如何在 TreeView 中查找 TreeViewItem
TreeView 控件提供了一种显示分层数据的便捷方法。 如果 TreeView 绑定到数据源，SelectedItem 属性提供了一种方便的方法，可用于快速检索所选数据对象。 通常最好使用基础数据对象，但有时可能需要编程地操控数据的容器 TreeViewItem。 例如，可能需要以编程方式展开 TreeViewItem，或在 TreeView中选择其他项。
若要查找包含特定数据对象的 TreeViewItem，必须遍历 TreeView的每个级别。 还可以虚拟化 TreeView 中的项以提高性能。 在可能虚拟化项的情况下，还必须实现 TreeViewItem 来检查它是否包含数据对象。
例
描述
以下示例在 TreeView 中搜索特定对象，并返回包含该对象的 TreeViewItem。 该示例确保实例化每个 TreeViewItem，以便搜索其子项。 如果 TreeView 不使用虚拟化项，则此示例也有效。
备注
以下示例在无视基础数据模型的情况下适用于所有 TreeView，并将每个 TreeViewItem 搜索直到找到对象。 另一种性能更好的技术是搜索指定对象的数据模型，跟踪其数据层次结构中的位置，然后在 TreeView中找到相应的 TreeViewItem。 但是，具有更好性能的技术需要了解数据模型，并且不能对任何给定 TreeView进行通用化。
代码
/// <summary>
/// Recursively search for an item in this subtree.
/// </summary>
/// <param name="container">
/// The parent ItemsControl. This can be a TreeView or a TreeViewItem.
/// </param>
/// <param name="item">
/// The item to search for.
/// </param>
/// <returns>
/// The TreeViewItem that contains the specified item.
/// </returns>
private TreeViewItem GetTreeViewItem(ItemsControl container, object item)
{
if (container != null)
{
if (container.DataContext == item)
{
return container as TreeViewItem;
}
// Expand the current container
if (container is TreeViewItem && !((TreeViewItem)container).IsExpanded)
{
container.SetValue(TreeViewItem.IsExpandedProperty, true);
}
// Try to generate the ItemsPresenter and the ItemsPanel.
// by calling ApplyTemplate. Note that in the
// virtualizing case even if the item is marked
// expanded we still need to do this step in order to
// regenerate the visuals because they may have been virtualized away.
container.ApplyTemplate();
ItemsPresenter itemsPresenter =
(ItemsPresenter)container.Template.FindName("ItemsHost", container);
if (itemsPresenter != null)
{
itemsPresenter.ApplyTemplate();
}
else
{
// The Tree template has not named the ItemsPresenter,
// so walk the descendents and find the child.
itemsPresenter = FindVisualChild<ItemsPresenter>(container);
if (itemsPresenter == null)
{
container.UpdateLayout();
itemsPresenter = FindVisualChild<ItemsPresenter>(container);
}
}
Panel itemsHostPanel = (Panel)VisualTreeHelper.GetChild(itemsPresenter, 0);
// Ensure that the generator for this panel has been created.
UIElementCollection children = itemsHostPanel.Children;
MyVirtualizingStackPanel virtualizingPanel =
itemsHostPanel as MyVirtualizingStackPanel;
for (int i = 0, count = container.Items.Count; i < count; i++)
{
TreeViewItem subContainer;
if (virtualizingPanel != null)
{
// Bring the item into view so
// that the container will be generated.
virtualizingPanel.BringIntoView(i);
subContainer =
(TreeViewItem)container.ItemContainerGenerator.
ContainerFromIndex(i);
}
else
{
subContainer =
(TreeViewItem)container.ItemContainerGenerator.
ContainerFromIndex(i);
// Bring the item into view to maintain the
// same behavior as with a virtualizing panel.
subContainer.BringIntoView();
}
if (subContainer != null)
{
// Search the next level for the object.
TreeViewItem resultContainer = GetTreeViewItem(subContainer, item);
if (resultContainer != null)
{
return resultContainer;
}
else
{
// The object is not under this TreeViewItem
// so collapse it.
subContainer.IsExpanded = false;
}
}
}
}
return null;
}
/// <summary>
/// Search for an element of a certain type in the visual tree.
/// </summary>
/// <typeparam name="T">The type of element to find.</typeparam>
/// <param name="visual">The parent element.</param>
/// <returns></returns>
private T FindVisualChild<T>(Visual visual) where T : Visual
{
for (int i = 0; i < VisualTreeHelper.GetChildrenCount(visual); i++)
{
Visual child = (Visual)VisualTreeHelper.GetChild(visual, i);
if (child != null)
{
T correctlyTyped = child as T;
if (correctlyTyped != null)
{
return correctlyTyped;
}
T descendent = FindVisualChild<T>(child);
if (descendent != null)
{
return descendent;
}
}
}
return null;
}
''' <summary>
''' Recursively search for an item in this subtree.
''' </summary>
''' <param name="container">
''' The parent ItemsControl. This can be a TreeView or a TreeViewItem.
''' </param>
''' <param name="item">
''' The item to search for.
''' </param>
''' <returns>
''' The TreeViewItem that contains the specified item.
