WPF中ItemsControl应用虚拟化时找到子元素的方法

原文:WPF中ItemsControl应用虚拟化时找到子元素的方法

 wpf的虚拟化技术会使UI的控件只初始化看的到的子元素， 而不是所有子元素都被初始化，这样会提高UI性能。

但是我们经常会遇到一个问题：

应用虚拟化后看不见的子元素因为没有实际产生导致ItemContainerGenerator的查找元素方法（ContainerFromIndex / ContainerFromItem）失效。

---

解决办法1：

（1）监听ItemsControl的ItemContainerGenerator的StatusChanged事件， 当GeneratorStatus为ContainerGenerated时再进行查找，

（2）遍历ItemsControl的Items，获取当前需要查找元素的Index，

（3）利用反射调用VirtualizingPanel的BringIndexIntoView，或者直接调用BringIntoView，然后强制滚动以产生Item（具体可以参考TreeViewItem的ExpandRecursicve的内部实现）。

需要注意的是：

（1）ItemContainerGenerator的StatuChanged事件会多次被触发， 原因是每次初始化的Item数量就是当前空间所能看到的数量，StatusChanged触发的次数就是总共Items除以每次初始的Item数量。

（2）调用BringIndexInToView不正确会导致InvalidOperationException，具体为“Cannot call StartAt when content generation is in progress.”  或者 ”无法在正在进行内容生成时调用StartAt。”。 可以用Dispatcher.BeginInvoke来解决， 如下面代码。

（3）当然ItemsControl中的虚拟化模式设置为Recycling， 即 VirtualizingStackPanel.VirtualizationMode ="Recycling"时，后端存储的子元素选中项会在ItermContainerGenerator重新产生子项时变为DisconnectedItem。可以把模式设置为Standard解决。

具体代码如下：

• 1. 查找入口

        private ItemsControl _currentSelectedItem = null;

        private void BtnFind_Click( object sender , System. Windows.RoutedEventArgs e)

        {

            if (string .IsNullOrEmpty( txtContent.Text ))

            {

                return;

            }

            if (_currentSelectedItem == null)

            {

                _currentSelectedItem = _treeView ;

            }

            else

            {

                if (_currentSelectedItem is TreeViewItem)

                {

                    ( _currentSelectedItem as TreeViewItem). IsExpanded = true ;

                }

            }

            if (_currentSelectedItem .ItemContainerGenerator. Status != GeneratorStatus .ContainersGenerated)

            {

                _currentSelectedItem.ItemContainerGenerator .StatusChanged -= new EventHandler(ItemContainerGenerator_StatusChanged );

                _currentSelectedItem.ItemContainerGenerator .StatusChanged += new EventHandler(ItemContainerGenerator_StatusChanged );

            }

            else

            {

                treeViewItem_BringIntoView(txtContent .Text);

            }

        }

• 2.StatusChanged事件的处理

      void ItemContainerGenerator_StatusChanged (object sender, EventArgs e)

        {

            var generator = sender as ItemContainerGenerator ;

            if (null == generator)

            {

                return;

            }

            //once the children have been generated, expand those children's children then remove the event handler

            if (generator .Status == GeneratorStatus.ContainersGenerated && _currentSelectedItem .ItemContainerGenerator. Status == GeneratorStatus .ContainersGenerated)

            {

                treeViewItem_BringIntoView(txtContent .Text);

            }

        }

• 3.具体虚拟化时的强制产生子元素及查找处理

 private void treeViewItem_BringIntoView(string findItem)

        {

            System.Diagnostics. Debug.WriteLine("enter treeViewItem_BringIntoview" );

            try

            {

                _currentSelectedItem.ApplyTemplate();

                ItemsPresenter itemsPresenter = (ItemsPresenter)_currentSelectedItem.Template.FindName("ItemsHost", (FrameworkElement)_currentSelectedItem);

                if (itemsPresenter != null )

                    itemsPresenter.ApplyTemplate();

                else

                    _currentSelectedItem.UpdateLayout();

                VirtualizingPanel virtualizingPanel = _currentSelectedItem.GetItemsHost() as VirtualizingPanel;

                virtualizingPanel.CallEnsureGenerator();

                int selectedIndex = -1;

                int count1 = _currentSelectedItem.Items.Count;

                for (int i = 0; i < count1; i++)

