Load on Demand

The performance of the Telerik RadTreeView control when operating with huge amount of items is significantly optimized through its load on demand feature. This mechanism lets the nodes load their child nodes as the user expands the parent by clicking on the expander icon.

This tutorial will walk you through the following common tasks:

• Enable load on demand behavior declaratively and programmatically.
• Using events to manage the load on demand process.
• Using the IsLoadingOnDemand property.

For the purpose of this tutorial will be used the following treeview declaration:

<telerik:RadTreeView Margin="8" x:Name="radTreeView"> 
    <telerik:RadTreeViewItem Header="Sport Categories"> 
        <telerik:RadTreeViewItem Header="Football"/> 
        <telerik:RadTreeViewItem Header="Tennis"/> 
        <telerik:RadTreeViewItem Header="Cycling"/> 
    </telerik:RadTreeViewItem> 
</telerik:RadTreeView> 

Enable Load on Demand Declaratively

To enable this feature you can set the IsLoadOnDemandEnabled property to the RadTreeViewItem to True.

<telerik:RadTreeView Margin="8" x:Name="radTreeView"> 
    <telerik:RadTreeViewItem Header="Sport Categories"> 
        <telerik:RadTreeViewItem Header="Football" IsLoadOnDemandEnabled="True"/> 
        <telerik:RadTreeViewItem Header="Tennis" IsLoadOnDemandEnabled="True"/> 
        <telerik:RadTreeViewItem Header="Cycling" IsLoadOnDemandEnabled="True"/> 
    </telerik:RadTreeViewItem> 
</telerik:RadTreeView> 

Enable Load on Demand Programmatically

The same effect can be achieved if you set the IsLoadOnDemandEnabled property of an instance of the RadTreeViewItem class in the code-behind:

private void EnableLoadOnDemand( RadTreeViewItem radTreeViewItem ) 
{ 
    radTreeViewItem.IsLoadOnDemandEnabled = true; 
} 

Private Sub EnableLoadOnDemand(ByVal radTreeViewItem As RadTreeViewItem) 
    radTreeViewItem.IsLoadOnDemandEnabled = True 
End Sub 

Events

When the expand icon is clicked the LoadOnDemand event is fired. This event provides the opportunity to add new items based on the identity of the clicked-on Item.

• Here is an example of how to handle the LoadOnDemand event when attached to a RadTreeViewItem:

      <telerik:RadTreeView Margin="8" x:Name="radTreeView"> 
          <telerik:RadTreeViewItem Header="Sport Categories"> 
              <telerik:RadTreeViewItem Header="Football" IsLoadOnDemandEnabled="True" LoadOnDemand="RadTreeViewItem_LoadOnDemand"/> 
              <telerik:RadTreeViewItem Header="Tennis" IsLoadOnDemandEnabled="True"/> 
              <telerik:RadTreeViewItem Header="Cycling" IsLoadOnDemandEnabled="True"/> 
          </telerik:RadTreeViewItem> 
      </telerik:RadTreeView> 
  

      private void RadTreeViewItem_LoadOnDemand( object sender, Telerik.Windows.RadRoutedEventArgs e ) 
      { 
          // get the clicked Item 
          RadTreeViewItem clickedItem = sender as RadTreeViewItem; 
          // add the new items 
          RadTreeViewItem newItem = new RadTreeViewItem() 
          { 
              Header = "Soccer" 
          }; 
          clickedItem.Items.Add( newItem ); 
      } 
  

      Private Sub RadTreeViewItem_LoadOnDemand(ByVal sender As Object, ByVal e As Telerik.Windows.RadRoutedEventArgs) 
          ' get the clicked Item ' 
          Dim clickedItem As RadTreeViewItem = TryCast(sender, RadTreeViewItem) 
          ' add the new items ' 
          Dim newItem As New RadTreeViewItem() 
          clickedItem.Items.Add(newItem) 
      End Sub 
  

• If you want to enable the Load on demand functionality globally for the entire RadTreeView you can set the IsLoadOnDemandEnabled property to the RadTreeView. This will make all Items in the RadTreeView to have the expander icon enabled.

  Here is an example of how to handle the LoadOnDemand event when attached to the RadTreeView:

      <telerik:RadTreeView Margin="8" x:Name="radTreeView" IsLoadOnDemandEnabled="True" LoadOnDemand="radTreeView_LoadOnDemand"> 
          <telerik:RadTreeViewItem Header="Sport Categories"> 
              <telerik:RadTreeViewItem Header="Football"/> 
              <telerik:RadTreeViewItem Header="Tennis"/> 
              <telerik:RadTreeViewItem Header="Cycling"/> 
          </telerik:RadTreeViewItem> 
      </telerik:RadTreeView> 
  

      private void radTreeView_LoadOnDemand( object sender, RadRoutedEventArgs e ) 
      { 
          // get the treeview 
          Telerik.Windows.Controls.RadTreeView tree = sender as Telerik.Windows.Controls.RadTreeView; 
          // get the clicked Item 
          RadTreeViewItem clickedItem = e.OriginalSource as RadTreeViewItem; 
          // add the new items 
          RadTreeViewItem newItem = new RadTreeViewItem() 
          { 
              Header = "New Item" 
          }; 
          clickedItem.Items.Add( newItem ); 
          clickedItem.IsLoadOnDemandEnabled = false; 
      } 
  

      Private Sub radTreeView_LoadOnDemand(sender As Object, e As RadRoutedEventArgs) 
       ' get the treeview ' 
       Dim tree As Telerik.Windows.Controls.RadTreeView = TryCast(sender, Telerik.Windows.Controls.RadTreeView) 
       ' get the clicked Item ' 
       Dim clickedItem As RadTreeViewItem = TryCast(e.OriginalSource, RadTreeViewItem) 
       ' add the new items ' 
       Dim newItem As New RadTreeViewItem() 
       clickedItem.Items.Add(newItem) 
       clickedItem.IsLoadOnDemandEnabled = False 
      End Sub 
  

The RadRoutedEventArgs expose both the Source and the OriginalSource of the event.The Source property points to the RadTreeView control that is currently handling the event, but the OriginalSource property points to the RadTreeViewItem that was clicked.

The IsLoadingOnDemand Property

When there are no items to add, and you want to stop the loading animation, set the IsLoadingOnDemand property to False to the RadTreeViewItem that has fired the LoadOnDemand event.

When there are no items to add, and you want to prevent the LoadOnDemand event to fire again, set the IsLoadOnDemandEnabled property to False to the RadTreeViewItem that has fired the LoadOnDemand event.

See Also

• Data Binding
• CheckBox Support
• Keyboard Support