Need to move caret to end of a textbox when its bound property is updated by the viewmodel

Question

I am struggling to find a good solution to a nagging problem we are experiencing. We have a multiline textbox in a WPF app using MvvmLight to implement the MVVM design pattern. The textbox is bound to a property in the viewmodel via a two-way binding. Both the end user and the viewmodel can update the bound property. Whenever the viewmodel updates the bound property, the NotifyPropertyChanged event is raised (via the MvvmLight Set() method.) When this happens, the textbox is properly updated but the caret moves to the front of the text. We have a method in the viewmodel that invokes an event that has an eventhandler in the codebehind. The eventhandler moves the caret to the end of the text. The problem is, there are many places throughout the code that we have to remember to execute that method. It seems to me there ought to be a way to have the view automatically move the caret to the end every time the NotifyPropertyChanged event is raised. I found one 12 year old example that does what I want here. But I've run into a conflict. Our view defines the DataContext thusly:

<UserControl x:Class="CaptionNet.Wpf.Captioning.CaptioningView"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         xmlns:local="clr-namespace:CaptionNet.Wpf.Captioning"
         d:DataContext="{Binding CaptioningViewModel, Source={StaticResource Locator}}"
         DataContext="{Binding CaptioningViewModel, Source={StaticResource Locator}}"
         xmlns:dp="clr-namespace:CaptionNet.Wpf.DependencyProperties"
         xmlns:cmd="http://www.galasoft.ch/mvvmlight"
         xmlns:interactivity="http://schemas.microsoft.com/expression/2010/interactivity"
         mc:Ignorable="d" 
         d:DesignHeight="500" d:DesignWidth="500"
         AutomationProperties.AutomationId="CaptioningPane">

The sample code defines the DataContext this way:

<UserControl.DataContext>
    <local:CaptioningViewModel CaptionsChanged="ScrollToEnd" />
</UserControl.DataContext>

The CaptionsChanged event is defined in the CaptioningViewModel. The ScrollToEnd is the eventhandler defined in the backend.

I can only define the DataContext one way. Either the original way or the example's way but not both. If I use the original way, I can't figure out how to subscribe the ScrollToEnd eventhandler to the CaptionsChanged event. If I use the example's way, I can't figure out how to bind the datacontext to the CaptioningViewModel. I've searched all day and have been unable to figure this out. Your help would be greatly appreciated.

Answer 1

Add a binding on TextBox.CaretIndex.

Answer 2

Since we already had a way of forcing the caret to the end of the text after a user had typed something somewhere in the middle of the text, all I really needed was a way to cause the caret to move to the end of the text when the bound property was updated by the viewmodel. I was able to accomplish this by setting NotifyonTargetUpdated to true in the textbox binding and then adding the TargetUpdatad event to the text box. In the event handler, I set the caret position to the end of the text and call Keyboard.Focus() on the textbox.

<TextBox
             Text="{Binding Captions, UpdateSourceTrigger=PropertyChanged, NotifyOnTargetUpdated=true}" 
             Name="CaptionTextBox" 
             AutomationProperties.AutomationId="CaptioningTextBox"
             Margin="20"
             BorderThickness="0"
             SelectionChanged="CaptionTextBox_SelectionChanged"
             PreviewKeyDown="CaptionTextBox_PreviewKeyDown"
             MouseRightButtonUp="CaptionTextBox_MouseRightButtonUp"
             AcceptsReturn="True" 
             TextWrapping="Wrap"
             FontSize="{DynamicResource CaptionFontSize}"
             Background="{DynamicResource FontBackgroundColor}"
             Foreground="{DynamicResource FontForegroundColor}"
             VerticalScrollBarVisibility="Hidden" 
             HorizontalScrollBarVisibility="Disabled"
             IsEnabled="{Binding IsEnabled}"
             AllowDrop="False"
             TextChanged="CaptionTextBox_TextChanged"
             Focusable="True"
            TargetUpdated="CaptionTextBox_TargetUpdated"

Then in the codebehind:

    private void CaptionTextBox_TargetUpdated(object sender, System.Windows.Data.DataTransferEventArgs e)
    {
        CaptionTextBox.SelectionLength = 0;
        CaptionTextBox.Select(CaptionTextBox.Text.Length, 0);
        Keyboard.Focus(CaptionTextBox);
    }