Load On Demand

Starting with the Q2 2011 release RadTreeList supports a new Load-on-Demand functionality. It allows child nodes to be added on the fly as parent nodes are expanded. This mode is useful when you need to fill sub nodes only when the parent node is expanded or the data source contains thousands of records:

Load On Demand mechanism

To use the Load-On-Demand mechanism:

1. Set AllowLoadOnDemand property to true

2. Get the root items from the datasource and assign them to the RadTreeList.DataSource into RadTreeList.NeedDataSource event handler:

C#

protected void RadTreeList1_NeedDataSource(object sender, TreeListNeedDataSourceEventArgs e)
{
   RadTreeList1.DataSource = GetDataTable("SELECT * FROM TestItems WHERE ParentID IS NULL", null);
}

VB.NET

Protected Sub RadTreeList1_NeedDataSource(ByVal sender As Object, ByVal e As TreeListNeedDataSourceEventArgs)
	RadTreeList1.DataSource = GetDataTable("SELECT * FROM TestItems WHERE ParentID IS NULL", Nothing)
End Sub

1. Handle the RadTreeList.ChildItemsDataBind event and select the subset of items related to the expanded item. Assign them to the child items datasource property that is available through the second argument passed to the event handler:

C#

protected void RadTreeList1_ChildItemsDataBind(object sender, TreeListChildItemsDataBindEventArgs e)
{
	int id = Convert.ToInt32(e.ParentDataKeyValues["ID"].ToString());
	e.ChildItemsDataSource = GetDataTable("SELECT * FROM TestItems WHERE ParentID = " + id);
}

VB.NET

Protected Sub RadTreeList1_ChildItemsDataBind(ByVal sender As Object, ByVal e As TreeListChildItemsDataBindEventArgs)
	Dim id As Integer = Convert.ToInt32(e.ParentDataKeyValues("ID").ToString())
	e.ChildItemsDataSource = GetDataTable("SELECT * FROM TestItems WHERE ParentID = " & id)
End Sub

In addition, the RadTreeList control always shows the ExpandCollapse button in front of each item. When the item is expanded and there are no nested items, nothing will be displayed below the expanded item, even though there was a button to expand it.

Limitations

The data source of the control and items is not fully resolved when load-on-demand is used, and this poses a couple of limitations:

• The HideExpandCollapseButtonIfNoChildren property is not applicable when LoadOnDemand is enabled.
• Aggregates cannot be used with load-on-demand.