Proper way to use CollectionViewSource in ViewModel

Question

I used Drag and Drop to bind Data Source object (a DB model) to DataGrid (basically following this example in Entity Framework Databinding with WPF.

Everything works fine with this implementation.

XAML

<Window.Resources>    
<CollectionViewSource x:Key="categoryViewSource"  
    d:DesignSource="{d:DesignInstance {x:Type local:Category}, CreateList=True}"/>
</Window.Resources>
<Grid DataContext="{StaticResource categoryViewSource}">
..

Code Behind

private void Window_Loaded(object sender, RoutedEventArgs e)
{
   System.Windows.Data.CollectionViewSource categoryViewSource =
      ((System.Windows.Data.CollectionViewSource)(this.FindResource("categoryViewSource")));

  _context.Categories.Load();
  categoryViewSource.Source = _context.Categories.Local;        
}

ViewModel

public MainWindow()
{
    InitializeComponent();
    this.DataContext = new MyViewModel();
}

---

However, when I try to use the same code from within ViewModel, it doesn‘t work (FindResource is not available), besides, I don’t think this is the right approach (i.e. to use x:Key in MVVM).

I would really appreciate any help to point me what is the right way to implement CollectionViewSource and DataBinding with DataGrid.

Answer 1

I found that it is handy to have a CollectionViewSource in my ViewModel and bind the ListBox (in my case) to the CollectionViewSource.View while setting the CollectionViewSource.Source to be the list I want to use.

Like so:

ViewModel:

    public DesignTimeVM()  //I'm using this as a Design Time VM 
    {
        Items = new List<Foo>();
        Items.Add(new Foo() { FooProp= "1", FooPrep= 20.0 });
        Items.Add(new Foo() { FooProp= "2", FooPrep= 30.0 });

        FooViewSource = new CollectionViewSource();
        FooViewSource.Source = Items;

        SelectedFoo = Items.First();

        //More code as needed
    }

XAML:

<ListBox ItemsSource="{Binding FooViewSource.View}" SelectedItem="{Binding SelectedFoo}"/>

This means I can do neat stuff in the VM as needed (from https://blogs.msdn.microsoft.com/matt/2008/08/28/collectionview-deferrefresh-my-new-best-friend/ ):

        using (FooViewSource.DeferRefresh())
        {
            //Remove an old Item
            //add New Item
            //sort list anew, etc. 
        }

I suppose this is possible when using the ICollectionView object also, but the demo code in the blog link seems to be some codebehind stuff, refering the listbox directly, which I'm trying to avoid.

BTW before you ask, here's how you use a Design Time VM: WPF Design Time View Model

Answer 2

Just for reference, another way is to use an attached property on the CollectionViewSource which then pipes the functions to the ViewModel (Implementing an Interface).

This is a very basic Demonstration just for filtering, it would need some work for e.g. a second Collection on the VM but i think it's enough to show the general technique.

If this is better or worse than the other methods is up for discussion, i just wanted to point out, that there's another way of doing this

Definition of attached Property:

public static class CollectionViewSourceFilter
{
    public static IFilterCollectionViewSource GetFilterObject(CollectionViewSource obj)
    {
        return (IFilterCollectionViewSource)obj.GetValue(FilterObjectProperty);
    }

    public static void SetFilterObject(CollectionViewSource obj, IFilterCollectionViewSource value)
    {
        obj.SetValue(FilterObjectProperty, value);
    }

    public static void FilterObjectChanged(object sender, DependencyPropertyChangedEventArgs e)
    {
        if (e.OldValue is IFilterCollectionViewSource oldFilterObject
            && sender is CollectionViewSource oldCvs)
        {
            oldCvs.Filter -= oldFilterObject.Filter;
            oldFilterObject.FilterRefresh -= (s, e2) => oldCvs.View.Refresh();
        }

        if (e.NewValue is IFilterCollectionViewSource filterObject
            && sender is CollectionViewSource cvs)
        {
            cvs.Filter += filterObject.Filter;
            filterObject.FilterRefresh += (s,e2) => cvs.View.Refresh();
        }
    }

    public static readonly DependencyProperty FilterObjectProperty = DependencyProperty.RegisterAttached(
        "FilterObject",
        typeof(Interfaces.IFilterCollectionViewSource),
        typeof(CollectionViewSourceFilter),
        new PropertyMetadata(null,FilterObjectChanged)
    );
}

Interface:

public interface IFilterCollectionViewSource
{
    void Filter(object sender, FilterEventArgs e);
    event EventHandler FilterRefresh;
}

usage in xaml:

<CollectionViewSource
        x:Key="yourKey"
        Source="{Binding YourCollection}"
        classes:CollectionViewSourceFilter.FilterObject="{Binding}" />

and usage in the ViewModel:

class YourViewModel : IFilterCollectionViewSource
{
    public event EventHandler FilterRefresh;

    private string _SearchTerm = string.Empty;
    public string SearchTerm
    {
        get { return _SearchTerm; }
        set {
            SetProperty(ref _SearchTerm, value);
            FilterRefresh?.Invoke(this, null);
        }
    }

    private ObservableCollection<YourItemType> _YourCollection = new ObservableCollection<YourItemType>();
    public ObservableCollection<YourItemType> YourCollection
    {
        get { return _YourCollection; }
        set { SetProperty(ref _YourCollection, value); }
    }

    public void Filter(object sender, FilterEventArgs e)
    {
        e.Accepted = (e.Item as YourItemType)?.YourProperty?.ToLower().Contains(SearchTerm.ToLower()) ?? true;
    }
}