qamp/Qrakhen.TilingFrames/Controls/DragAndDrop/DragAndDropControl.cs

using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Media;
using System.Xml.Linq;

namespace CopaData.FileInspector.GUI.TilingPanels.Controls.DragAndDrop;

/// <summary>
///   Used to handle drag and drop callbacks via the abstract methods <see cref="OnDragStart(object, UIElement)"/> and <see cref="OnDrop(object, UIElement)"/>.
///   Can be used as a parent to any amount of <see cref="UIElement"/>s that refer to the their handler via <see cref="HandlerProperty"/>.
///   If no Handler is provided, the element itself will be considered its own handler, as long as it is a <see cref="DragAndDropControl"/>.
///   In order to make an element draggable, set <see cref="IsDraggableProperty"/> to true within the element's XAML attributes.
///   To mark a FrameworkElement as a drop zone, set <see cref="IsDropZoneProperty"/> to true.
///   If a draggable element is dropped on a drop zone, the handler's <see cref="DragAndDropControl.OnDrop(object, UIElement)" /> is called.
/// </summary>
public abstract class DragAndDropControl : Control
{
  /// <summary>
  ///   Called when a DragDrop operation starts.
  /// </summary>
  protected virtual void OnElementDragStart(UIElement draggedElement) { }

  /// <summary>
  ///   Called during a DragDrop operation in order to update the cursor, if needed.
  /// </summary>
  protected virtual void OnElementDragGiveCursorFeedback(object sender, GiveFeedbackEventArgs args) { }

  /// <summary>
  ///   Called during a DragDrop operation.
  /// </summary>
  protected virtual void OnElementDrag(UIElement draggedElement, MouseEventArgs args) { }

  /// <summary>
  ///   Called when an element was dropped without being on an element marked as a DropZone.
  /// </summary>
  /// <param name="draggedElement"></param>
  protected virtual void OnElementDragStop(UIElement draggedElement) { }

  /// <summary>
  ///   Called when this element is the potential target of a DragDrop operation.
  /// </summary>
  protected virtual void OnElementDragOver(DragAndDropData data, UIElement targetElement) { }

  /// <summary>
  ///   Called when this element stops being a potential target of a DragDrop operation.
  /// </summary>
  protected virtual void OnElementDragLeave(UIElement targetElement) { }

  /// <summary>
  ///   Required callback to handle <paramref name="targetElement"/>, 
  ///   provided with <see cref="DragAndDropData"/> containing both the source element being dragged and it's original data context.
  /// </summary>
  protected abstract void OnElementDropped(DragAndDropData data, UIElement targetElement);

  #region Attached Properties 

  /* === IsDraggable === */

  public static readonly DependencyProperty IsDraggableProperty = DependencyProperty
    .RegisterAttached(
      "IsDraggable",
      typeof(bool),
      typeof(DragAndDropControl),
      new PropertyMetadata(false, OnIsDraggableChanged));

  public static bool GetIsDraggable(DependencyObject obj) => (bool)obj.GetValue(IsDraggableProperty);
  public static void SetIsDraggable(DependencyObject obj, bool value) => obj.SetValue(IsDraggableProperty, value);

  /// <summary>
  ///   Adds event listeners to the element's GiveFeedback, MouseLeftButtonUp and MouseMove events.
  /// </summary>
  private static void OnIsDraggableChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
  {
    if (sender is not FrameworkElement element)
    {
      throw new InvalidOperationException($"IsDraggable may only be attached to FrameworkElements, got {sender.GetType()} instead.");
    }

    if ((bool)e.NewValue)
    {
      element.GiveFeedback += HandleGiveFeedback;
      element.MouseLeftButtonUp += HandleLeftMouseButtonUp;
      element.MouseMove += HandleMouseMove;
    }
    else
    {
      element.GiveFeedback -= HandleGiveFeedback;
      element.MouseLeftButtonUp -= HandleLeftMouseButtonUp;
      element.MouseMove -= HandleMouseMove;
    }
  }

  /* === IsDropZone === */

  public static readonly DependencyProperty IsDropZoneProperty = DependencyProperty
    .RegisterAttached(
      "IsDropZone",
      typeof(bool),
      typeof(DragAndDropControl),
      new PropertyMetadata(false, OnIsDropZoneChanged));

  public static bool GetIsDropZone(DependencyObject obj) => (bool)obj.GetValue(IsDropZoneProperty);
  public static void SetIsDropZone(DependencyObject obj, bool value) => obj.SetValue(IsDropZoneProperty, value);

  /// <summary>
  ///   Adds an event listener to this element's <see cref="UIElement.Drop"/> event.
  /// </summary>
  private static void OnIsDropZoneChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
  {
    if (sender is not FrameworkElement element)
    {
      throw new InvalidOperationException($"IsDraggable may only be attached to FrameworkElements, got {sender.GetType()} instead.");
    }

    if ((bool)e.NewValue)
    {
      element.AllowDrop = true;
      element.DragOver += HandleDragOver;
      element.DragLeave += HandleDragLeave;
      element.Drop += HandleDrop;
    }
    else
    {
      element.DragOver -= HandleDragOver;
      element.DragLeave -= HandleDragLeave;
      element.Drop -= HandleDrop;
    }
  }

