TreeView.BeforeCheck 事件
定义
重要
一些信息与预发行产品相关，相应产品在发行之前可能会进行重大修改。 对于此处提供的信息，Microsoft 不作任何明示或暗示的担保。
在选中树节点复选框前发生。
public:
event System::Windows::Forms::TreeViewCancelEventHandler ^ BeforeCheck;public event System.Windows.Forms.TreeViewCancelEventHandler BeforeCheck;public event System.Windows.Forms.TreeViewCancelEventHandler? BeforeCheck;member this.BeforeCheck : System.Windows.Forms.TreeViewCancelEventHandler Public Custom Event BeforeCheck As TreeViewCancelEventHandler 事件类型
示例
下面的代码示例在用户更改其选中状态时更新 的所有子树节点 TreeNode 。 此代码要求具有 Form 的 ，TreeView其 TreeNodeCollection中具有 TreeNode 对象。 应 TreeNodeCollection 具有具有子节点的树节点。
// Updates all child tree nodes recursively.
void CheckAllChildNodes( TreeNode^ treeNode, bool nodeChecked )
{
IEnumerator^ myEnum = treeNode->Nodes->GetEnumerator();
while ( myEnum->MoveNext() )
{
TreeNode^ node = safe_cast<TreeNode^>(myEnum->Current);
node->Checked = nodeChecked;
if ( node->Nodes->Count > 0 )
{
// If the current node has child nodes, call the CheckAllChildsNodes method recursively.
this->CheckAllChildNodes( node, nodeChecked );
}
}
}
// NOTE This code can be added to the BeforeCheck event handler instead of the AfterCheck event.
// After a tree node's Checked property is changed, all its child nodes are updated to the same value.
void node_AfterCheck( Object^ /*sender*/, TreeViewEventArgs^ e )
{
// The code only executes if the user caused the checked state to change.
if ( e->Action != TreeViewAction::Unknown )
{
if ( e->Node->Nodes->Count > 0 )
{
/* Calls the CheckAllChildNodes method, passing in the current
Checked value of the TreeNode whose checked state changed. */
this->CheckAllChildNodes( e->Node, e->Node->Checked );
}
}
}
// Updates all child tree nodes recursively.
private void CheckAllChildNodes(TreeNode treeNode, bool nodeChecked)
{
foreach(TreeNode node in treeNode.Nodes)
{
node.Checked = nodeChecked;
if(node.Nodes.Count > 0)
{
// If the current node has child nodes, call the CheckAllChildsNodes method recursively.
this.CheckAllChildNodes(node, nodeChecked);
}
}
}
// NOTE This code can be added to the BeforeCheck event handler instead of the AfterCheck event.
// After a tree node's Checked property is changed, all its child nodes are updated to the same value.
private void node_AfterCheck(object sender, TreeViewEventArgs e)
{
// The code only executes if the user caused the checked state to change.
if(e.Action != TreeViewAction.Unknown)
{
if(e.Node.Nodes.Count > 0)
{
/* Calls the CheckAllChildNodes method, passing in the current
Checked value of the TreeNode whose checked state changed. */
this.CheckAllChildNodes(e.Node, e.Node.Checked);
}
}
}
' Updates all child tree nodes recursively.
Private Sub CheckAllChildNodes(treeNode As TreeNode, nodeChecked As Boolean)
Dim node As TreeNode
For Each node In treeNode.Nodes
node.Checked = nodeChecked
If node.Nodes.Count > 0 Then
' If the current node has child nodes, call the CheckAllChildsNodes method recursively.
Me.CheckAllChildNodes(node, nodeChecked)
End If
Next node
End Sub
' NOTE This code can be added to the BeforeCheck event handler instead of the AfterCheck event.
' After a tree node's Checked property is changed, all its child nodes are updated to the same value.
Private Sub node_AfterCheck(sender As Object, e As TreeViewEventArgs) Handles treeView1.AfterCheck
' The code only executes if the user caused the checked state to change.
If e.Action <> TreeViewAction.Unknown Then
If e.Node.Nodes.Count > 0 Then
' Calls the CheckAllChildNodes method, passing in the current
' Checked value of the TreeNode whose checked state changed.
Me.CheckAllChildNodes(e.Node, e.Node.Checked)
End If
End If
End Sub
注解
注意
TreeNode.Checked从 或 AfterCheck 事件中BeforeCheck设置 属性会导致事件多次引发，并可能导致意外行为。 例如，在以递归方式更新子节点时，可以在事件处理程序中设置 Checked 属性，以便用户不必单独展开和检查每个节点。 若要防止多次引发事件，请将逻辑添加到事件处理程序，该处理程序仅在 的 属性TreeViewEventArgs未设置为 TreeViewAction.Unknown时Action执行递归代码。
有关如何处理事件的详细信息，请参阅 处理和引发事件。
