How to: Load Data Dynamically via Events

  • 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.

VirtualTreeEvents

// 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) {
    //...
}