TreeView.BeforeCollapse Evento

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

public event System.Windows.Forms.TreeViewCancelEventHandler BeforeCollapse;

public event System.Windows.Forms.TreeViewCancelEventHandler? BeforeCollapse;

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

Public Custom Event BeforeCollapse As TreeViewCancelEventHandler 

Tipo evento

Esempio

Nell'esempio di codice seguente viene illustrato come modificare lo stato di compressione di un oggetto TreeView in modo che tutti i nodi controllati siano visibili. Prima di tutto, tutti i nodi vengono espansi e viene aggiunto un gestore per l'evento BeforeCollapse . Successivamente, tutti i nodi vengono compressi. Il BeforeCollapse gestore eventi determina se un determinato nodo ha nodi figlio controllati. Se un nodo ha controllato gli elementi figlio, il collasso viene annullato per tale nodo. Per consentire la compressione del nodo standard quando si fa clic sul segno meno accanto a un nodo, il BeforeCollapse gestore eventi viene quindi rimosso.

Questo comportamento può essere implementato anche gestendo l'evento BeforeExpand , come illustrato nell'esempio per tale argomento.

Per l'esempio completo, vedere l'argomento di riferimento del TreeView costruttore.

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->ExpandAll();
      
      // Add the checkForCheckedChildren event handler to the BeforeExpand event.
      treeView1->BeforeCollapse += checkForCheckedChildren;
      
      // Expand all nodes of treeView1. Nodes without checked children are 
      // prevented from expanding by the checkForCheckedChildren event handler.
      treeView1->CollapseAll();
      
      // Remove the checkForCheckedChildren event handler from the BeforeExpand 
      // event so manual node expansion will work correctly.
      treeView1->BeforeCollapse -= checkForCheckedChildren;
      
      // Enable redrawing of treeView1.
      treeView1->EndUpdate();
   }

   // Prevent collapse of a node that has 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.ExpandAll();

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

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

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

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

// Prevent collapse of a node that has 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.ExpandAll()

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

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

    ' Remove the checkForCheckedChildren event handler from the BeforeExpand 
    ' event so manual node expansion will work correctly.
    RemoveHandler treeView1.BeforeCollapse, AddressOf CheckForCheckedChildren
    ' Enable redrawing of treeView1.
    treeView1.EndUpdate()
End Sub


' Prevent collapse of a node that has checked child nodes.
Private Sub CheckForCheckedChildren(ByVal sender As Object, ByVal e As TreeViewCancelEventArgs)
    If 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

Commenti

Per altre informazioni su come gestire gli eventi, vedere la gestione e generazione di eventi.

Si applica a

Vedi anche

• OnBeforeCollapse(TreeViewCancelEventArgs)
• AfterCollapse
• OnAfterCollapse(TreeViewEventArgs)