将 TabControl ItemsSource 绑定到 ViewModels 的 ObservableCollection 会导致内容在焦点上刷新

Question

我正在使用 MVVM 框架创建一个 WPF 应用程序，并且我采用了 Josh Smith 关于 MVVM here... 的文章中的几个功能

最重要的是，我将 TabControl 绑定到 ViewModel 的 ObservableCollection。这意味着正在使用一个选项卡式 MDI 界面，该界面将 UserControl 显示为 TabItem 的内容。我在应用程序中看到的问题是，当我有多个选项卡并且在选项卡之间来回切换时，每次更改选项卡时都会引用内容。

如果您下载 Josh Smith 的源代码，您会发现他的应用也存在同样的问题。例如，单击“查看所有客户”按钮并向下滚动到 ListView 的底部。接下来单击“创建新客户”按钮。当您切换回 All Customer 视图时，您会注意到 ListView 滚动回顶部。如果您切换回 New Customer 选项卡并将光标放在其中一个 TextBoxes 中，然后切换到 All Customers 选项卡并返回，您会注意到光标现在消失了。

我想这是因为我使用的是 ObservableCollection，但我不能确定。有什么办法可以防止tab的内容在收到焦点时刷新？

编辑： 当我在我的应用程序上运行分析器时，我发现了我的问题。我正在为我的 ViewModels 定义一个 DataTemplate，因此它知道如何在 ViewModel 显示在选项卡中时呈现它......就像这样：

<DataTemplate DataType="{x:Type vm:CustomerViewModel}">
    <vw:CustomerView/>
</DataTemplate>

所以每当我切换到不同的选项卡时，它都必须重新创建 ViewModel。我通过将 ViewModels 的 ObservableCollection 更改为 UserControls 的 ObservableCollection 来临时修复它。但是，如果可能的话，我真的还是想使用 DataTemplates。有没有办法让 DataTemplate 工作？

Answer 1

WPF 的默认行为是卸载不可见的项目，包括卸载不可见的 TabItem。这意味着当您返回选项卡时，TabItem 会重新加载，并且任何未绑定的内容（例如滚动位置）都会被重置。

有一个很好的网站here，其中包含扩展 TabControl 并阻止它在切换选项卡时破坏其 TabItems 的代码，但它现在似乎不再存在。

这是我用来防止该问题的代码。它最初来自该站点，尽管我对其进行了一些更改。它在切换选项卡时保留 TabItems 的ContentPresenter，并在您返回页面时使用它来重绘 TabItem。它占用了更多内存，但我发现它的性能更好，因为 TabItem 不再需要重新创建它上面的所有控件。

// Extended TabControl which saves the displayed item so you don't get the performance hit of 
// unloading and reloading the VisualTree when switching tabs

// Obtained from http://eric.burke.name/dotnetmania/2009/04/26/22.09.28
// and made a some modifications so it reuses a TabItem's ContentPresenter when doing drag/drop operations

[TemplatePart(Name = "PART_ItemsHolder", Type = typeof(Panel))]
public class TabControlEx : System.Windows.Controls.TabControl
{
    // Holds all items, but only marks the current tab's item as visible
    private Panel _itemsHolder = null;

    // Temporaily holds deleted item in case this was a drag/drop operation
    private object _deletedObject = null;

    public TabControlEx()
        : base()
    {
        // this is necessary so that we get the initial databound selected item
        this.ItemContainerGenerator.StatusChanged += ItemContainerGenerator_StatusChanged;
    }

    /// <summary>
    /// if containers are done, generate the selected item
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    void ItemContainerGenerator_StatusChanged(object sender, EventArgs e)
    {
        if (this.ItemContainerGenerator.Status == GeneratorStatus.ContainersGenerated)
        {
            this.ItemContainerGenerator.StatusChanged -= ItemContainerGenerator_StatusChanged;
            UpdateSelectedItem();
        }
    }

    /// <summary>
    /// get the ItemsHolder and generate any children
    /// </summary>
    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();
        _itemsHolder = GetTemplateChild("PART_ItemsHolder") as Panel;
        UpdateSelectedItem();
    }

    /// <summary>
    /// when the items change we remove any generated panel children and add any new ones as necessary
    /// </summary>
    /// <param name="e"></param>
    protected override void OnItemsChanged(NotifyCollectionChangedEventArgs e)
    {
        base.OnItemsChanged(e);

        if (_itemsHolder == null)
        {
            return;
        }

        switch (e.Action)
        {
            case NotifyCollectionChangedAction.Reset:
                _itemsHolder.Children.Clear();

                if (base.Items.Count > 0)
                {
                    base.SelectedItem = base.Items[0];
                    UpdateSelectedItem();
                }

                break;

            case NotifyCollectionChangedAction.Add:
            case NotifyCollectionChangedAction.Remove:

                // Search for recently deleted items caused by a Drag/Drop operation
                if (e.NewItems != null && _deletedObject != null)
                {
                    foreach (var item in e.NewItems)
                    {
                        if (_deletedObject == item)
                        {
                            // If the new item is the same as the recently deleted one (i.e. a drag/drop event)
                            // then cancel the deletion and reuse the ContentPresenter so it doesn't have to be 
                            // redrawn. We do need to link the presenter to the new item though (using the Tag)
                            ContentPresenter cp = FindChildContentPresenter(_deletedObject);
                            if (cp != null)
                            {
                                int index = _itemsHolder.Children.IndexOf(cp);

