VirtualizingStackPanel + MVVM + multiple selection

问题:

I have implemented a selection pattern similar to the one described in this post using a ViewModel to store the IsSelected value, and by binding the ListViewItem.IsSelected to the ViewModel IsSelected:

<ListView.ItemContainerStyle>
    <Style TargetType="{x:Type ListViewItem}">
        <Setter Property="IsSelected" Value="{Binding Mode=TwoWay, Path=IsSelected}"/>
    </Style>
</ListView.ItemContainerStyle>

It works in general, but I encounter a severe problem. By using the a VirtualizingStackPanel as the panel in the list view, only the visible ListViewItem are getting created. If I use "Ctrl+A" to select all items, or by using shortcut combination such "Shift+Ctrl+End" on the first item, all items get selected, but for the non visible items, the ViewModel does not get its IsSelected set to true. That is logical, because if the ListViewItem are not created, the binding can't work.

Anybody experienced the same issue, and found a solution (apart from not using a VirtualizingStackPanel)?

回答1:

I found another way of handling selection in the MVVM pattern, which solved my issue. Instead of maintaining the selection in the viewmodel, the selection is retrieved from the ListView/ListBox, and passed as a parameter to the Command. All done in XAML:

<ListView 
    x:Name="_items"
    ItemsSource="{Binding Items}" ... />

<Button 
    Content="Remove Selected" 
    Command="{Binding RemoveSelectedItemsCommand}" 
    CommandParameter="{Binding ElementName=_items, Path=SelectedItems}"/>

in my ViewModel:

private void RemoveSelection(object parameter)
{
    IList selection = (IList)parameter;
    ...
}

回答2:

In my case, I ended up solving this by deriving a ListBoxEx class from ListBox, and adding code to respond to selection changes, enforcing the selection state on the item view models:

private readonly List<IListItemViewModelBase> selectedItems = new List<IListItemViewModelBase>();

protected override void OnSelectionChanged(SelectionChangedEventArgs e)
{
    base.OnSelectionChanged(e);

    bool isVirtualizing = VirtualizingStackPanel.GetIsVirtualizing(this);
    bool isMultiSelect = (SelectionMode != SelectionMode.Single);

    if (isVirtualizing && isMultiSelect)
    {
        var newSelectedItems = SelectedItems.Cast<IListItemViewModelBase>();

        foreach (var deselectedItem in this.selectedItems.Except(newSelectedItems))
        {
            deselectedItem.IsSelected = false;
        }

        this.selectedItems.Clear();
        this.selectedItems.AddRange(newSelectedItems);

        foreach (var newlySelectedItem in this.selectedItems)
        {
            newlySelectedItem.IsSelected = true;
        }
    }
}

回答3:

Apart from not using VirtualizingStackPanel, the only thing I can think of is to capture those keyboard shortcuts and have methods for modifying a certain range of your ViewModel items so that their IsSelected property is set to True (e.g., SelectAll(), SelectFromCurrentToEnd()). Basically bypassing the Binding on ListViewItem for controlling the selection for such cases.