DiscordChatExporter/DiscordChatExporter.Gui/Behaviors/MultiSelectionListBoxBehavior.cs

using System.Collections;
using System.Collections.Specialized;
using System.Linq;
using System.Windows;
using System.Windows.Controls;
using Microsoft.Xaml.Behaviors;

namespace DiscordChatExporter.Gui.Behaviors;

public class MultiSelectionListBoxBehavior<T> : Behavior<ListBox>
{
    public static readonly DependencyProperty SelectedItemsProperty = DependencyProperty.Register(
        nameof(SelectedItems),
        typeof(IList),
        typeof(MultiSelectionListBoxBehavior<T>),
        new FrameworkPropertyMetadata(
            null,
            FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
            OnSelectedItemsChanged
        )
    );

    private static void OnSelectedItemsChanged(
        DependencyObject sender,
        DependencyPropertyChangedEventArgs args
    )
    {
        var behavior = (MultiSelectionListBoxBehavior<T>)sender;
        if (behavior._modelHandled)
            return;

        if (behavior.AssociatedObject is null)
            return;

        behavior._modelHandled = true;
        behavior.SelectItems();
        behavior._modelHandled = false;
    }

    private bool _viewHandled;
    private bool _modelHandled;

    public IList? SelectedItems
    {
        get => (IList?)GetValue(SelectedItemsProperty);
        set => SetValue(SelectedItemsProperty, value);
    }

    // Propagate selected items from the model to the view
    private void SelectItems()
    {
        _viewHandled = true;

        AssociatedObject.SelectedItems.Clear();
        if (SelectedItems is not null)
        {
            foreach (var item in SelectedItems)
                AssociatedObject.SelectedItems.Add(item);
        }

        _viewHandled = false;
    }

    // Propagate selected items from the view to the model
    private void OnListBoxSelectionChanged(object? sender, SelectionChangedEventArgs args)
    {
        if (_viewHandled)
            return;
        if (AssociatedObject.Items.SourceCollection is null)
            return;

        SelectedItems = AssociatedObject.SelectedItems.Cast<T>().ToArray();
    }

    private void OnListBoxItemsChanged(object? sender, NotifyCollectionChangedEventArgs args)
    {
        if (_viewHandled)
            return;
        if (AssociatedObject.Items.SourceCollection is null)
            return;
        SelectItems();
    }

    protected override void OnAttached()
    {
        base.OnAttached();

        AssociatedObject.SelectionChanged += OnListBoxSelectionChanged;
        ((INotifyCollectionChanged)AssociatedObject.Items).CollectionChanged +=
            OnListBoxItemsChanged;
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();

        if (AssociatedObject is not null)
        {
            AssociatedObject.SelectionChanged -= OnListBoxSelectionChanged;
            ((INotifyCollectionChanged)AssociatedObject.Items).CollectionChanged -=
                OnListBoxItemsChanged;
        }
    }
}