                                (_itemsHolder.Children[index] as ContentPresenter).Tag =
                                    (item is TabItem) ? item : (this.ItemContainerGenerator.ContainerFromItem(item));
                            }
                            _deletedObject = null;
                        }
                    }
                }

                if (e.OldItems != null)
                {
                    foreach (var item in e.OldItems)
                    {

                        _deletedObject = item;

                        // We want to run this at a slightly later priority in case this
                        // is a drag/drop operation so that we can reuse the template
                        this.Dispatcher.BeginInvoke(DispatcherPriority.DataBind,
                            new Action(delegate()
                        {
                            if (_deletedObject != null)
                            {
                                ContentPresenter cp = FindChildContentPresenter(_deletedObject);
                                if (cp != null)
                                {
                                    this._itemsHolder.Children.Remove(cp);
                                }
                            }
                        }
                        ));
                    }
                }

                UpdateSelectedItem();
                break;

            case NotifyCollectionChangedAction.Replace:
                throw new NotImplementedException("Replace not implemented yet");
        }
    }

    /// <summary>
    /// update the visible child in the ItemsHolder
    /// </summary>
    /// <param name="e"></param>
    protected override void OnSelectionChanged(SelectionChangedEventArgs e)
    {
        base.OnSelectionChanged(e);
        UpdateSelectedItem();
    }

    /// <summary>
    /// generate a ContentPresenter for the selected item
    /// </summary>
    void UpdateSelectedItem()
    {
        if (_itemsHolder == null)
        {
            return;
        }

        // generate a ContentPresenter if necessary
        TabItem item = GetSelectedTabItem();
        if (item != null)
        {
            CreateChildContentPresenter(item);
        }

        // show the right child
        foreach (ContentPresenter child in _itemsHolder.Children)
        {
            child.Visibility = ((child.Tag as TabItem).IsSelected) ? Visibility.Visible : Visibility.Collapsed;
        }
    }

    /// <summary>
    /// create the child ContentPresenter for the given item (could be data or a TabItem)
    /// </summary>
    /// <param name="item"></param>
    /// <returns></returns>
    ContentPresenter CreateChildContentPresenter(object item)
    {
        if (item == null)
        {
            return null;
        }

        ContentPresenter cp = FindChildContentPresenter(item);

        if (cp != null)
        {
            return cp;
        }

        // the actual child to be added.  cp.Tag is a reference to the TabItem
        cp = new ContentPresenter();
        cp.Content = (item is TabItem) ? (item as TabItem).Content : item;
        cp.ContentTemplate = this.SelectedContentTemplate;
        cp.ContentTemplateSelector = this.SelectedContentTemplateSelector;
        cp.ContentStringFormat = this.SelectedContentStringFormat;
        cp.Visibility = Visibility.Collapsed;
        cp.Tag = (item is TabItem) ? item : (this.ItemContainerGenerator.ContainerFromItem(item));
        _itemsHolder.Children.Add(cp);
        return cp;
    }

    /// <summary>
    /// Find the CP for the given object.  data could be a TabItem or a piece of data
    /// </summary>
    /// <param name="data"></param>
    /// <returns></returns>
    ContentPresenter FindChildContentPresenter(object data)
    {
        if (data is TabItem)
        {
            data = (data as TabItem).Content;
        }

        if (data == null)
        {
            return null;
        }

        if (_itemsHolder == null)
        {
            return null;
        }

        foreach (ContentPresenter cp in _itemsHolder.Children)
        {
            if (cp.Content == data)
            {
                return cp;
            }
        }

        return null;
    }

    /// <summary>
    /// copied from TabControl; wish it were protected in that class instead of private
    /// </summary>
    /// <returns></returns>
    protected TabItem GetSelectedTabItem()
    {
        object selectedItem = base.SelectedItem;
        if (selectedItem == null)
        {
            return null;
        }

        if (_deletedObject == selectedItem)
        { 

        }

        TabItem item = selectedItem as TabItem;
        if (item == null)
        {
            item = base.ItemContainerGenerator.ContainerFromIndex(base.SelectedIndex) as TabItem;
        }
        return item;
    }
}

我通常使用的 TabControl 模板如下所示：

<Style x:Key="TabControlEx_NoHeadersStyle" TargetType="{x:Type local:TabControlEx}">
    <Setter Property="SnapsToDevicePixels" Value="true"/>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type localControls:TabControlEx}">
                <DockPanel>
                    <!-- This is needed to draw TabControls with Bound items -->
                    <StackPanel IsItemsHost="True" Height="0" Width="0" />
                    <Grid x:Name="PART_ItemsHolder" />
                </DockPanel>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

Answer 2

我相信这与绑定模式有关 - 您是否尝试过 {Binding Mode=OneWay}？ 绑定到任何“DataSource”对象也会导致同样的问题。

我不确定单向绑定是否是您正在寻找的所需行为，但我会从那里开始。

Answer 3

它与 ObservableCollection 无关。这是因为客户的视图被重用，而不是 WPF 为每个客户创建一个新的视图。

您可以查看 WPF Application Framework (WAF) 的 Writer 示例应用程序。它还为选项卡式 MDI 界面实现了 TabControl，但不会遇到您在帖子中提到的问题。这个问题在 Writer 中通过为每个文档“选项卡”创建一个自己的 UserControl 来解决。