使用附加属性从 View 更新 ViewModel

Question

我想在DataGrid 上保存/加载列订单。我正在尝试在我的 ViewModel 和 View 之间绑定一个包含 DataGrid 列索引的 StringCollection。我可以在我的 ViewModel 中成功设置StringCollection，它使用附加属性更新视图，但是当我使用 UI 更改视图中的列顺序时，ViewModel 不会更新以反映新的列顺序。如何让我的 ViewModel 监听 View 的变化？

我的 xaml

<DataGrid ItemsSource="{Binding MyObservableCollection}"
          attach:DataGridColumnChanger.ColumnOrder="{Binding ColumnOrders, Mode=TwoWay}">
</DataGrid>

我的附属财产

public class DataGridColumnChanger : DependencyObject
{
   #region dependency properties
   public static StringCollection GetColumnOrder(DependencyObject obj)
   {
      return (StringCollection)obj.GetValue(ColumnOrderProperty);
   }
   public static void SetColumnOrder(DependencyObject obj, StringCollection value)
   {
      obj.SetValue(ColumnOrderProperty, value);
   }
   public static readonly DependencyProperty ColumnOrderProperty = DependencyProperty.RegisterAttached("ColumnOrder",
      typeof(StringCollection), typeof(DataGridColumnChanger), new UIPropertyMetadata(new StringCollection(), new PropertyChangedCallback(OnColumnOrderChange)));

   #endregion

   private static void OnColumnOrderChange(DependencyObject obj, DependencyPropertyChangedEventArgs e)
   {
      var element = obj as DataGrid;
      if (element != null)
      {
         try
         {
            StringCollection collection = (StringCollection)e.NewValue;
            if (collection != null && collection.Count > 0)
            {
               for (int j = 0; j <= element.Columns.Count - 1; j++)
               {
                  int index = Convert.ToInt32(collection[j]);
                  element.Columns[j].DisplayIndex = index;
               }
            }
         }
         catch
         {
            Console.WriteLine("Error");
         }
      }
   }
}

我的视图模型

private StringCollection columnOrders = new StringCollection();
public System.Collections.Specialized.StringCollection ColumnOrders
{
   get => this.columnOrders;
   set
   {
      this.columnOrders = value;
      this.RaisePropertyChanged("ColumnOrders");
   }
}

Answer 1

您的附加行为必须监听附加的DataGrid.ColumnDisplayIndexChanged 事件。处理此事件并同步附加DataGrid的源集合：

private static ConditionalWeakTable<DataGrid, DataGrid> RegisteredInstances { get; } = new ConditionalWeakTable<DataGrid, DataGrid>();

private static void OnColumnOrderChange(DependencyObject obj, DependencyPropertyChangedEventArgs e)
{
  if (element is DataGrid dataDrid)
  {
    if (!RegisteredInstances.TryGetValue(dataDrid, out _))
    {
      RegisteredInstances.Add(dataDrid, dataDrid);
      dataDrid.ColumnDisplayIndexChanged += OnDataGridColumnOrderChanged;
    }

    try
    {
      ...
    }
    catch
    {
      ...
    }              
  }
}

private static void OnDataGridColumnOrderChanged(object sender, DataGridColumnEventArgs e)
{
  var dataGrid = sender as DataGrid;
  if (RegisteredInstances.TryGetValue(dataDrid, out _))
  {
    StringCollection newColumnOrderCollection = new StringCollection();

    // TODO::Populate newColumnOrderCollection using e.Column.DisplayIndex 
    // and raise property changed by assigning the collection to the OrderCollection property
  }
}

Answer 2

您的附加属性实现仅对集合的更改做出反应。为了对用户界面的变化做出反应，您必须订阅DataGrid.ColumnDisplayIndexChanged 事件。

以下是如何从您的代码开始执行此操作的示例。您没有指定是否要在每次显示索引更改时创建一个新集合，因此在下文中，我重用该集合。

请注意，如果您绑定的集合为空或没有all 列的项目，则集合现在从DataGrid 初始化，这很容易出错。该集合在 Loaded 处理程序事件中初始化，因为在完全加载 DataGrid 之前，这些列会将 DisplayIndex 设置为 -1。

DisplayIndex 属性在添加到 DataGrid.Columns 集合之前的默认值为 -1。当列添加到 DataGrid 时，该值会更新。

public class DataGridColumnChanger : DependencyObject
{
   #region dependency properties
   public static StringCollection GetColumnOrder(DependencyObject obj)
   {
      return (StringCollection)obj.GetValue(ColumnOrderProperty);
   }
   public static void SetColumnOrder(DependencyObject obj, StringCollection value)
   {
      obj.SetValue(ColumnOrderProperty, value);
   }
   public static readonly DependencyProperty ColumnOrderProperty = DependencyProperty.RegisterAttached("ColumnOrder",
      typeof(StringCollection), typeof(DataGridColumnChanger), new UIPropertyMetadata(new StringCollection(), OnColumnOrderChange));

   #endregion

   private static void OnColumnOrderChange(DependencyObject obj, DependencyPropertyChangedEventArgs e)
   {
      var dataGrid = (DataGrid)obj;

      if (e.NewValue == null)
      {
         // No collection set, unsubscribe from the event.
         dataGrid.ColumnDisplayIndexChanged -= OnDisplayIndexChanged;
      }
      else
      {
         // A new collection was set, subscribe to the event to sync it with user driven reordering.
         dataGrid.ColumnDisplayIndexChanged += OnDisplayIndexChanged;

         var columnOrder = (StringCollection)e.NewValue;

         // The collection is not fully initialized.
         if (columnOrder == null || columnOrder.Count <= dataGrid.Columns.Count)
         {
            // Must be updated after load, otherwise the indices are -1.
            dataGrid.Loaded += OnDataGridLoaded;
            return;
         }

         try
         {
            for (var i = 0; i <= dataGrid.Columns.Count - 1; i++)
            {
               var index = Convert.ToInt32(columnOrder[i]);
               dataGrid.Columns[i].DisplayIndex = index;
            }
         }
         catch
         {
            Console.WriteLine("Error");
         }
      }
   }

   private static void OnDataGridLoaded(object sender, RoutedEventArgs e)
   {
      var dataGrid = (DataGrid)sender;
      var columnOrder = GetColumnOrder(dataGrid);

      // Since the collection is not fully initialized, clear it as it could be invalid.
      columnOrder.Clear();

      // Initialize the collection with the current column display indices.
      foreach (var column in dataGrid.Columns)
         columnOrder.Add(column.DisplayIndex.ToString());

      dataGrid.Loaded -= OnDataGridLoaded;
   }

   private static void OnDisplayIndexChanged(object sender, DataGridColumnEventArgs e)
   {
      var dataGrid = (DataGrid)sender;
      var columnOrder = GetColumnOrder(dataGrid);
      var columnIndex = dataGrid.Columns.IndexOf(e.Column);

      // Sync the changed column display index.
      columnOrder[columnIndex] = e.Column.DisplayIndex.ToString();
   }
}

我还想建议您使用int 的集合而不是StringCollection，因为显示索引的类型为int，并且代码中的转换是不必要的。

接下来，您可能有兴趣创建一个Behavior<DataGrid> 而不是附加的属性类，因为它已经提供了对关联对象和其他有用方法的访问，所以它更容易编写。为此，您可以安装Microsoft.Xaml.Behaviors.Wpf NuGet 包。