  /* === Handler === */

  public static readonly DependencyProperty HandlerProperty = DependencyProperty
    .RegisterAttached(
      "Handler",
      typeof(DependencyObject),
      typeof(DragAndDropControl),
      new PropertyMetadata(null, OnHandlerChanged));

  public static DependencyObject GetHandler(DependencyObject obj) => (DependencyObject)obj.GetValue(HandlerProperty);
  public static void SetHandler(DependencyObject obj, bool value) => obj.SetValue(HandlerProperty, value);

  private static void OnHandlerChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
  {
    // Simple validation
    if (sender is not UIElement element || e.NewValue is not DragAndDropControl dndControl)
    {
      throw new InvalidOperationException($"Can not set {e.NewValue} as handler. Handler needs to be of type DragAndDropControl.");
    }
  }

  #endregion

  /// <summary>
  ///   Attempts to retrieve the correct <see cref="DragAndDropControl"/> handler for the provided element.<br/>
  ///   This may either be via the defined Handler property, or the element itself if no handler property was attached. 
  /// </summary>
  private static DragAndDropControl RetrieveHandler(DependencyObject obj)
  {
    if (obj.GetValue(HandlerProperty) is not DragAndDropControl handler)
    {
      if (obj is DragAndDropControl)
      {
        handler = (DragAndDropControl)obj;
      }
      else // Todo: last resort if ((handler = FindVisualParent<DragAndDropControl>(obj)!) == null)
      {
        throw new InvalidOperationException($"Could not find a DragAndDropControl for element {obj}. You may explicitly provide one using the DragAndDropControl.Handler attached property.");
      }
    }

    return handler;
  }

  /// <summary>
  ///   Dynamic handler for the <see cref="UIElement.MouseMove"/> event.
  ///   Called when the mouse cursor moves while on an element.
  ///   This also calls the <see cref="OnElementDrag(UIElement, MouseEventArgs)"/> method in order to handle any specific behaviour <b>during</b> the operation.
  /// </summary>
  private static void HandleMouseMove(object sender, MouseEventArgs args)
  {
    if (args.LeftButton == MouseButtonState.Pressed)
    {
      FrameworkElement element = (FrameworkElement)sender;
      DragAndDropControl handler = RetrieveHandler(element);
      DragAndDropData data = new(element, element.DataContext);
      DragDrop.DoDragDrop(element, data, DragDropEffects.All);
      handler.OnElementDrag(element, args);
    }
  }

  /// <summary>
  ///   Dynamic handler for the <see cref="UIElement.GiveFeedback"/> event.
  /// </summary>
  private static void HandleGiveFeedback(object sender, GiveFeedbackEventArgs args)
  {
    FrameworkElement element = (FrameworkElement)sender;
    DragAndDropControl handler = RetrieveHandler(element);
    handler.OnElementDragGiveCursorFeedback(element, args);
  }

  /// <summary>
  ///   Dynamic handler for the <see cref="UIElement.MouseLeftButtonUp"/> event.
  /// </summary>
  private static void HandleLeftMouseButtonUp(object sender, MouseButtonEventArgs args)
  {
    FrameworkElement element = (FrameworkElement)sender;
    DragAndDropControl handler = RetrieveHandler(element);
    handler.OnElementDragStop(element);
  }

  /// <summary>
  ///   Dynamic handler for the <see cref="UIElement.DragOver"/> event.
  /// </summary>
  private static void HandleDragOver(object sender, DragEventArgs args)
  {
    FrameworkElement element = (FrameworkElement)sender;
    DragAndDropControl handler = RetrieveHandler(element);
    handler.OnElementDragOver((DragAndDropData)args.Data.GetData(typeof(DragAndDropData)), element);
  }

  /// <summary>
  ///   Dynamic handler for the <see cref="UIElement.DragLeave"/> event.
  /// </summary>
  private static void HandleDragLeave(object sender, DragEventArgs args)
  {
    FrameworkElement element = (FrameworkElement)sender;
    DragAndDropControl handler = RetrieveHandler(element);
    handler.OnElementDragLeave(element);
  }

  /// <summary>
  ///   Dynamic handler for the <see cref="UIElement.Drop"/> event.
  /// </summary>
  private static void HandleDrop(object sender, DragEventArgs args)
  {
    FrameworkElement element = (FrameworkElement)sender;
    DragAndDropControl handler = RetrieveHandler(element);
    handler.OnElementDropped((DragAndDropData)args.Data.GetData(typeof(DragAndDropData)), element);
  }

  private static T? FindVisualParent<T>(DependencyObject child) where T : DependencyObject
  {
    // Todo: find a better solution to this, if there even is one.
    // There are, in fact, many controls even in the WPF standard that do similar lookups,
    // so I assume it can't be _that_ bad, but every clean solution kind of seems to need some ugly spot in it. :(

    DependencyObject parent = VisualTreeHelper.GetParent(child);
    while (parent != null)
    {
      if (parent is T expected) return expected;
      parent = VisualTreeHelper.GetParent(parent);
    }
    return null;
  }
}