Shaker/Avalonia/Avalonia.Xaml.Interactions.DragAndDrop/TypedDragBehavior.cs

using System;
using System.Diagnostics.CodeAnalysis;
using System.Threading.Tasks;
using Avalonia.Controls;
using Avalonia.Input;
using Avalonia.Interactivity;
using Avalonia.Xaml.Interactivity;

namespace Avalonia.Xaml.Interactions.DragAndDrop;

/// <summary>
/// 
/// </summary>
public class TypedDragBehavior : StyledElementBehavior<Control>
{
    private Point _dragStartPoint;
    private PointerEventArgs? _triggerEvent;
    private object? _value;
    private bool _lock;

    /// <summary>
    /// 
    /// </summary>
    public static readonly StyledProperty<Type?> DataTypeProperty =
        AvaloniaProperty.Register<TypedDragBehavior, Type?>(nameof(DataType));

    /// <summary>
    /// 
    /// </summary>
    public static readonly StyledProperty<IDragHandler?> HandlerProperty =
        AvaloniaProperty.Register<TypedDragBehavior, IDragHandler?>(nameof(Handler));

    /// <summary>
    /// 
    /// </summary>
    public Type? DataType
    {
        get => GetValue(DataTypeProperty);
        set => SetValue(DataTypeProperty, value);
    }

    /// <summary>
    /// 
    /// </summary>
    public IDragHandler? Handler
    {
        get => GetValue(HandlerProperty);
        set => SetValue(HandlerProperty, value);
    }

    /// <inheritdoc />
    protected override void OnAttachedToVisualTree()
    {
        AssociatedObject?.AddHandler(InputElement.PointerPressedEvent, AssociatedObject_PointerPressed, RoutingStrategies.Direct | RoutingStrategies.Tunnel | RoutingStrategies.Bubble);
        AssociatedObject?.AddHandler(InputElement.PointerReleasedEvent, AssociatedObject_PointerReleased, RoutingStrategies.Direct | RoutingStrategies.Tunnel | RoutingStrategies.Bubble);
        AssociatedObject?.AddHandler(InputElement.PointerMovedEvent, AssociatedObject_PointerMoved, RoutingStrategies.Direct | RoutingStrategies.Tunnel | RoutingStrategies.Bubble);
    }

    /// <inheritdoc />
    protected override void OnDetachedFromVisualTree()
    {
        AssociatedObject?.RemoveHandler(InputElement.PointerPressedEvent, AssociatedObject_PointerPressed);
        AssociatedObject?.RemoveHandler(InputElement.PointerReleasedEvent, AssociatedObject_PointerReleased);
        AssociatedObject?.RemoveHandler(InputElement.PointerMovedEvent, AssociatedObject_PointerMoved);
    }

    private async Task DoDragDrop(PointerEventArgs triggerEvent, object? value)
    {
        var data = new DataObject();
        data.Set(ContextDropBehavior.DataFormat, value!);

        var effect = DragDropEffects.None;

        if (triggerEvent.KeyModifiers.HasFlag(KeyModifiers.Alt))
        {
            effect |= DragDropEffects.Link;
        }
        else if (triggerEvent.KeyModifiers.HasFlag(KeyModifiers.Shift))
        {
            effect |= DragDropEffects.Move;
        }
        else if (triggerEvent.KeyModifiers.HasFlag(KeyModifiers.Control))
        {
            effect |= DragDropEffects.Copy;
        }
        else
        {
            effect |= DragDropEffects.Move;
        }

        await DragDrop.DoDragDrop(triggerEvent, data, effect);
    }

    private void AssociatedObject_PointerPressed(object? sender, PointerPressedEventArgs e)
    {
        var properties = e.GetCurrentPoint(AssociatedObject).Properties;
        if (properties.IsLeftButtonPressed)
        {
            if (e.Source is Control control 
                && AssociatedObject?.DataContext == control.DataContext
                && DataType is not null
                && DataType.IsInstanceOfType(control.DataContext))
            {
                _dragStartPoint = e.GetPosition(null);
                _triggerEvent = e;
                _value = control.DataContext;
                _lock = true;
            }
        }
    }

    private void AssociatedObject_PointerReleased(object? sender, PointerReleasedEventArgs e)
    {
        var properties = e.GetCurrentPoint(AssociatedObject).Properties;
        if (properties.PointerUpdateKind == PointerUpdateKind.LeftButtonReleased && _triggerEvent is not null)
        {
            _triggerEvent = null;
            _value = null;
            _lock = false;
        }
    }

    private async void AssociatedObject_PointerMoved(object? sender, PointerEventArgs e)
    {
        var properties = e.GetCurrentPoint(AssociatedObject).Properties;
        if (properties.IsLeftButtonPressed && _triggerEvent is not null)
        {
            var point = e.GetPosition(null);
            var diff = _dragStartPoint - point;
            if (Math.Abs(diff.X) > 3 || Math.Abs(diff.Y) > 3)
            {
                if (_lock)
                {
                    _lock = false;
                }
                else
                {
                    return;
                }

                Handler?.BeforeDragDrop(sender, _triggerEvent, _value);

                await DoDragDrop(_triggerEvent, _value);

                Handler?.AfterDragDrop(sender, _triggerEvent, _value);

                _triggerEvent = null;
                _value = null;
            }
        }
    }
}