AutoScrollListBoxBehavior.cs

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using System.Collections.Specialized;
using System.Windows;
using System.Windows.Controls;
 
namespace Dreamine.UI.Wpf.Behaviors;

public static class AutoScrollListBoxBehavior
{
    public static readonly DependencyProperty IsEnabledProperty =
        DependencyProperty.RegisterAttached(
            "IsEnabled",
            typeof(bool),
            typeof(AutoScrollListBoxBehavior),
            new PropertyMetadata(false, OnIsEnabledChanged));

    public static void SetIsEnabled(DependencyObject element, bool value)
        => element.SetValue(IsEnabledProperty, value);

    public static bool GetIsEnabled(DependencyObject element)
        => (bool)element.GetValue(IsEnabledProperty);

    private static readonly DependencyProperty SubscriptionProperty =
        DependencyProperty.RegisterAttached(
            "Subscription",
            typeof(NotifyCollectionChangedEventHandler),
            typeof(AutoScrollListBoxBehavior),
            new PropertyMetadata(null));

    private static void OnIsEnabledChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (d is not ListBox listBox)
            return;
 
        if ((bool)e.NewValue)
        {
            listBox.Loaded += OnLoaded;
            listBox.Unloaded += OnUnloaded;
        }
        else
        {
            listBox.Loaded -= OnLoaded;
            listBox.Unloaded -= OnUnloaded;
            Detach(listBox);
        }
    }

    private static void OnLoaded(object sender, RoutedEventArgs e)
    {
        if (sender is not ListBox listBox)
            return;
 
        Attach(listBox);
        ScrollToLast(listBox);
    }

    private static void OnUnloaded(object sender, RoutedEventArgs e)
    {
        if (sender is not ListBox listBox)
            return;
 
        Detach(listBox);
    }

    private static void Attach(ListBox listBox)
    {
        Detach(listBox);
 
        if (listBox.ItemsSource is not INotifyCollectionChanged incc)
            return;
 
        NotifyCollectionChangedEventHandler handler = (_, __) =>
        {
            listBox.Dispatcher.InvokeAsync(() => ScrollToLast(listBox));
        };
 
        incc.CollectionChanged += handler;
        listBox.SetValue(SubscriptionProperty, handler);
    }

    private static void Detach(ListBox listBox)
    {
        if (listBox.ItemsSource is INotifyCollectionChanged incc &&
            listBox.GetValue(SubscriptionProperty) is NotifyCollectionChangedEventHandler handler)
        {
            incc.CollectionChanged -= handler;
        }
 
        listBox.ClearValue(SubscriptionProperty);
    }

    private static void ScrollToLast(ListBox listBox)
    {
        if (listBox.Items.Count <= 0)
            return;
 
        var last = listBox.Items[listBox.Items.Count - 1];
        listBox.ScrollIntoView(last);
    }
}