问添加新行时，如何使RichTextBox滚动到末尾？

EN

    <Window x:Class="CheckRichTextBox.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="170" Width="300">
    <StackPanel>
        <RichTextBox Height="100" Name="richTextBox1" IsReadOnly="True" VerticalScrollBarVisibility="Visible"/>
        <Button Name="btnAdd" Content="Click me to add text" VerticalAlignment="Bottom" Click="BtnAddClick" />
    </StackPanel>
</Window>

using System.Windows;

namespace CheckRichTextBox
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void BtnAddClick(object sender, RoutedEventArgs e)
        {
            richTextBox1.AppendText("You had Clicked the button for adding text\n");
            richTextBox1.ScrollToEnd();
        }
    }
}

public class ScrollToBottomAction : TriggerAction<RichTextBox>
{
    protected override void Invoke(object parameter)
    {
        AssociatedObject.ScrollToEnd();
    }
}

<RichTextBox IsReadOnly="True" VerticalScrollBarVisibility="Auto">
     <i:Interaction.Triggers>
            <i:EventTrigger EventName="TextChanged">
                <interactivity:ScrollToBottomAction/>
            </i:EventTrigger>
     </i:Interaction.Triggers>
</RichTextBox>

public partial class MainWindow
{
    private bool AutoScroll = true;

    public MainWindow()
    {
        InitializeComponent();

        yourRichTextBox.Loaded += (s, e) =>
          {
              var scrollViewer = VisualTreeHelper.GetChild(VisualTreeHelper.GetChild(yourRichTextBox, 0), 0) as ScrollViewer;
              scrollViewer.ScrollChanged += (scroller, eScroller) => ScrollViewer_ScrollChanged(scroller, eScroller);
          };
    }

    private void ScrollViewer_ScrollChanged(object sender, ScrollChangedEventArgs e)
    {
        // User scroll event : set or unset autoscroll mode
        if (e.Source as ScrollViewer != null && e.ExtentHeightChange == 0)
        {   // Content unchanged : user scroll event
            if ((e.Source as ScrollViewer).VerticalOffset == (e.Source as ScrollViewer).ScrollableHeight)
            {   // Scroll bar is in bottom
                // Set autoscroll mode
                AutoScroll = true;
            }
            else
            {   // Scroll bar isn't in bottom
                // Unset autoscroll mode
                AutoScroll = false;
            }
        }

        // Content scroll event : autoscroll eventually
        if (AutoScroll && e.ExtentHeightChange != 0 && e.Source as ScrollViewer != null)
        {   // Content changed and autoscroll mode set
            // Autoscroll
            (e.Source as ScrollViewer).ScrollToVerticalOffset((e.Source as ScrollViewer).ExtentHeight);
        }
    }
}