The DropDownList has built-in support to filter the data source, when allowFiltering is enabled. It performs when characters are typed in the search box. In the filtering event, you can filter down the data source and return the resulted data to DropDownList via updateData method to display its list items. This sample illustrates that, query the data source and pass the resulted data to DropDownList through the updateData method in filtering event. More information on the filtering feature configuration can be found in the documentation section.