命令绑定到用户控件拖放

Question

如何通过使用 WPF 中的命令模式来创建响应用户控件的拖放事件的 UI？

Answer 1

在用户控件上

实现一个有参数的命令。我将 ICommand 与 Josh Smiths RelayCommand 一起使用，但我对其进行了扩展以提供参数。 （此答案末尾的代码）

  /// <summary>
  /// Gets and Sets the ICommand that manages dragging and dropping.
  /// </summary>
  /// <remarks>The CanExecute will be called to determin if a drop can take place, the Executed is called when a drop takes place</remarks>
  public ICommand DragDropCommand {
     get { return (ICommand)GetValue(DragDropCommandProperty); }
     set { SetValue(DragDropCommandProperty, value); }

现在您可以将视图模型绑定到此命令。

为我们的实体拖动类型设置另一个属性（您可以对此进行硬编码），但我将此用户控件重复用于不同的事物，并且我不希望一个控件在放置时接受错误的实体类型。

  /// <summary>
  /// Gets and Sets the Name of the items we are dragging
  /// </summary>
  public String DragEntityType {
     get { return (String)GetValue(DragEntityTypeProperty); }
     set { SetValue(DragEntityTypeProperty, value); }
  }

覆盖 OnPreviewLeftMouseButtonDown

   protected override void OnPreviewMouseLeftButtonDown(MouseButtonEventArgs e) {
      //find the item the mouse is over, i.e. the one you want to drag.
      var itemToDrag = FindItem(e);

      //move the selected items, using the drag entity type
      DataObject data = new DataObject(this.DragEntityType, itemToDrag);
      //use the helper class to initiate the drag
      DragDropEffects de = DragDrop.DoDragDrop(this, data, DragDropEffects.Move);

      //call the base
      base.OnPreviewMouseLeftButtonDown(e);
   }

当您调用 DragDrop.DoDragDrop 时，将在适当的时候调用以下方法

重写 OnDragOver 和 OnDragDrop 方法，并使用命令询问是否可以拖放

protected override void OnDragOver(DragEventArgs e) {

     //if we can accept the drop
     if (this.DragDropCommand != null && this.DragDropCommand.CanExecute(e.Data)) {

        // Console.WriteLine(true);
     }
     //otherwise
     else {
        e.Effects = DragDropEffects.None;
        e.Handled = true;
     }
     base.OnDragOver(e);
  }

  protected override void OnDrop(DragEventArgs e) {

     if (this.DragDropCommand == null) { }
     //if we dont allow dropping on ourselves and we are trying to do it
     //else if (this.AllowSelfDrop == false && e.Source == this) { }
     else {
        this.DragDropCommand.Execute(e.Data);
     }
     base.OnDrop(e);
  }

在视图模型中

然后当你在视图模型中设置你的命令时，使用类似的东西，然后将命令绑定到你的用户控件

     this.MyDropCommand = new ExtendedRelayCommand((Object o) => AddItem(o), (Object o) => { return ItemCanBeDragged(o); });

通常您是从一个用户控件拖动到另一个用户控件，因此您将为一个用户控件设置一个命令，为另一个用户控件设置一个命令，每个命令都有您可以接受的不同 DragEntityType。两个用户控件，一个用于拖动，一个用于放置，反之亦然。每个用户控件都有不同的 DragEntityType，因此您可以分辨出拖动的来源。

  private Boolean ItemCanBeDragged(object o) {
     Boolean returnValue = false;

     //do they have permissions to dragt
     if (this.HasPermissionToDrag) {

        IDataObject data = o as IDataObject;

        if (data == null) { }
        //this line looks up the DragEntityType
        else if (data.GetDataPresent("ItemDragEntityTypeForItemWeAreDragging")) {
           returnValue = true;
        }
     }
     return returnValue;
  }

当我们放下时

  private void AddItem(object o) {
     IDataObject data = o as IDataObject;

     if (data == null) { }
     else {
        MyDataObject myData = data.GetData("ItemDragEntityTypeForItemWeAreDroppingHere") as MyDataObject ;

        if (myData == null) { }
        else {
            //do something with the dropped data
        }
     }
  }

我可能遗漏了一些东西，但是这种技术让我可以询问视图模型是否可以拖动一个项目，并让我询问视图模型是否可以放下（如果视图模型将接受该项目）它的可绑定项，并且它很好地分离了视图/视图模型。如果您有任何问题，请随时提出。

扩展中继命令，感谢 Josh Smith...

   /// <summary>
   /// A command whose sole purpose is to 
   /// relay its ExtendedFunctionality to other
   /// objects by invoking delegates. The
   /// default return value for the CanExecute
   /// method is 'true'.
   /// </summary>
   public class ExtendedRelayCommand : ICommand {
      #region Constructors

      /// <summary>
      /// Creates a new command that can always execute.
      /// </summary>
      /// <param name="execute">The execution logic.</param>
      public ExtendedRelayCommand(Action<Object> execute)
         : this(execute, null) {
      }

      /// <summary>
      /// Creates a new command.
      /// </summary>
      /// <param name="execute">The execution logic.</param>
      /// <param name="canExecute">The execution status logic.</param>
      public ExtendedRelayCommand(Action<Object> execute, Func<Object, bool> canExecute) {
         if (execute == null)
            throw new ArgumentNullException("execute");

         _execute = execute;
         _canExecute = canExecute;
      }

      #endregion // Constructors

      #region ICommand Members

      [DebuggerStepThrough]
      public bool CanExecute(object parameter) {
         return _canExecute == null ? true : _canExecute(parameter);
      }

      public event EventHandler CanExecuteChanged {
         add {
            if (_canExecute != null)
               CommandManager.RequerySuggested += value;
         }
         remove {
            if (_canExecute != null)
               CommandManager.RequerySuggested -= value;
         }
      }

      public void Execute(object parameter) {
         _execute(parameter);
      }

      #endregion // ICommand Members

      #region Fields

      readonly Action<Object> _execute;
      readonly Func<Object, bool> _canExecute;

      #endregion // Fields
   }