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。
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。
/// <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>
- 提高树视图 的性能
