annotations/Azaion.Annotator/Extensions/ScrollViewerExtensions.cs

using System.Windows;
using System.Windows.Controls;

namespace Azaion.Annotator.Extensions;

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)
    {
        if (sender is not ScrollViewer scroll)
            throw new InvalidOperationException("The attached AlwaysScrollToEnd property can only be applied to ScrollViewer instances.");

        var alwaysScrollToEnd = e.NewValue != null && (bool)e.NewValue;
        if (alwaysScrollToEnd)
        {
            scroll.ScrollToEnd();
            scroll.ScrollChanged += ScrollChanged;
        }
        else
            scroll.ScrollChanged -= ScrollChanged;
    }

    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)
    {
        var scroll = sender as ScrollViewer;
        if (scroll == null)
            throw new InvalidOperationException("The attached AlwaysScrollToEnd property can only be applied to ScrollViewer instances.");

        if (e.ExtentHeightChange == 0)
            _autoScroll = scroll.VerticalOffset == scroll.ScrollableHeight;

        if (_autoScroll && e.ExtentHeightChange != 0)
            scroll.ScrollToVerticalOffset(scroll.ExtentHeight);
    }
}