How to: Load Data Dynamically via Events

• Nov 13, 2018
• 4 minutes to read

The example shows how to display information on directories and files on your system in a Tree List control dynamically, using only events. The full code for this example is available in the Explorer (Virtual Tree) module of the Tree List Main Demo. Here, only a part of the code is listed, giving you a basic understanding of how the Tree List events for dynamic loading work.

To provide data for the Tree List control, the TreeList.VirtualTreeGetChildNodes and TreeList.VirtualTreeGetCellValue events are handled. The first event is used to provide lists of root and child nodes. The second event is used to provide values for node cells. To save the changes made by an end-user to node cells, the TreeList.VirtualTreeSetCellValue event is handled. However, its implementation is omitted here. In this example, it’s assumed that the Tree List control contains three columns, displaying a directory/file name, a directory flag and a file size. This data is provided via the VirtualTreeGetCellValue event.

To support dynamic data loading, the TreeList.DataSource property must be set to any custom object, except an IList object or an object implementing the IVirtualTreeListData interface. In the example, the DataSource property is set to a new Object instance.

The image below shows the result.

C#

// Specifies whether the root nodes that represent the local drivers are created.
bool loadDrives = false;
private void Form1_Load(object sender, System.EventArgs e) {
    treeList1.DataSource = new object();
}

// Indicates whether the processed node corresponds to the file.
bool IsFile(DirectoryInfo info){
    return (info.Attributes & FileAttributes.Directory) == 0;
}

// Initializes cell values.
private void treeList1_VirtualTreeGetCellValue(object sender, 
DevExpress.XtraTreeList.VirtualTreeGetCellValueInfo e) {
    DirectoryInfo di = new DirectoryInfo((string)e.Node);
    if (e.Column == treeListColumn1) 
            e.CellData = di.Name;
    if (e.Column == treeListColumn2) {
        if (!IsFile(di))
            e.CellData = "Folder";
        else 
            e.CellData = "File";
    }
    if (e.Column == treeListColumn3) {
        if (IsFile(di)){
            e.CellData = new FileInfo((string)e.Node).Length;
        }
        else e.CellData = null;
    }
}

// Creates and initializes child nodes.
private void treeList1_VirtualTreeGetChildNodes(object sender, 
DevExpress.XtraTreeList.VirtualTreeGetChildNodesInfo e) {
    if (!loadDrives){ // create drives
        string[] root = Directory.GetLogicalDrives();
        e.Children = root;
        loadDrives = true;
    }
    else {
        try{
            string path  = (string)e.Node;
            if(Directory.Exists(path)){
                string[] dirs = Directory.GetDirectories(path);
                string[] files = Directory.GetFiles(path);
                string[] arr = new string[dirs.Length + files.Length];
                dirs.CopyTo(arr,0);
                files.CopyTo(arr,dirs.Length);
                e.Children = arr;
            }
            else e.Children = new object[]{};
        }
        catch { e.Children = new object[]{}; }
    }
}

private void treeList1_VirtualTreeSetCellValue(object sender, VirtualTreeSetCellValueInfo e) {
    //...
}

VB.NET

Dim loadDrives As Boolean = False
Private Sub Form1_Load(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles MyBase.Load
    TreeList1.DataSource = New Object()
End Sub

Function IsFile(ByVal info As DirectoryInfo) As Boolean
    Return (info.Attributes And FileAttributes.Directory) = 0
End Function

Private Sub TreeList1_VirtualTreeGetCellValue(ByVal sender As Object, _
ByVal e As DevExpress.XtraTreeList.VirtualTreeGetCellValueInfo) _
Handles TreeList1.VirtualTreeGetCellValue
    Dim di As New DirectoryInfo(CStr(e.Node))
    If e.Column Is TreeListColumn1 Then
        e.CellData = di.Name
    End If
    If e.Column Is TreeListColumn2 Then
        If Not IsFile(di) Then
            e.CellData = "Folder"
        Else
            e.CellData = "File"
        End If
    End If
    If e.Column Is TreeListColumn3 Then
        If IsFile(di) Then
            e.CellData = New FileInfo(CStr(e.Node)).Length
        Else
            e.CellData = Nothing
        End If
    End If
End Sub

Private Sub TreeList1_VirtualTreeGetChildNodes(ByVal sender As Object, _
ByVal e As DevExpress.XtraTreeList.VirtualTreeGetChildNodesInfo) _
Handles TreeList1.VirtualTreeGetChildNodes
    If loadDrives = False Then
        Dim root As String() = Directory.GetLogicalDrives()
        e.Children = root
        loadDrives = True
    Else
        Try
            Dim path As String = CStr(e.Node)
            If Directory.Exists(path) Then
                Dim dirs As String() = Directory.GetDirectories(path)
                Dim files As String() = Directory.GetFiles(path)
                Dim arr(dirs.Length + files.Length) As String
                dirs.CopyTo(arr, 0)
                files.CopyTo(arr, dirs.Length)
                e.Children = arr
            Else
                e.Children = New Object() {}
            End If
        Catch
        End Try
    End If
End Sub

Private Sub TreeList1_VirtualTreeSetCellValue(ByVal sender As System.Object, _
ByVal e As DevExpress.XtraTreeList.VirtualTreeSetCellValueInfo) _
Handles TreeList1.VirtualTreeSetCellValue
    '...
End Sub