DataContextChanged of a Tab in a TabControl is raised too early

I have a TabControl binding to some items. Underneath it is a Button where I can add items dynamically. On adding an item, the new item should become the active Tab (works fine with TabControl.SelectedItem):

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:this="clr-namespace:WpfApplication1"
        Title="MainWindow" Height="350" Width="525">
    <StackPanel>
        <TabControl ItemsSource="{Binding Items}"
                    SelectedItem="{Binding SelectedItem, Mode=OneWay}">
            <TabControl.ContentTemplate>
                <DataTemplate>
                    <this:UserControl1 />
                </DataTemplate>
            </TabControl.ContentTemplate>
        </TabControl>
        <Button Content="Foo" Click="Button_Click"/>
    </StackPanel>
</Window开发者_运维问答>

Code-behind:

using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;
using System.Windows;

namespace WpfApplication1
{
    public partial class MainWindow : INotifyPropertyChanged
    {
        public ObservableCollection<Foo> Items { get; set; }
        public Foo SelectedItem { get { return Items.Last(); } }
        public event PropertyChangedEventHandler PropertyChanged;

        public MainWindow()
        {
            Items = new ObservableCollection<Foo>();
            Items.Add(new Foo {Bar = "bar"});

            InitializeComponent();
            DataContext = this;
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            Items.Add(new Foo {Bar = "bar"});

            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs("Items"));
                PropertyChanged(this, new PropertyChangedEventArgs("SelectedItem"));
            }
        }
    }

    public class Foo { public string Bar { get; set; } }
}

The UserControl1 looks like this:

<UserControl x:Class="WpfApplication1.UserControl1"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <StackPanel>
        <TextBox/>
        <TextBox x:Name="_textBox"
             DataContextChanged="OnDataContextChanged"
             Text="{Binding Bar}" />
    </StackPanel>
</UserControl>

And the code-behind of it should focus _textBox and selectAll its text when the user clicks on the tab:

using System.Windows;

namespace WpfApplication1
{
    public partial class UserControl1
    {
        public UserControl1()
        {
            InitializeComponent();
        }

        private void OnDataContextChanged(object sender,
                                          DependencyPropertyChangedEventArgs e)
        {
            _textBox.Focus();
            _textBox.SelectAll();
        }
    }
}

I try to achieve that with the DataContextChanged-event, but due to its unpredictability (s.f. http://msdn.microsoft.com/en-us/library/system.windows.frameworkelement.datacontextchanged.aspx), it doesn't work all the time. I also tried it with the Loaded-event, but this will be called only once when the DataTemplate is loaded.

So, I think I need to receive the Loaded-event every time the DataContext has changed and the data-binding engine has finished its job. Is there such an event?

Are you wanting to select the text when the user adds a tab AND when the user clicks on a different tab?

If this is the case you may want to to handle this with two event handlers - The tab changed event for the tab control - and then setting it in code when you add a new item.

The DataContext according to your code does not change. It is set to the main window and then inherited down to the child controls.

    public MainWindow()
    {
        Items = new ObservableCollection<Foo>();
        Items.Add(new Foo {Bar = "bar"});

        InitializeComponent();
        DataContext = this;
    }