custom listbox control event handling

Question

I'm following MVVM pattern. I have created custom listbox control naming it as ExtendedListbox.

in xmal, I have define listbox and define ItemSource property.

<ExtendedListBox ItemsSource="{Binding Students}">

in my viewmodel, I have defined students as ObservableCollection.

public ObservableCollection <Student> Students;

Runtime, I'm adding/removing student object in Students collection.

In my ExtendedListBox control class, i want to know which object get added/removed from datasource of listbox.

is there any event that get triggered when any item get added or removed from datasource of listbox?

Answer 1

If you want an MVVM style then I would suggest using System.Windows.Interactivity and use `Behaviors.

Sample usage.

Behavior class

public class ListBoxBehavior : Behavior<ListBox>
{
    public static readonly DependencyProperty AddedItemsProperty =
        DependencyProperty.Register("AddedItems", typeof (List<object>), typeof (ListBoxBehavior), new PropertyMetadata(new List<object>()));

    public List<object> AddedItems
    {
        get { return (List<object>) GetValue(AddedItemsProperty); }
        set { SetValue(AddedItemsProperty, value); }
    }

    public static readonly DependencyProperty RemovedItemsProperty =
        DependencyProperty.Register("RemovedItems", typeof(List<object>), typeof(ListBoxBehavior), new PropertyMetadata(new List<object>));

    public List<object> RemovedItems
    {
        get { return (List<object>) GetValue(RemovedItemsProperty); }
        set { SetValue(RemovedItemsProperty, value); }
    }

    protected override void OnAttached()
    {
        base.OnAttached();
        var observableCollection = AssociatedObject.ItemsSource as ObservableCollection<object>;
        if (observableCollection != null)
        {
            observableCollection.CollectionChanged += ItemsSourceOnCollectionChanged;
        }
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();
        var observableCollection = AssociatedObject.ItemsSource as ObservableCollection<object>;
        if (observableCollection != null)
        {
            observableCollection.CollectionChanged -= ItemsSourceOnCollectionChanged;
        }
    }

    private void ItemsSourceOnCollectionChanged(object sender, NotifyCollectionChangedEventArgs notifyCollectionChangedEventArgs)
    {
        AddedItems.Clear();
        RemovedItems.Clear();
        switch (notifyCollectionChangedEventArgs.Action)
        {
            case NotifyCollectionChangedAction.Add:
                foreach (var newItem in notifyCollectionChangedEventArgs.NewItems)
                {
                    AddedItems.Add(newItem);
                }
                break;
            case NotifyCollectionChangedAction.Remove:
                foreach (var newItem in notifyCollectionChangedEventArgs.NewItems)
                {
                    RemovedItems.Add(newItem);
                }
                break;
        }
    }
}

XAML

    <ListBox>
        <i:Interaction.Behaviors>
            <ListBoxBehavior AddedItems="{Binding AddedItems}"/>
            <ListBoxBehavior AddedItems="{Binding RemovedItems}"/>
        </i:Interaction.Behaviors>
    </ListBox>

What happens is that the events that are subscribed is encapsulated in the behavior class and you can now create a binding to the dependency property that is associated with the class which is the ListBox.