WPF Datagrid: MVVM friendly way to bind SelectedCells to my ViewModel

Question

I'm using the WPF datagrid, and have SelectionUnit="Cell" and SelectionMode="Extended". I'm also trying to adhere to the MVVM principals as much as I can.

I need my ViewModel to keep track of the current SelectedCells.

Life would be easy if I could just Bind its SelectedCells property to my ViewModel. Oddly enough, SelectedCells is only raised once - when we first select any cell in the grid.

MS explains it here: http://social.msdn.microsoft.com/Forums/en/wpf/thread/737117f4-6d20-4232-88cf-e52cc44d4431

Can anyone think of an MVVM-friendly approach to get around it?

Thanks!

Answer 1

Do you need the SelectedCells constantly data-binded, or just when the user hits the OK/Accept button? If you only need it at the end of whatever process the user is in you can bind the SelectedCells to the CommandParameter property of a Button, for example. The SelectedCells is an IList, and you know enough to just do a cast to whatever object type the selection actually is. The other option is messier, you can use an attached property, keeping the event-handling out of your Views. This attached property would handle either a ListBox or in your case a DataGrid (MultiSelector).

public class Attach 
{
    public static IList GetSelectedItems(DependencyObject obj)
    {
        return (IList)obj.GetValue(SelectedItemsProperty);
    }

    public static void SetSelectedItems(DependencyObject obj, IList value)
    {
        obj.SetValue(SelectedItemsProperty, value);
    }

    /// <summary>
    /// Attach this property to expose the read-only SelectedItems property of a MultiSelector for data binding.
    /// </summary>
    public static readonly DependencyProperty SelectedItemsProperty =
        DependencyProperty.RegisterAttached("SelectedItems", typeof(IList), typeof(Attach), new UIPropertyMetadata(new List<object>() as IList, OnSelectedItemsChanged));



    static SelectionChangedEventHandler GetSelectionChangedHandler(DependencyObject obj)
    {
        return (SelectionChangedEventHandler)obj.GetValue(SelectionChangedHandlerProperty);
    }
    static void SetSelectionChangedHandler(DependencyObject obj, SelectionChangedEventHandler value)
    {
        obj.SetValue(SelectionChangedHandlerProperty, value);
    }
    static readonly DependencyProperty SelectionChangedHandlerProperty =
        DependencyProperty.RegisterAttached("SelectionChangedHandler", typeof(SelectionChangedEventHandler), typeof(Attach), new UIPropertyMetadata(null));


    //d is MultiSelector (d as ListBox not supported)
    static void OnSelectedItemsChanged(DependencyObject d, DependencyPropertyChangedEventArgs args)
    {
        if (GetSelectionChangedHandler(d) != null)
            return;

        if (d is MultiSelector)//DataGrid
        {
            MultiSelector multiselector = d as MultiSelector;
            SelectionChangedEventHandler selectionchanged = null;
            foreach (var selected in GetSelectedItems(d) as IList)
                multiselector.SelectedItems.Add(selected);

            selectionchanged = (sender, e) =>
            {
                SetSelectedItems(d, multiselector.SelectedItems);
            };
            SetSelectionChangedHandler(d, selectionchanged);
            multiselector.SelectionChanged += GetSelectionChangedHandler(d);
        }
        else if (d is ListBox)
        {
            ListBox listbox = d as ListBox;
            SelectionChangedEventHandler selectionchanged = null;

            selectionchanged = (sender, e) =>
            {
                SetSelectedItems(d, listbox.SelectedItems);
            };
            SetSelectionChangedHandler(d, selectionchanged);
            listbox.SelectionChanged += GetSelectionChangedHandler(d);
        }}}

Usage in XAML:

<DataGrid ItemsSource="{Binding Path=SourceList}"
              myControls:Attach.SelectedItems="{Binding Path=myMvvmSelectedItems, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" 
              SelectionMode="Extended" />

Answer 2

You might be interested in the BookLibrary sample application of the WPF Application Framework (WAF). It shows how to synchronize the DataGrid.SelectedItems with the ViewModel. This might be very similar to SelectedCells.