WPF TabControl - 防止选项卡更改时卸载？

Question

当 WPF 选项卡控件中的选项卡发生更改时，是否有办法防止选项卡卸载/重新加载？或者，如果这是不可能的，是否有推荐的方法来缓存选项卡内容，以便不必在每次选项卡更改时重新生成它们？

例如，一个选项卡的 UI 是完全可定制的并存储在数据库中。当用户选择要处理的对象时，自定义布局中的项目将填充该对象的数据。用户期望初始加载或检索数据时出现轻微延迟，但在选项卡之间来回更改时不会出现延迟，并且更改选项卡时的延迟非常明显。

Answer 1

我在这里找到了一个解决方法（web-archive-link，因为网站已关闭）： https://web.archive.org/web/20120429044747/http://eric.burke.name/dotnetmania/2009/04/26/22.09。 28

它基本上存储选项卡的 ContentPresenter 并在切换选项卡时加载它而不是重绘它。拖/放选项卡时它仍然会导致延迟，因为这是一个删除/添加操作，但是经过一些修改，我也让它消失了（以较低的调度程序优先级运行删除代码，然后添加代码，所以添加操作有机会取消删除操作并使用旧的 ContentPresenter 而不是绘制新的）

编辑：上面的链接似乎不再起作用，所以我将在此处粘贴代码的副本。它已进行了一些修改以允许拖放，但它仍然应该以相同的方式工作。

using System;
using System.Windows;
using System.Windows.Threading;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;
using System.Collections.Specialized;

// 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;
    }
}

Answer 2

除此之外，我遇到了类似的问题，并设法通过缓存表示后面代码中选项卡项内容的用户控件来解决它。

在我的项目中，我有一个绑定到集合（MVVM）的选项卡控件。但是，第一个选项卡是概述，显示列表视图中所有其他选项卡的摘要。我遇到的问题是，每当用户将其选择从项目选项卡移至概述选项卡时，概述就会使用所有摘要数据重新绘制，这可能需要 10-15 秒，具体取决于集合中的项目数量。 （请注意，它们没有从数据库或其他任何东西重新加载实际数据，这纯粹是花费时间绘制摘要视图）。

我想要的是，摘要视图的加载仅在首次加载数据上下文时发生一次，并且选项卡之间的任何后续切换都是即时的。

解决方案：

涉及的类：
MainWindow.xaml - 包含选项卡控件的主页。
MainWindow.xaml.cs - 上面的代码隐藏。
MainWindowViewModel.cs - 上述视图的视图模型，包含集合。
Overview.xaml - 绘制概述选项卡项内容的用户控件。
OverviewViewModel.cs - 上述视图的视图模型。

步骤：

1. 用名为“OverviewPlaceholder”的空白用户控件替换“MainWindow.xaml”中用于绘制概述选项卡项的数据模板

2. 在“MainWindowViewModel.cs”中公开对“OverviewViewModel”的引用

3. 添加静态引用“MainWindow.xaml.cs”中的“Overview”

4. 将事件处理程序添加到用户控件“OverviewPlaceholder”的加载事件中，仅当该方法为 null 时，才会实例化对“Overview”的静态引用，将此引用的数据上下文设置为当前数据上下文中的“OverviewViewModel”引用（即“MainWindowViewModel”），并将占位符的内容设置为静态引用向“Overview”

现在，概述页面仅绘制一次，因为每次加载它（即用户单击概述选项卡）时，都会将已呈现的静态用户控件放回到页面上。

Answer 3

我有一个非常简单的解决方案来避免选项卡更改时选项卡重新加载，
在 tabItem 中使用 contentPresenter 而不是 content 属性。

例如（MVVM风格）

替换

经过