Synchronizing Two Rich Text Box Scroll bars in WPF
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Synchronizing Two RichTextBox Scroll Bars in WPF
Learn how to effectively synchronize the vertical scroll positions of two RichTextBox controls in WPF, enabling a unified viewing experience for related content.
Synchronizing the scroll bars of two or more controls is a common requirement in WPF applications, especially when displaying related content side-by-side. For RichTextBox controls, this can be particularly useful for comparing documents, displaying code with line numbers, or showing original and translated texts. This article will guide you through the process of achieving seamless vertical scroll synchronization between two RichTextBox instances.
Understanding the Challenge
The RichTextBox control in WPF does not directly expose its internal ScrollViewer component, which is responsible for handling scrolling. This makes direct binding or manipulation of scroll properties challenging. To overcome this, we need to access the ScrollViewer within the RichTextBox's visual tree and then hook into its ScrollChanged event. When one RichTextBox scrolls, we'll programmatically update the scroll position of the other.
Synchronization mechanism for two RichTextBox controls
Accessing the ScrollViewer
The first step is to find the ScrollViewer instance within the RichTextBox. Since ScrollViewer is a templated part of RichTextBox, we can use the VisualTreeHelper to traverse the visual tree and locate it. A helper method can simplify this process.
using System.Windows.Controls;
using System.Windows.Media;
public static class RichTextBoxHelper
{
public static ScrollViewer GetScrollViewer(DependencyObject depObj)
{
if (depObj is ScrollViewer scrollViewer)
{
return scrollViewer;
}
for (int i = 0; i < VisualTreeHelper.GetChildrenCount(depObj); i++)
{
var child = VisualTreeHelper.GetChild(depObj, i);
var result = GetScrollViewer(child);
if (result != null)
{
return result;
}
}
return null;
}
}
Helper method to find the ScrollViewer within a DependencyObject
GetScrollViewer helper method is a generic way to find a ScrollViewer within any control's visual tree. It uses recursion to search through all child elements until a ScrollViewer is found.Implementing Scroll Synchronization
Once we can access the ScrollViewer for both RichTextBox controls, we can subscribe to their ScrollChanged events. In the event handler, we will check which RichTextBox initiated the scroll and then update the vertical scroll offset of the other RichTextBox accordingly. To prevent an infinite loop of events, we'll use a flag to temporarily disable event handling.
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
public partial class MainWindow : Window
{
private ScrollViewer _scrollViewer1;
private ScrollViewer _scrollViewer2;
private bool _isScrolling;
public MainWindow()
{
InitializeComponent();
this.Loaded += MainWindow_Loaded;
}
private void MainWindow_Loaded(object sender, RoutedEventArgs e)
{
_scrollViewer1 = RichTextBoxHelper.GetScrollViewer(richTextBox1);
_scrollViewer2 = RichTextBoxHelper.GetScrollViewer(richTextBox2);
if (_scrollViewer1 != null) _scrollViewer1.ScrollChanged += ScrollViewer1_ScrollChanged;
if (_scrollViewer2 != null) _scrollViewer2.ScrollChanged += ScrollViewer2_ScrollChanged;
// Example content
richTextBox1.Document.Blocks.Add(new Paragraph(new Run("This is content for RichTextBox 1.\n")));
for (int i = 0; i < 50; i++)
{
richTextBox1.Document.Blocks.Add(new Paragraph(new Run($"Line {i + 1} in RichTextBox 1.")));
}
richTextBox2.Document.Blocks.Add(new Paragraph(new Run("This is content for RichTextBox 2.\n")));
for (int i = 0; i < 50; i++)
{
richTextBox2.Document.Blocks.Add(new Paragraph(new Run($"Line {i + 1} in RichTextBox 2.")));
}
}
private void ScrollViewer1_ScrollChanged(object sender, ScrollChangedEventArgs e)
{
if (_isScrolling) return;
_isScrolling = true;
if (_scrollViewer2 != null) _scrollViewer2.ScrollToVerticalOffset(e.VerticalOffset);
_isScrolling = false;
}
private void ScrollViewer2_ScrollChanged(object sender, ScrollChangedEventArgs e)
{
if (_isScrolling) return;
_isScrolling = true;
if (_scrollViewer1 != null) _scrollViewer1.ScrollToVerticalOffset(e.VerticalOffset);
_isScrolling = false;
}
}
C# code-behind for synchronizing RichTextBox scrollbars
<Window x:Class="WpfApp.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Synchronized RichTextBoxes" Height="450" Width="800">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<RichTextBox x:Name="richTextBox1" Grid.Column="0" Margin="5" VerticalScrollBarVisibility="Visible"/>
<RichTextBox x:Name="richTextBox2" Grid.Column="1" Margin="5" VerticalScrollBarVisibility="Visible"/>
</Grid>
</Window>
XAML layout for two RichTextBox controls
_isScrolling flag is crucial to prevent a StackOverflowException. Without it, scrolling one RichTextBox would trigger the other to scroll, which would then trigger the first, creating an endless loop of ScrollChanged events.1. Set up your WPF project
Create a new WPF Application project in Visual Studio.
2. Define XAML layout
Add two RichTextBox controls to your MainWindow.xaml with distinct x:Name attributes and set VerticalScrollBarVisibility="Visible".
3. Create RichTextBoxHelper class
Add a new static class named RichTextBoxHelper.cs to your project and paste the GetScrollViewer method into it.
4. Implement synchronization logic
In your MainWindow.xaml.cs code-behind, add the _scrollViewer1, _scrollViewer2, and _isScrolling fields. Implement the MainWindow_Loaded event to find the ScrollViewer instances and subscribe to their ScrollChanged events. Finally, implement the ScrollViewer1_ScrollChanged and ScrollViewer2_ScrollChanged methods to handle the synchronization logic.
5. Run the application
Execute your application. You should now be able to scroll either RichTextBox, and the other will follow its vertical scroll position.