wpf - 在选项卡之间切换时，WPF 选项卡控件中遇到延迟(每次切换选项卡时都会创建新的 View 实例)

Question

我是 WPF 的初学者，在编码时我必须做很多学习，所以请耐心等待。我搜索了文件，但找不到任何适合我的问题的答案。

我当前有一个包含选项卡控件的 View ，该控件最初加载了 2 个默认选项卡，然后根据用户操作以编程方式添加和删除其他选项卡。

我这样做的方法是为不同类型的 View 创建一个可观察的 ViewModel 集合，我可以添加和删除这些 View 。

在 View 的 Xaml 中，我有一些不同的 DataTemplates，用于确定要加载哪种类型的 View 。

我面临的问题是，当我从一个选项卡单击到另一个选项卡时，我看到了延迟 - 接近一秒。我在 View 的代码后面放置了一个断点，并注意到每次单击该选项卡时都会为 View 调用 InitializeComponent() 方法。

ViewModel 的集合称为 ActiveContents，其中包含 ActiveContent 类型的项目。 ActiveContent 类是我创建的，它包含一个名为 ContentItem 的对象，该对象将保存 ViewModel。

以下是我的项目中的一些代码:

xaml 中的数据模板之一:

<UserControl.Resources>
    <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
        .
        .
        .
            </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>

        <DataTemplate DataType="{x:Type viewmodels:InstallQueueViewModel}">
            <local:InstallQueueView DataContext="{Binding Path=DataContext.InstallQueueVM, 
                RelativeSource={RelativeSource AncestorType={x:Type UserControl}}}" />
        </DataTemplate>
    .
    .
    .
        <DataTemplate x:Key="DataTemplateTabControlContent">
            <ContentControl Content="{Binding ContentItem}"></ContentControl>
        </DataTemplate>
</UserControl.Resources>

<Grid>
    <TabControl ItemsSource="{Binding ActiveContents}"
                SelectedIndex="{Binding SelectedTab, Mode=TwoWay}"
                IsSynchronizedWithCurrentItem="True"
                ItemContainerStyle="{DynamicResource TabItemContainerStyleSpacedMedium}"
                ItemTemplate="{StaticResource DataTemplateTabControlHeader}"
                ContentTemplate="{StaticResource DataTemplateTabControlContent}">
    </TabControl>

有没有办法阻止它每次加载 View 的新实例？就像我说的，这是我第一次，所以完全有可能我只是以错误的方式来做这件事 - 如果我应该改变我做某事的方式，请告诉我。

我想我应该提到的一件事是，我使用 MahApps 来获得应用程序的 Metro 感觉。我不确定这是否是造成我所看到的延迟的原因。

主要是当我切换到包含 DataGrid 的 View 时，我发现了延迟。我正在考虑将其更改为 ListView 因为数据是只读的，我希望这会给我带来一些改进，但根本原因仍然没有解决。

任何帮助将不胜感激。

Answer 1

这对我的一个项目来说是一个巨大的问题，我最终创建了一个扩展的 TabControl 来保存每个 TabItem 的 ContentPresenter > 切换选项卡时，并在返回选项卡时重新加载。

原版代码来自here ，或者您可以在不久前关于该问题的相关问题中找到我的代码版本:WPF TabControl - Preventing Unload on Tab Change?

// 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;
    }
}