                {

                    ItemsItem1 tviItem = _currentSelectedItem.Items.GetItemAt(i) as ItemsItem1;

                    if (null != tviItem && tviItem.Label.Equals(findItem))

                    {

                        selectedIndex = i;

                        break;

                    }

                }

                if (selectedIndex < 0)

                {

                    return;

                }

                Action action = () =>

                {

                    TreeViewItem itemSelected = null ;

                    //Force to generate every treeView item by using scroll item

                    if (virtualizingPanel != null )

                    {

                        try

                        {

                            virtualizingPanel.CallBringIndexIntoView(selectedIndex);

                        }

                        catch (System.Exception ex)

                        {

                            System.Diagnostics. Debug.WriteLine("CallBringIndexIntoView exception : " + ex.Message);

                        }

                        itemSelected = (TreeViewItem)_currentSelectedItem.ItemContainerGenerator.ContainerFromIndex(selectedIndex);

                    }

                    else

                    {

                        itemSelected = (TreeViewItem)_currentSelectedItem.ItemContainerGenerator.ContainerFromIndex(selectedIndex);

                        itemSelected.BringIntoView();

                    }

                    if (null != itemSelected)

                    {

                        _currentSelectedItem = itemSelected;

                        (_currentSelectedItem as TreeViewItem ).IsSelected = true;

                        _currentSelectedItem.BringIntoView();

                    }

                };

                Dispatcher.BeginInvoke( DispatcherPriority.Background, action);

            }

            catch (System.Exception ex)

            {

                //

            }

        }

• 4.xaml代码

<Window

    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

    xmlns:x ="http://schemas.microsoft.com/winfx/2006/xaml"

    xmlns:d ="http://schemas.microsoft.com/expression/blend/2008"

    xmlns:mc ="http://schemas.openxmlformats.org/markup-compatibility/2006"

    mc:Ignorable ="d"

    x:Class ="WpfApplication1.MainWindow"

    x:Name ="Window"

    Title="MainWindow"

    Width="640"

    Height="480" >

    <Window.Resources>

        <HierarchicalDataTemplate

            x:Key ="ItemsItem1Template"

            ItemsSource="{Binding Items}" >

            <StackPanel>

                <TextBlock

                    Text="{Binding Label}" />

            </StackPanel>

        </HierarchicalDataTemplate>

    </Window.Resources>

    <Grid

        x:Name ="LayoutRoot" >

        <TreeView

            x:Name ="_treeView"

            HorizontalAlignment="Left"

            Width="287"

            d:DataContext ="{Binding}"

            ItemsSource="{Binding Items, Source ={StaticResource SampleDataSource }}"

            ItemTemplate="{DynamicResource ItemsItem1Template}"

            VirtualizingStackPanel.IsVirtualizing ="True"

             VirtualizingStackPanel.VirtualizationMode ="Standard"

              />

        <Button

            x:Name ="btnFind"

            Content="Find"

            HorizontalAlignment="Right"

            Margin="0,8,8,0"

            VerticalAlignment="Top"

            Width="75"

            Click="BtnFind_Click" />

        <TextBox

            x:Name ="txtContent"

            Margin="291,8,87,0"

            TextWrapping="Wrap"

            VerticalAlignment="Top"

            Height="21.837" />

    </Grid>

</Window>

• 5.反射系统控件私有方法的类

public static class WPFUIElementExtension

    {

        #region Functions to get internal members using reflection

        // Some functionality we need is hidden in internal members, so we use reflection to get them

        #region ItemsControl.ItemsHost

        static readonly PropertyInfo ItemsHostPropertyInfo = typeof (ItemsControl). GetProperty("ItemsHost" , BindingFlags.Instance | BindingFlags. NonPublic);

        public static Panel GetItemsHost(this ItemsControl itemsControl)

        {

            Debug.Assert (itemsControl != null);

            return ItemsHostPropertyInfo .GetValue( itemsControl, null ) as Panel;

        }

        #endregion ItemsControl.ItemsHost

        #region Panel.EnsureGenerator

        private static readonly MethodInfo EnsureGeneratorMethodInfo = typeof(Panel ).GetMethod( "EnsureGenerator", BindingFlags .Instance | BindingFlags.NonPublic );

        public static void CallEnsureGenerator(this Panel panel)

