How to: Load Data Dynamically via Events

Nov 13, 2018
2 minutes to read

The following demo shows how to use the TreeList.VirtualTreeGetChildNodes and TreeList.VirtualTreeGetCellValue events to specify data for a Tree List control.

Run Demo: Explorer(Virtual Tree)

The navigationTreeList control in this demo displays directories of the file system. Instead of loading the entire directory structure on application startup, this control loads directories on demand (when a user expands a specific directory).

The TreeList.VirtualTreeGetChildNodes event is handled to dynamically load child items (directories) for a specific node. The Tree List control automatically creates nodes for all child items that the VirtualTreeGetChildNodes event handler supplies.

The TreeList.VirtualTreeGetCellValue event handler specifies values for loaded children.

C#
VB.NET

csharp

navigationTreeList.VirtualTreeGetChildNodes += OnNavigationTreeListGetChildNodes;
navigationTreeList.VirtualTreeGetCellValue += OnNavigationTreeListGetCellValue;
//...
void OnNavigationTreeListGetChildNodes(object sender, VirtualTreeGetChildNodesInfo e) {
    Cursor current = Cursor.Current;
    Cursor.Current = Cursors.WaitCursor;
    e.Children = ((Item)e.Node).GetDirectories();
    Cursor.Current = current;
}

void OnNavigationTreeListGetCellValue(object sender, VirtualTreeGetCellValueInfo e) {
    e.CellData = ((Item)e.Node).DisplayName;
}

//...
public abstract class Item : IFileImage {
    //...
    public string DisplayName {get; private set;}
    public abstract List<Item> GetDirectories();
}

AddHandler Me.navigationTreeList.VirtualTreeGetChildNodes, AddressOf Me.OnNavigationTreeListGetChildNodes
AddHandler Me.navigationTreeList.VirtualTreeGetCellValue, AddressOf Me.OnNavigationTreeListGetCellValue
'...
Private Sub OnNavigationTreeListGetChildNodes(ByVal sender As Object, ByVal e As DevExpress.XtraTreeList.VirtualTreeGetChildNodesInfo)
    Dim current As System.Windows.Forms.Cursor = System.Windows.Forms.Cursor.Current
    System.Windows.Forms.Cursor.Current = System.Windows.Forms.Cursors.WaitCursor
    e.Children = CType(e.Node, DevExpress.XtraTreeList.Demos.Item).GetDirectories()
    System.Windows.Forms.Cursor.Current = current
End Sub

Private Sub OnNavigationTreeListGetCellValue(ByVal sender As Object, ByVal e As DevExpress.XtraTreeList.VirtualTreeGetCellValueInfo)
    e.CellData = CType(e.Node, DevExpress.XtraTreeList.Demos.Item).DisplayName
End Sub

Public MustInherit Class Item
    Implements DevExpress.XtraTreeList.Demos.IFileImage
    '...
    Private _DisplayName As String, _Name As String, _FullName As String
    Public MustOverride Function GetDirectories() As List(Of DevExpress.XtraTreeList.Demos.Item)
End Class

Handle the TreeList.VirtualTreeSetCellValue event for the opposite task: if the Tree List is editable, you can write new cell values entered by users to a data source.

C#
VB.NET

csharp

using DevExpress.XtraTreeList;

void OnVirtualTreeSetCellValue(object sender, VirtualTreeSetCellValueInfo e) {
    ((Item)e.Node).DisplayName = e.NewCellData.ToString();
}

Imports DevExpress.XtraTreeList

Private Sub OnVirtualTreeSetCellValue(ByVal sender As Object, ByVal e As VirtualTreeSetCellValueInfo)
    CType(e.Node, Item).DisplayName = e.NewCellData.ToString()
End Sub

See the Explorer (Virtual Tree) demo for the complete code.