In viewmodel I have method:

private void GetFriendsListCompleted(object sender, ResultsArgs<Friend> e)
    {
        if (!e.HasError)
        {
            var curr = e.Results;
            if (curr != null)
            {
                this.FriendsList= new CollectionViewSource();

                this.FriendsList.Source = list;
                this.FriendsList.Filter += this.FriendFilter;
                FilterText = "";

                Dispatcher.Invoke(DispatcherPriority.Normal, new Action(

                        () => this.FriendsList.View.Refresh()));
            }
    }

Can anybody please help me with this ? Thank you

Answer 1

You haven't shown any of the code that's actually running on the background thread on completion but I'm guessing that in it you're creating a collection object that you're then trying to assign to your CollectionView. When the CV tries to update (on the UI thread) from your Refresh call it would then try to use the collection that's owned by the other thread.

If you include the relevant code it would be easier to say for sure.

Answer 2

You are creating CollectionViewSource in one thread and refreshing that in another thread (dispatcher thread). Update your GetFriendsListCompleted to

private void GetFriendsListCompleted(object sender, ResultsArgs<Friend> e)
{
    if (!e.HasError)
    {
        var curr = e.Results;
        if (curr != null)
        {
            Dispatcher.Invoke(DispatcherPriority.Normal, new Action(
                    () => {
                     this.FriendsList= new CollectionViewSource();
                     this.FriendsList.Source = list;
                     this.FriendsList.Filter += this.FriendFilter;
                     FilterText = "";
                     this.FriendsList.View.Refresh();
                     }));
        }
    }
}