        {

            Debug.Assert (panel != null);

            EnsureGeneratorMethodInfo.Invoke (panel, null);

        }

        #endregion Panel.EnsureGenerator

        #region VirtualizingPanel. BringIndexIntoView

        private static readonly MethodInfo BringIndexIntoViewMethodInfo = typeof(VirtualizingPanel ).GetMethod( "BringIndexIntoView", BindingFlags .Instance | BindingFlags.NonPublic );

        public static void CallBringIndexIntoView(this VirtualizingPanel virtualizingPanel, int index)

        {

            Debug.Assert (virtualizingPanel != null);

            BringIndexIntoViewMethodInfo.Invoke (virtualizingPanel, new object [] { index });

        }

        #endregion VirtualizingPanel. BringIndexIntoView

        #endregion Functions to get internal members using reflection

    }

---

解决方法2：

（1）参考方法1的第一步解决方法

（2）遍历ItemsControl的Items， 根据ContainerFromIndex去找到当前可见的元素的index。

（3）利用BringInoView去滚动现有的Item以便UI产生后续的子元素， 然后循环直到找见要查找的子元素。（遍历分为2部分，向前遍历和向后遍历）

注意事项：

（1）参考方法1的第一注意事项

（2）因为比方法1多了一次循环遍历，当items很多时有点卡顿，不过还在可以忍受的范围。

具体代码：

1.参考方法1的代码，具体只有强制生成子元素的方法有区别， 即与方法1中的步骤3有区别。

2.如下：

  private void treeViewItem_BringIntoView2(string findItem)

        {

            System.Diagnostics. Debug .WriteLine("enter treeViewItem_BringIntoview" );

            try

            {

                _currentSelectedItem.ApplyTemplate();

                ItemsPresenter itemsPresenter = (ItemsPresenter )_currentSelectedItem.Template.FindName( "ItemsHost", (FrameworkElement )_currentSelectedItem);

                if (itemsPresenter != null )

                    itemsPresenter.ApplyTemplate();

                else

                    _currentSelectedItem.UpdateLayout();

                VirtualizingPanel virtualizingPanel = _currentSelectedItem.GetItemsHost() as VirtualizingPanel ;

                virtualizingPanel.CallEnsureGenerator();

                TreeViewItem itemTemp = null ;

                ItemsItem1 objTemp = null ;

                int visiableIndex = -1;

                int findIndex = -1;

                int count1 = _currentSelectedItem.Items.Count;

                for (int i = 0; i < count1; i++)

                {

                    itemTemp = ( TreeViewItem )_currentSelectedItem.ItemContainerGenerator.ContainerFromIndex(i);

                    if (null != itemTemp)

                    {

                        visiableIndex = i;

                    }

                    objTemp = _currentSelectedItem.Items.GetItemAt(i) as ItemsItem1 ;

                    if (null != objTemp && objTemp.Label.Equals(findItem))

                    {

                        findIndex = i;

                    }

                }

                if (findIndex == -1 || visiableIndex == -1)

                {

                    return ;

                }

                if (findIndex < visiableIndex)

                {

                    for (int j = visiableIndex; j >= findIndex; j--)

                    {

                        itemTemp = (TreeViewItem )_currentSelectedItem.ItemContainerGenerator.ContainerFromIndex(j);

                        if (null != itemTemp)

                        {

                            itemTemp.BringIntoView();

                        }

                    }

                }

                else if (findIndex > visiableIndex)

                {

                    for (int j = visiableIndex; j <= findIndex; j++)

                    {

                        itemTemp = (TreeViewItem )_currentSelectedItem.ItemContainerGenerator.ContainerFromIndex(j);

                        if (null != itemTemp)

                        {

                            itemTemp.BringIntoView();

                        }

                    }

                }

                else

                {

                    itemTemp = (TreeViewItem )_currentSelectedItem.ItemContainerGenerator.ContainerFromIndex(visiableIndex);

                    if (null != itemTemp)

                    {

                        itemTemp.BringIntoView();

                    }

                }

                if (null != itemTemp)

                {

                    _currentSelectedItem = itemTemp;

                    (_currentSelectedItem as TreeViewItem ).IsSelected = true;

                    _currentSelectedItem.BringIntoView();

                }

            }

            catch (System.Exception ex)

            {

                //

            }

        }