TreeView.BeforeExpand 事件

定义

命名空间:

System.Windows.Forms

程序集:

System.Windows.Forms.dll

在展开树节点前发生。

public:
 event System::Windows::Forms::TreeViewCancelEventHandler ^ BeforeExpand;

public event System.Windows.Forms.TreeViewCancelEventHandler BeforeExpand;

member this.BeforeExpand : System.Windows.Forms.TreeViewCancelEventHandler 

Public Custom Event BeforeExpand As TreeViewCancelEventHandler 

事件类型

TreeViewCancelEventHandler

示例

下面的代码示例演示如何更改 的 TreeView 折叠状态，以便所有选中的节点都可见。 首先，折叠所有节点，并为 事件添加 BeforeExpand 处理程序。 接下来，展开所有节点。 事件处理程序 BeforeExpand 确定给定节点是否具有已检查的子节点。 如果节点没有选中的子级，则会取消该节点的扩展。 为了在单击节点旁边的加号时允许标准节点扩展， BeforeExpand 然后删除事件处理程序。

还可以通过处理 BeforeCollapse 事件来实现此行为，如该主题的示例所示。

有关完整示例，请参阅 CheckBoxes 参考主题。

private:
   void showCheckedNodesButton_Click( Object^ /*sender*/, EventArgs^ /*e*/ )
   {
      // Disable redrawing of treeView1 to prevent flickering 
      // while changes are made.
      treeView1->BeginUpdate();
      
      // Collapse all nodes of treeView1.
      treeView1->CollapseAll();
      
      // Add the checkForCheckedChildren event handler to the BeforeExpand event.
      treeView1->BeforeExpand += checkForCheckedChildren;
      
      // Expand all nodes of treeView1. Nodes without checked children are 
      // prevented from expanding by the checkForCheckedChildren event handler.
      treeView1->ExpandAll();
      
      // Remove the checkForCheckedChildren event handler from the BeforeExpand 
      // event so manual node expansion will work correctly.
      treeView1->BeforeExpand -= checkForCheckedChildren;
      
      // Enable redrawing of treeView1.
      treeView1->EndUpdate();
   }

   // Prevent expansion of a node that does not have any checked child nodes.
   void CheckForCheckedChildrenHandler( Object^ /*sender*/, TreeViewCancelEventArgs^ e )
   {
      if (  !HasCheckedChildNodes( e->Node ) )
            e->Cancel = true;
   }


   // Returns a value indicating whether the specified 
   // TreeNode has checked child nodes.
   bool HasCheckedChildNodes( TreeNode^ node )
   {
      if ( node->Nodes->Count == 0 )
            return false;

      System::Collections::IEnumerator^ myEnum = node->Nodes->GetEnumerator();
      while ( myEnum->MoveNext() )
      {
         TreeNode^ childNode = safe_cast<TreeNode^>(myEnum->Current);
         if ( childNode->Checked )
                  return true;

         // Recursively check the children of the current child node.
         if ( HasCheckedChildNodes( childNode ) )
                  return true;
      }

      return false;
   }

private void showCheckedNodesButton_Click(object sender, EventArgs e)
{
    // Disable redrawing of treeView1 to prevent flickering 
    // while changes are made.
    treeView1.BeginUpdate();

    // Collapse all nodes of treeView1.
    treeView1.CollapseAll();

    // Add the checkForCheckedChildren event handler to the BeforeExpand event.
    treeView1.BeforeExpand += checkForCheckedChildren;

    // Expand all nodes of treeView1. Nodes without checked children are 
    // prevented from expanding by the checkForCheckedChildren event handler.
    treeView1.ExpandAll();

    // Remove the checkForCheckedChildren event handler from the BeforeExpand 
    // event so manual node expansion will work correctly.
    treeView1.BeforeExpand -= checkForCheckedChildren;

    // Enable redrawing of treeView1.
    treeView1.EndUpdate();
}

// Prevent expansion of a node that does not have any checked child nodes.
private void CheckForCheckedChildrenHandler(object sender, 
    TreeViewCancelEventArgs e)
{
    if (!HasCheckedChildNodes(e.Node)) e.Cancel = true;
}

// Returns a value indicating whether the specified 
// TreeNode has checked child nodes.
private bool HasCheckedChildNodes(TreeNode node)
{
    if (node.Nodes.Count == 0) return false;
    foreach (TreeNode childNode in node.Nodes)
    {
        if (childNode.Checked) return true;
        // Recursively check the children of the current child node.
        if (HasCheckedChildNodes(childNode)) return true;
    }
    return false;
}

Private Sub showCheckedNodesButton_Click(ByVal sender As Object, ByVal e As EventArgs)
    ' Disable redrawing of treeView1 to prevent flickering 
    ' while changes are made.
    treeView1.BeginUpdate()

    ' Collapse all nodes of treeView1.
    treeView1.CollapseAll()

    ' Add the CheckForCheckedChildren event handler to the BeforeExpand event.
    AddHandler treeView1.BeforeExpand, AddressOf CheckForCheckedChildren

    ' Expand all nodes of treeView1. Nodes without checked children are 
    ' prevented from expanding by the checkForCheckedChildren event handler.
    treeView1.ExpandAll()

    ' Remove the checkForCheckedChildren event handler from the BeforeExpand 
    ' event so manual node expansion will work correctly.
    RemoveHandler treeView1.BeforeExpand, AddressOf CheckForCheckedChildren

    ' Enable redrawing of treeView1.
    treeView1.EndUpdate()
End Sub

' Prevent expansion of a node that does not have any checked child nodes.
Private Sub CheckForCheckedChildren(ByVal sender As Object, ByVal e As TreeViewCancelEventArgs)
    If Not HasCheckedChildNodes(e.Node) Then
        e.Cancel = True
    End If
End Sub

' Returns a value indicating whether the specified 
' TreeNode has checked child nodes.
Private Function HasCheckedChildNodes(ByVal node As TreeNode) As Boolean
    If node.Nodes.Count = 0 Then
        Return False
    End If
    Dim childNode As TreeNode
    For Each childNode In node.Nodes
        If childNode.Checked Then
            Return True
        End If
        ' Recursively check the children of the current child node.
        If HasCheckedChildNodes(childNode) Then
            Return True
        End If
    Next childNode
    Return False
End Function 'HasCheckedChildNodes

注解

有关如何处理事件的详细信息，请参阅 处理和引发事件。

另请参阅

• OnBeforeExpand(TreeViewCancelEventArgs)
• AfterExpand
• OnAfterExpand(TreeViewEventArgs)