Items added to the ListBox at runtime never gets focused

Question

As you can see in the image above, I have three columns in listbox.

First Column is FirstName, Second Column is LastName and Third Column is Age.

When I hit Enter on the Age Column a new Person is added to the list. Which eventually reflects the changes in ListBox.

Problem:

When I Press Enter on Age Column I get a new Person added as expected. But the focus does not go to the next ListItem. No matter how many times I press Enter I never get focus to the items added Programmatically.

Sample:

I have created a sample project that reproduces the issue:

Download Sample Project

I have a ListBox as follows:

<ListBox ItemsSource="{Binding People}" SelectedItem="{Binding SelectedPerson}">

    <ListBox.Resources>
        <Style TargetType="ListBoxItem" BasedOn="{StaticResource {x:Type ListBoxItem}}">
            <Style.Triggers>
                <Trigger Property="IsKeyboardFocusWithin" Value="True">
                    <Setter Property="IsSelected" Value="True"></Setter>
                </Trigger>
            </Style.Triggers>
            <Setter Property="Focusable" Value="False" />
        </Style>
    </ListBox.Resources>

    <ListBox.ItemTemplate>

        <DataTemplate>

            <Grid x:Name="CurrentItemGrid">

                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="200" />
                    <ColumnDefinition Width="200" />
                    <ColumnDefinition Width="100" />
                </Grid.ColumnDefinitions>

                <TextBox Text="{Binding FirstName, UpdateSourceTrigger=PropertyChanged}" Tag="IgnoreEnterKeyTraversal">

                    <TextBox.InputBindings>
                        <KeyBinding Command="{Binding DataContext.DeleteUnwantedOrderItemTransactionCommand, 
                                                              RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type Window}}}"
                                            Gesture="Return" />
                        <KeyBinding Command="{Binding DataContext.DeleteUnwantedOrderItemTransactionCommand, 
                                                              RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type Window}}}"
                                            Gesture="Tab" />
                    </TextBox.InputBindings>

                </TextBox>

                <TextBox Grid.Column="1" Text="{Binding LastName, UpdateSourceTrigger=PropertyChanged}" Margin="5,0"/>

                <TextBox Grid.Column="2" Text="{Binding Age, UpdateSourceTrigger=PropertyChanged}" Margin="5,0" Tag="IgnoreEnterKeyTraversal">

                    <TextBox.InputBindings>
                        <KeyBinding Command="{Binding DataContext.AddNewOrderItemTransactionCommand, 
                                                              RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type Window}}}"
                                            Gesture="Return" />
                        <KeyBinding Command="{Binding DataContext.AddNewOrderItemTransactionCommand, 
                                                              RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type Window}}}"
                                            Gesture="Tab" />
                    </TextBox.InputBindings>
                </TextBox>

            </Grid>

        </DataTemplate>

    </ListBox.ItemTemplate>

</ListBox>

In the ViewModel:

public class MainWindowViewModel : INotifyPropertyChanged
{
    IEventAggregator eventAggregator;

    public MainWindowViewModel(IEventAggregator _eventAggregator)
    {
        People = new ObservableCollection<Person>();
        People.Add(new Person());
        eventAggregator = _eventAggregator;

        DeleteUnwantedOrderItemTransactionCommand = new RelayCommand(DeleteUnwantedOrderItemTransaction);
        AddNewOrderItemTransactionCommand = new RelayCommand(AddNewOrderItemTransaction);
    }

    public RelayCommand DeleteUnwantedOrderItemTransactionCommand { get; set; }
    public RelayCommand AddNewOrderItemTransactionCommand { get; set; }

    private ObservableCollection<Person> _People;

    public ObservableCollection<Person> People
    {
        get
        {
            return _People;
        }
        set
        {
            if (_People != value)
            {
                _People = value;
                OnPropertyChanged("People");
            }
        }
    }

    private Person _SelectedPerson;

    public Person SelectedPerson
    {
        get
        {
            return _SelectedPerson;
        }
        set
        {
            if (_SelectedPerson != value)
            {
                _SelectedPerson = value;
                OnPropertyChanged("SelectedPerson");
            }
        }
    }

    protected void DeleteUnwantedOrderItemTransaction(object obj)
    {
        if (SelectedPerson.FirstName == "")
        {
            People.Remove(SelectedPerson);
        }

        if (People.Count == 0)
        {
            People.Add(new Person());
        }

        eventAggregator.GetEvent<ChangeFocusToNextUIElementEvent>().Publish(true);
    }

    protected void AddNewOrderItemTransaction(object obj)
    {
        if (SelectedPerson == People.Last())
            People.Add(new Person());

        eventAggregator.GetEvent<ChangeFocusToNextUIElementEvent>().Publish(true);
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private void OnPropertyChanged(String info)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(info));
        }
    }
}

In Code-Behind:

public partial class MainWindow : Window
{
    IEventAggregator _eventAggregator = new EventAggregator();

    public MainWindow()
    {
        InitializeComponent();
        this.DataContext = new MainWindowViewModel(_eventAggregator);
        _eventAggregator.GetEvent<ChangeFocusToNextUIElementEvent>().Subscribe(MoveToNextUIElement);
    }

    void MoveToNextUIElement(bool obj)
    {
        // Gets the element with keyboard focus.
        UIElement elementWithFocus = Keyboard.FocusedElement as UIElement;

        if (elementWithFocus != null)
        {
            elementWithFocus.MoveFocus(new TraversalRequest(FocusNavigationDirection.Next));
        }
    }
}

Answer 1

The moment you fire your event you have added your element to your VM, but it might not have been bound and created by the View (ListBox) yet. Dispatching the focus request with a low priority might help. Also check if you can access the element by pressing TAB, so you know the traversal works.

elementWithFocus.Dispatcher.Invoke(() => 
    elementWithFocus.MoveFocus(new TraversalRequest(FocusNavigationDirection.Next)),
    DispatcherPriority.Input); // !

Virtualization of the ListBox Elements could also be an issue. You have to bring the added item into view.

cheeers.