TreeViewCancelEventHandler Delegate
Assembly: System.Windows.Forms (in system.windows.forms.dll)
'Declaration Public Delegate Sub TreeViewCancelEventHandler ( _ sender As Object, _ e As TreeViewCancelEventArgs _ ) 'Usage Dim instance As New TreeViewCancelEventHandler(AddressOf HandlerMethod)
/** @delegate */ public delegate void TreeViewCancelEventHandler ( Object sender, TreeViewCancelEventArgs e )
JScript supports the use of delegates, but not the declaration of new ones.
Parameters
- sender
The source of the event.
- e
A TreeViewCancelEventArgs that contains the event data.
When you create a TreeViewCancelEventArgs delegate, you identify the method that will handle the event. To associate the event with your event handler, add an instance of the delegate to the event. The event handler is called whenever the event occurs, unless you remove the delegate. For more information about event handler delegates, see Events and Delegates.
The following example demonstrates how to change the collapse state of a TreeView so that all the checked nodes are visible. First, all the nodes are collapsed, and a handler is added to the TreeView.BeforeExpand event. Next, all the nodes are expanded. The TreeView.BeforeExpand event handler determines whether a given node has child nodes that are checked. If a node does not have checked children, the expansion is canceled for that node. In order to allow normal node expansion when the plus sign next to a node is clicked, the TreeView.BeforeExpand event handler is then removed.
This behavior can also be implemented by handling the TreeView.BeforeCollapse event, as illustrated in the example for that topic.
Imports System Imports System.Drawing Imports System.Windows.Forms Public Class Form1 Inherits Form Private treeView1 As TreeView Private showCheckedNodesButton As Button Public Sub New() treeView1 = New TreeView showCheckedNodesButton = New Button Me.SuspendLayout() ' Initialize treeView1. treeView1.Location = New Point(0, 25) treeView1.Size = New Size(292, 248) treeView1.Anchor = AnchorStyles.Top Or AnchorStyles.Left Or AnchorStyles.Bottom Or AnchorStyles.Right treeView1.CheckBoxes = True ' Add nodes to treeView1. Dim node As TreeNode Dim x As Integer For x = 0 To 3 ' Add a root node. node = treeView1.Nodes.Add(String.Format("Node{0}", x * 4)) Dim y As Integer For y = 1 To 4 ' Add a node as a child of the previously added node. node = node.Nodes.Add(String.Format("Node{0}", x * 4 + y)) Next y Next x ' Set the checked state of one of the nodes to ' demonstrate the showCheckedNodesButton button behavior. treeView1.Nodes(1).Nodes(0).Nodes(0).Checked = True ' Initialize showCheckedNodesButton. showCheckedNodesButton.Size = New Size(144, 24) showCheckedNodesButton.Text = "Show Checked Nodes" AddHandler showCheckedNodesButton.Click, AddressOf showCheckedNodesButton_Click ' Initialize the form. Me.ClientSize = New Size(292, 273) Me.Controls.AddRange(New Control() {showCheckedNodesButton, treeView1}) Me.ResumeLayout(False) End Sub 'New <STAThreadAttribute()> _ Shared Sub Main() Application.Run(New Form1) End Sub 'Main 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 'showCheckedNodesButton_Click ' 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 'CheckForCheckedChildren ' 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 End Class 'Form1
import System.*;
import System.Drawing.*;
import System.Windows.Forms.*;
public class Form1 extends Form
{
private TreeView treeView1;
private Button showCheckedNodesButton;
private TreeViewCancelEventHandler checkForCheckedChildren;
public Form1()
{
treeView1 = new TreeView();
showCheckedNodesButton = new Button();
checkForCheckedChildren =
new TreeViewCancelEventHandler(CheckForCheckedChildrenHandler);
this.SuspendLayout();
// Initialize treeView1.
treeView1.set_Location(new Point(0, 25));
treeView1.set_Size(new Size(292, 248));
treeView1.set_Anchor(AnchorStyles.Top | AnchorStyles.Left
| AnchorStyles.Bottom | AnchorStyles.Right);
treeView1.set_CheckBoxes(true);
// Add nodes to treeView1.
TreeNode node;
for (int x = 0; x < 3; ++x) {
// Add a root node.
node = treeView1.get_Nodes().Add(String.Format
("Node{0}", (Int32)(x * 4)));
for (int y = 1; y < 4; ++y) {
// Add a node as a child of the previously added node.
node = node.get_Nodes().Add(String.Format
("Node{0}", (Int32)(x * 4 + y)));
}
}
// Set the checked state of one of the nodes to
// demonstrate the showCheckedNodesButton button behavior.
treeView1.get_Nodes().get_Item(1).get_Nodes().get_Item(0).
get_Nodes().get_Item(0).set_Checked(true);
// Initialize showCheckedNodesButton.
showCheckedNodesButton.set_Size(new Size(144, 24));
showCheckedNodesButton.set_Text("Show Checked Nodes");
showCheckedNodesButton.add_Click(
new EventHandler(showCheckedNodesButton_Click));
// Initialize the form.
this.set_ClientSize(new Size(292, 273));
this.get_Controls().AddRange(new Control[]
{ showCheckedNodesButton, treeView1 });
this.ResumeLayout(false);
} //Form1
public static void main(String[] args)
{
Application.Run(new Form1());
} //main
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.add_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.remove_BeforeExpand(checkForCheckedChildren);
// Enable redrawing of treeView1.
treeView1.EndUpdate();
} //showCheckedNodesButton_Click
// Prevent expansion of a node that does not have any checked child nodes.
private void CheckForCheckedChildrenHandler(Object sender,
TreeViewCancelEventArgs e)
{
if (!(HasCheckedChildNodes(e.get_Node()))) {
e.set_Cancel(true);
}
} //CheckForCheckedChildrenHandler
// Returns a value indicating whether the specified
// TreeNode has checked child nodes.
private boolean HasCheckedChildNodes(TreeNode node)
{
if (node.get_Nodes().get_Count() == 0) {
return false;
}
for (int iCtr = 0; iCtr < node.get_Nodes().get_Count(); iCtr++) {
TreeNode childNode = node.get_Nodes().get_Item(iCtr);
if (childNode.get_Checked()) {
return true;
}
// Recursively check the children of the current child node.
if (HasCheckedChildNodes(childNode)) {
return true;
}
}
return false;
} //HasCheckedChildNodes
} //Form1
Windows 98, Windows 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition
The .NET Framework does not support all versions of every platform. For a list of the supported versions, see System Requirements.