Question

Scenario: A ListView is DataBound to an ObservableCollection<CustomClass> and is displaying it’s items through a custom ItemTemplate. The CustomClass just contains three string properties and one boolean property and already has INotifyPropertyChanged implemented on every of it’s four properties. The custom ItemTemplate of the ListView has One-Way bindings on the three string properties and a Two-Way binding on the boolean property, displaying it as a CheckBox.

Problem: I’m looking for the most elegant (in terms of WPF) way to display the count of all checked items in that ListView using a TextBlock – or in other words, all items that have their boolean property set to true in that collection. I want that TextBlock to immediately update the displayed count if one of the ListView items gets checked/unchecked. I know that there are (rather) ugly ways to achieve this with code behind and eventhandling, but I’d like to know if there’s a clever way to do this maybe completely in XAML with arcane DataBinding syntax.

Edit: Just as an example/clarification: The ListView displays 100 items, 90 items have their boolean property set to true, so the TextBlock will display ’90’. If the user unchecks one more item through it’s CheckBox and therefore sets it’s property to false through the Two-Way binding, the TextBlock should update to ’89’.

Answer 1

Thanks for all the answers I’ve got, these were all applicable solutions but unfortunately not really what I’ve tried to achieve. So this is how I’ve solved the problem now:

I’ve implemented a DependencyProperty on the Window containing the TextBlock:

public static readonly DependencyProperty ActiveItemCountProperty =
        DependencyProperty.Register("ActiveItemCount", typeof(int), typeof(CustomControl), new UIPropertyMetadata(0));

On the DataTemplate for the ListView items the CheckBox registered an EventHandler for the Click-Event:

<CheckBox IsChecked="{Binding Active, Mode=TwoWay}" Click="CheckBox_Click" />

The event handler in code behind looks something like this:

    private void CheckBox_Click(object sender, RoutedEventArgs e)
    {
        ObservableCollection<CustomClass> sourceCol = listView.DataContext as ObservableCollection<CustomClass>;
        if (sourceCol != null)
            ActiveItemCount = sourceCol.Count(x => x.Active);
    }

And obviously, the TextBlock is just data bound to this DependencyProperty:

<TextBlock Text="{Binding Path=ActiveItemCount, ElementName=ControlRoot}" />

With ControlRoot being the name of the Window.