''' </returns>
Private Function GetTreeViewItem(ByVal container As ItemsControl,
ByVal item As Object) As TreeViewItem
If container IsNot Nothing Then
If container.DataContext Is item Then
Return TryCast(container, TreeViewItem)
End If
' Expand the current container
If TypeOf container Is TreeViewItem AndAlso
Not DirectCast(container, TreeViewItem).IsExpanded Then
container.SetValue(TreeViewItem.IsExpandedProperty, True)
End If
' Try to generate the ItemsPresenter and the ItemsPanel.
' by calling ApplyTemplate. Note that in the
' virtualizing case, even if IsExpanded = true,
' we still need to do this step in order to
' regenerate the visuals because they may have been virtualized away.
container.ApplyTemplate()
Dim itemsPresenter As ItemsPresenter =
DirectCast(container.Template.FindName("ItemsHost", container), ItemsPresenter)
If itemsPresenter IsNot Nothing Then
itemsPresenter.ApplyTemplate()
Else
' The Tree template has not named the ItemsPresenter,
' so walk the descendents and find the child.
itemsPresenter = FindVisualChild(Of ItemsPresenter)(container)
If itemsPresenter Is Nothing Then
container.UpdateLayout()
itemsPresenter = FindVisualChild(Of ItemsPresenter)(container)
End If
End If
Dim itemsHostPanel As Panel =
DirectCast(VisualTreeHelper.GetChild(itemsPresenter, 0), Panel)
' Do this to ensure that the generator for this panel has been created.
Dim children As UIElementCollection = itemsHostPanel.Children
Dim virtualizingPanel As MyVirtualizingStackPanel =
TryCast(itemsHostPanel, MyVirtualizingStackPanel)
For index As Integer = 0 To container.Items.Count - 1
Dim subContainer As TreeViewItem
If virtualizingPanel IsNot Nothing Then
' Bring the item into view so
' that the container will be generated.
virtualizingPanel.BringIntoView(index)
subContainer =
DirectCast(container.ItemContainerGenerator.ContainerFromIndex(index),
TreeViewItem)
Else
subContainer =
DirectCast(container.ItemContainerGenerator.ContainerFromIndex(index),
TreeViewItem)
' Bring the item into view to maintain the
' same behavior as with a virtualizing panel.
subContainer.BringIntoView()
End If
If subContainer IsNot Nothing Then
' Search the next level for the object.
Dim resultContainer As TreeViewItem =
GetTreeViewItem(subContainer, item)
If resultContainer IsNot Nothing Then
Return resultContainer
Else
' The object is not under this TreeViewItem
' so collapse it.
subContainer.IsExpanded = False
End If
End If
Next
End If
Return Nothing
End Function
''' <summary>
''' Search for an element of a certain type in the visual tree.
''' </summary>
''' <typeparam name="T">The type of element to find.</typeparam>
''' <param name="visual">The parent element.</param>
''' <returns></returns>
Private Function FindVisualChild(Of T As Visual)(ByVal visual As Visual) As T
For i As Integer = 0 To VisualTreeHelper.GetChildrenCount(visual) - 1
Dim child As Visual = DirectCast(VisualTreeHelper.GetChild(visual, i), Visual)
If child IsNot Nothing Then
Dim correctlyTyped As T = TryCast(child, T)
If correctlyTyped IsNot Nothing Then
Return correctlyTyped
End If
Dim descendent As T = FindVisualChild(Of T)(child)
If descendent IsNot Nothing Then
Return descendent
End If
End If
Next
Return Nothing
End Function
前面的代码依赖于自定义的 VirtualizingStackPanel，该 VirtualizingStackPanel 公开一个名为 BringIntoView的方法。 以下代码定义了自定义标识 VirtualizingStackPanel。
public class MyVirtualizingStackPanel : VirtualizingStackPanel
{
/// <summary>
/// Publically expose BringIndexIntoView.
/// </summary>
public void BringIntoView(int index)
{
this.BringIndexIntoView(index);
}
}
Public Class MyVirtualizingStackPanel
Inherits VirtualizingStackPanel
''' <summary>
''' Publically expose BringIndexIntoView.
''' </summary>
Public Overloads Sub BringIntoView(ByVal index As Integer)
Me.BringIndexIntoView(index)
End Sub
End Class
以下 XAML 演示如何创建使用自定义 VirtualizingStackPanel的 TreeView。
<TreeView VirtualizingStackPanel.IsVirtualizing="True">
<!--Use the custom class MyVirtualizingStackPanel
as the ItemsPanel for the TreeView and
TreeViewItem object.-->
<TreeView.ItemsPanel>
<ItemsPanelTemplate>
<src:MyVirtualizingStackPanel/>
</ItemsPanelTemplate>
</TreeView.ItemsPanel>
<TreeView.ItemContainerStyle>
<Style TargetType="TreeViewItem">
<Setter Property="ItemsPanel">
<Setter.Value>
<ItemsPanelTemplate>
<src:MyVirtualizingStackPanel/>
</ItemsPanelTemplate>
</Setter.Value>
</Setter>
</Style>
</TreeView.ItemContainerStyle>
</TreeView>
另请参阅
- 提高树视图 的性能
