How to automatically scroll ScrollViewer - only if the user did not change scroll position
You can use ScrollChangedEventArgs.ExtentHeightChange to know if a ScrollChanged is due to a change in the content or to a user action... When the content is unchanged, the ScrollBar position sets or unsets the auto-scroll mode. When the content has changed you can apply auto-scrolling.
Code behind:
private Boolean AutoScroll = true;
private void ScrollViewer_ScrollChanged(Object sender, ScrollChangedEventArgs e)
{
// User scroll event : set or unset auto-scroll mode
if (e.ExtentHeightChange == 0)
{ // Content unchanged : user scroll event
if (ScrollViewer.VerticalOffset == ScrollViewer.ScrollableHeight)
{ // Scroll bar is in bottom
// Set auto-scroll mode
AutoScroll = true;
}
else
{ // Scroll bar isn't in bottom
// Unset auto-scroll mode
AutoScroll = false;
}
}
// Content scroll event : auto-scroll eventually
if (AutoScroll && e.ExtentHeightChange != 0)
{ // Content changed and auto-scroll mode set
// Autoscroll
ScrollViewer.ScrollToVerticalOffset(ScrollViewer.ExtentHeight);
}
}
Here is an adaptation from several sources.
public class ScrollViewerExtensions
{
public static readonly DependencyProperty AlwaysScrollToEndProperty = DependencyProperty.RegisterAttached("AlwaysScrollToEnd", typeof(bool), typeof(ScrollViewerExtensions), new PropertyMetadata(false, AlwaysScrollToEndChanged));
private static bool _autoScroll;
private static void AlwaysScrollToEndChanged(object sender, DependencyPropertyChangedEventArgs e)
{
ScrollViewer scroll = sender as ScrollViewer;
if (scroll != null)
{
bool alwaysScrollToEnd = (e.NewValue != null) && (bool)e.NewValue;
if (alwaysScrollToEnd)
{
scroll.ScrollToEnd();
scroll.ScrollChanged += ScrollChanged;
}
else { scroll.ScrollChanged -= ScrollChanged; }
}
else { throw new InvalidOperationException("The attached AlwaysScrollToEnd property can only be applied to ScrollViewer instances."); }
}
public static bool GetAlwaysScrollToEnd(ScrollViewer scroll)
{
if (scroll == null) { throw new ArgumentNullException("scroll"); }
return (bool)scroll.GetValue(AlwaysScrollToEndProperty);
}
public static void SetAlwaysScrollToEnd(ScrollViewer scroll, bool alwaysScrollToEnd)
{
if (scroll == null) { throw new ArgumentNullException("scroll"); }
scroll.SetValue(AlwaysScrollToEndProperty, alwaysScrollToEnd);
}
private static void ScrollChanged(object sender, ScrollChangedEventArgs e)
{
ScrollViewer scroll = sender as ScrollViewer;
if (scroll == null) { throw new InvalidOperationException("The attached AlwaysScrollToEnd property can only be applied to ScrollViewer instances."); }
// User scroll event : set or unset autoscroll mode
if (e.ExtentHeightChange == 0) { _autoScroll = scroll.VerticalOffset == scroll.ScrollableHeight; }
// Content scroll event : autoscroll eventually
if (_autoScroll && e.ExtentHeightChange != 0) { scroll.ScrollToVerticalOffset(scroll.ExtentHeight); }
}
}
Use it in your XAML like so:
<ScrollViewer Height="230" HorizontalScrollBarVisibility="Auto" extensionProperties:ScrollViewerExtension.AlwaysScrollToEnd="True">
<TextBlock x:Name="Trace"/>
</ScrollViewer>
This code will automatically scroll to end when the content grows if it was previously scrolled all the way down.
XAML:
<Window x:Class="AutoScrollTest.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Height="300" Width="300">
<ScrollViewer Name="_scrollViewer">
<Border BorderBrush="Red" BorderThickness="5" Name="_contentCtrl" Height="200" VerticalAlignment="Top">
</Border>
</ScrollViewer>
</Window>
Code behind:
using System;
using System.Windows;
using System.Windows.Threading;
namespace AutoScrollTest
{
public partial class Window1 : Window
{
public Window1()
{
InitializeComponent();
DispatcherTimer timer = new DispatcherTimer();
timer.Interval = new TimeSpan(0, 0, 2);
timer.Tick += ((sender, e) =>
{
_contentCtrl.Height += 10;
if (_scrollViewer.VerticalOffset == _scrollViewer.ScrollableHeight)
{
_scrollViewer.ScrollToEnd();
}
});
timer.Start();
}
}
}