Question

C＃XBap应用程序

我有一个带有四个TabItem的TabControl。其中两个TabItem只包含来自WPFToolkit的DataGrid，它从SQL Server数据库中提取相当小的数据表（100行乘4列）。我的问题是，当我加载我的应用程序并单击其中一个包含数据网格的TabItem时。在我将这个标签置于焦点之前，似乎有2-3秒的暂停。这仅在第一次单击选项卡时发生。它似乎是数据网格的渲染。

如何在应用程序加载时使这些选项卡呈现为预渲染，以便当用户单击选项卡时，在选项卡显示之前没有2-3秒的初始暂停。

由于

Answer 1

我们使用标准的WPF TabControl，问题是每次更改SelectedItem时都会破坏VisualTree。

我们最终做的是创建一个特殊的TabControl（我称之为TabControlEx），它保留所有渲染的项目，但选择只显示/隐藏TabItems的ContentPresenters。

以下是相关代码

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

namespace MVVM.Demo
{
    /// <summary>
    /// The standard WPF TabControl is quite bad in the fact that it only
    /// even contains the current TabItem in the VisualTree, so if you
    /// have complex views it takes a while to re-create the view each tab
    /// selection change.Which makes the standard TabControl very sticky to
    /// work with. This class along with its associated ControlTemplate
    /// allow all TabItems to remain in the VisualTree without it being Sticky.
    /// It does this by keeping all TabItem content in the VisualTree but
    /// hides all inactive TabItem content, and only keeps the active TabItem
    /// content shown.
    /// </summary>
    [TemplatePart(Name = "PART_ItemsHolder", Type = typeof(Panel))]
    public class TabControlEx : TabControl
    {
        #region Data
        private Panel itemsHolder = null;
        #endregion

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

        #region Public/Protected Methods
        /// <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();
                    break;

                case NotifyCollectionChangedAction.Add:
                case NotifyCollectionChangedAction.Remove:
                    if (e.OldItems != null)
                    {
                        foreach (var item in e.OldItems)
                        {
                            ContentPresenter cp = FindChildContentPresenter(item);
                            if (cp != null)
                            {
                                itemsHolder.Children.Remove(cp);
                            }
                        }
                    }

                    // don't do anything with new items because we don't want to
                    // create visuals that aren't being shown

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

        #region Private Methods

        /// <summary>
        /// in some scenarios we need to update when loaded in case the 
        /// ApplyTemplate happens before the databind.
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void TabControlEx_Loaded(object sender, RoutedEventArgs e)
        {
            UpdateSelectedItem();
        }

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

        /// <summary>
        /// generate a ContentPresenter for the selected item
        /// </summary>
        private 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>
        private 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>
        private 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;
        }
        #endregion
    }
}

你要模仿它的地方（你可能需要将它展开为左/右TabStripLocation）

<ControlTemplate x:Key="MainTabControlTemplateEx"
                TargetType="{x:Type controls:TabControlEx}">
    <Grid>

        <Grid.RowDefinitions>
            <RowDefinition x:Name="row0" Height="Auto"/>
            <RowDefinition x:Name="row1" Height="4"/>
            <RowDefinition x:Name="row2" Height="*"/>
        </Grid.RowDefinitions>

        <TabPanel x:Name="tabpanel" 
            Background="{StaticResource OutlookButtonHighlight}"
            Margin="0"
            Grid.Row="0"
            IsItemsHost="True" />

        <Grid x:Name="divider"
                Grid.Row="1" Background="Black" 
                HorizontalAlignment="Stretch"/>

        <Grid x:Name="PART_ItemsHolder"
                Grid.Row="2"/>
    </Grid>
    <!-- no content presenter -->
    <ControlTemplate.Triggers>
        <Trigger Property="TabStripPlacement" Value="Top">
            <Setter TargetName="tabpanel" Property="Grid.Row" Value="0"/>
            <Setter TargetName="divider" Property="Grid.Row" Value="1"/>
            <Setter TargetName="PART_ItemsHolder" Property="Grid.Row" Value="2" />
            <Setter TargetName="row0" Property="Height" Value="Auto" />
            <Setter TargetName="row1" Property="Height" Value="4" />
            <Setter TargetName="row2" Property="Height" Value="*" />
        </Trigger>
        <Trigger Property="TabStripPlacement" Value="Bottom">
            <Setter TargetName="tabpanel" Property="Grid.Row" Value="2" />
            <Setter TargetName="divider" Property="Grid.Row" Value="1" />
            <Setter TargetName="PART_ItemsHolder" Property="Grid.Row" Value="0" />
            <Setter TargetName="row0" Property="Height" Value="*" />
            <Setter TargetName="row1" Property="Height" Value="4" />
            <Setter TargetName="row2" Property="Height" Value="Auto" />
        </Trigger>
    </ControlTemplate.Triggers>

</ControlTemplate>

您可以像这样使用

<local:TabControlEx 
        IsSynchronizedWithCurrentItem="True" 
        ItemsSource="{Binding Path=Workspaces}" 
        Template="{StaticResource MainTabControlTemplateEx}">
</local:TabControlEx>

它运作良好，我们已经使用它很长时间了很长时间

Answer 2

问题不在于获取数据（您可以在单独的线程中预加载）但实际上是在数据网格中构建可视项目。

如果您使用Snoop查看，您可以看到有很多可视项目，如果您不需要所有数据网格功能，您可以使用更简单的表示（ListView / ItemsControl / Custom）

Answer 3

在选择时动态加载Tab内容，以便UI响应，使用类似于下面的代码：

 private void tab_Selected(object sender, EventArgs e)
{
   //Get the selected tab
 Action loadTab = delegate
{
  LoadSelectedTab(tabItem);
}
Dispatcher.BeginInvoke(DispatcherPriority.Background, loadTab);
}
public void LoadSelectedTab(TabItem item)
{
  item.Content = new EmployeeTab();
  .....
}

UI repsonse将非常快，UI开始加载非常快，你没有看到任何延迟暂停

Answer 4

无法添加评论，但要感谢sacha的回答，并将其扩大一点。该TabControl的加载存在问题，因此第一个选项卡未显示（在可视树中不存在）。将以下代码添加到“公共/受保护的方法”区域即可解决此问题。

    /// <summary>
    /// There was a flaky issue when first tab was uninitialized
    /// </summary>
    /// <param name="oldValue"></param>
    /// <param name="newValue"></param>
    protected override void OnItemsSourceChanged(IEnumerable oldValue, IEnumerable newValue)
    {
        base.OnItemsSourceChanged(oldValue, newValue);
        UpdateSelectedItem();
    }

如何在WPF中的TabItem上预呈现控件？

4 个答案: