Loading the list of items asynchronously in a WPF listbox using Dispatcher

Question

Hi All,

I am working on creating a WPF solution which uses MVVM pattern to load searched items in a search control asynchronously. The search control which is a WPF usercontrol is created with a textbox to enter search text and search button and a hidden listbox which would be visible when it loads the searched items list in it. This user control is in turn embedded into another WPF view which has a treeview of certain items. This view has a view model in which the logic to load the searched items of the tree view would be loaded in the search control. All the while, this has been happening synchronously without the use of any Dispatcher call. But, after a change request, I would like to make this happen asynchronously in a different thread using Dispatcher.

Could anyone please let me know how to get handle of the Dispatcher of the Search control in the view model class so as to call BeginInvoke on it using MVVM pattern wherein my View model is not aware of the view? Any clue would be highly appreciated.

public ObservableCollection<Details> CatalogSearchResults { get; private set; }

private void ExecuteSearchCommand(object parameter)
    {
        CatalogSearchResults.Clear();
        if (string.IsNullOrEmpty(parameter.ToString())) return;

        searchtext = (string)parameter;
        searchtext.Trim();

        SetSearchResults();
    }

private void SetSearchResults() 
    { 
    BackgroundWorker bw = new BackgroundWorker(); 

    bw.DoWork += LoadResults; 
    bw.RunWorkerCompleted += this.LoadResultsCompleted; 

    bw.RunWorkerAsync(); 
    } 

private void LoadResults(object sender, DoWorkEventArgs args) 
{ 
        IsSearchInProgress = true;
        foreach (var category in _rootCategory.Recurse(FindChildren))
        {
            if (category.CommentDetails != null)
            {
                //limitation - there is no direct way to add range to observable collection.
                //Using linq query would result in two loops rather than one.
                foreach (var node in category.Details)
                {
                    if (node.Name.IndexOf(searchtext, StringComparison.CurrentCultureIgnoreCase) >= 0
                        || node.PrecannedText.IndexOf(searchtext,            StringComparison.CurrentCultureIgnoreCase) >= 0)
                    {
                        Application.Current.Dispatcher.BeginInvoke(DispatcherPriority.Normal,
                            (ThreadStart)delegate { CatalogSearchResults.Add(node); }); 
                          Thread.Sleep(100); 
                    }
                }
            }
        }
        IsSearchInProgress = false;
}

In the xaml, I am biding the Items property of the Search control to the CatalogSearchResults:

 <ctrl:SearchControl x:Name="Ctrl" Grid.RowSpan="2" HorizontalAlignment="Stretch" VerticalAlignment="Top"   ToolTip="Search" Command="{Binding SearchCommand}"   Grid.ColumnSpan="3"                                                                             
            CommandParameter="{Binding Text, RelativeSource={RelativeSource Self}}"                                                                                
            Items ="{Binding CatalogSearchResults}" > </ctrl:SearchControl>

Thanks, Sowmya

Answer 1

It depends on a lot of factors (and your description is a bit confusing), but I've given a lengthy answer here that may shed some light on the matter. Basically, using the dispatcher alone will not automatically make the code multi-threaded; you'll need some real multi-threading mechanism like BackgroundWorker or the Task Parallel Library. Depending on how you have things set up and on exactly what you do in the other thread, you may indeed need to invoke some actions on the dispatcher thread - however BackgroundWorker does this automatically in most cases so I'd go with that for simple things. The Task Parallel Library also has special handling for the dispatcher, you should find more info on that on MSDN or any TPL tutorial.

The best advice I'd give if you didn't deal heavily with multi-threading until now is to gather as much information as possible on it, because, as it has been said countless times until now, multi-threading is hard! :)

Answer 2

All views in the application have the same dispatcher, you can access it with Application.Current.Dispatcher.

But anyway, you don't need the dispatcher to perform operations on a worker thread. You only need it to perform actions on the UI, because UI elements can only be accessed from the UI thread. But even then, you usually don't need to explicitly manipulate the dispatcher. You can update a property of your ViewModel from the worker thread, controls bound to this property will be updated alright, because the PropertyChanged event is automatically marshalled to the UI dispatcher.

What doesn't work is modifying an bound ObservableCollection<T> from a worker thread: you need to do it from the UI thread using Dispatcher.Invoke. You can also use a specialized ObservableCollection<T> that raises event on the UI thread.

Answer 3

Modify as necessary. 'Items' is just an observableCollection of strings exposed from the VM

    private void SetSearchResults()
    {
        BackgroundWorker bw = new BackgroundWorker();

        bw.DoWork += LoadResults;
        bw.RunWorkerCompleted += this.LoadResultsCompleted;

        bw.RunWorkerAsync();
    }

    private void LoadResultsCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
    }

    private void LoadResults(object sender, DoWorkEventArgs args)
    {
        List<string> results = GetResults();

        foreach (string result in results)
        {
             Application.Current.Dispatcher.Invoke(
                    DispatcherPriority.Normal, (ThreadStart)delegate { Items.Add(result); } //Dont worry about access to modified closure in this case
             Thread.Sleep(100);
        }
    }

In XAML

<ListBox ItemsSource={Binding Items}/>

Answer 4

Here's a simple implementation showing how to use BackgroundWorker to update objects on the UI thread while DoWork is running - in this example, there's a ListBox in the UI that's bound to FilteredItems, and ItemsSource is a property of the UserControl of type IEnumerable:

    FilteredItems = new ObservableCollection<object>();
    BackgroundWorker bw = new BackgroundWorker();
    bw.WorkerReportsProgress = true;
    bw.DoWork += bw_DoWork;
    bw.RunWorkerCompleted += bw_RunWorkerCompleted;
    bw.ProgressChanged += bw_ProgressChanged;
    bw.RunWorkerAsync();

    private void bw_DoWork(object sender, DoWorkEventArgs e)
    {
        BackgroundWorker bw = (BackgroundWorker) sender;
        var result = ItemsSource
           .OfType<object>()
           .Where(x => x.ToString().Contains(_FilterText));
        foreach (object o in result)
        {
            // Pass each object found to bw_ProgressChanged in the UserState argument.
            // This updates the UI as each item is found.
            bw.ReportProgress(0, o);
        }
    }

    void bw_ProgressChanged(object sender, ProgressChangedEventArgs e)
    {
        // FilteredItems is bound to the UI, but it's OK to update it here because
        // the ProgressChanged event handler runs on the UI thread.
        FilteredItems.Add(e.UserState);
    }

    private void bw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        if (e.Error != null)
        {
            MessageBox.Show(e.Error.Message);
        }
    }

Note that calling ReportProgress every time you find an item is pretty inefficient, as you're marshalling every item found across threads with an Invoke call. Depending on how long the filtering is actually taking, it may be better to accumulate a bunch of results and pass a List<object> to bw_ReportProgress instead of just a single object.