TreeView.BeforeCollapse Evento
Definição
Importante
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Ocorre antes de o nó da árvore colapsar.
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 de Evento
Exemplos
O exemplo de código seguinte demonstra como alterar o estado de colapso de um TreeView para que todos os nós verificados fiquem visíveis. Primeiro, todos os nós são expandidos e um handler é adicionado para o BeforeCollapse evento. De seguida, todos os nós são colapsados. O BeforeCollapse gestor de eventos determina se um dado nó tem nós filhos que estão verificados. Se um nó verificou filhos, o colapso é cancelado para esse nó. Para permitir o colapso padrão do nó quando o sinal menos ao lado de um nó é clicado, o BeforeCollapse manipulador de eventos é então removido.
Este comportamento também pode ser implementado ao lidar com o BeforeExpand evento, como ilustrado no exemplo para esse tópico.
Para o exemplo completo, veja o TreeView tópico de referência do construtor.
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
Observações
Para mais informações sobre como gerir eventos, consulte Gestão e Levantamento de Eventos.