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)

{

}

WPF中ItemsControl应用虚拟化时找到子元素的方法

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