Virtualization

Virtualization is useful for displaying large data sets.

The UI virtualization technique uses a fixed amount of list items in the popup list of the widget regardless of the dataset size. When the list is scrolled, the widget reuses the existing items to display the relevant data instead of creating new ones. For a runnable example, refer to the demo on virtualization in the DropDownList.

The DropDownList supports binding to primitive or enum value types.

1. Create the Read and ValueMapper actions.

       public ActionResult Index()
       {
           return View(new ProductViewModel
           {
               ProductID = 4,
               ProductName = "ProductName4"
           });
       }
   
       [HttpPost]
       public ActionResult ProductsVirtualization_Read([DataSourceRequest] DataSourceRequest request)
       {
           return Json(GetProducts().ToDataSourceResult(request));
       }
   
       public ActionResult Products_ValueMapper(int[] values)
       {
           var indices = new List<int>();
   
           if (values != null && values.Any())
           {
               var index = 0;
   
               foreach (var product in GetProducts())
               {
                   if (values.Contains(product.ProductID))
                   {
                       indices.Add(index);
                   }
   
                   index += 1;
               }
           }
   
           return Json(indices, JsonRequestBehavior.AllowGet);
       }
   
       private static IEnumerable<ProductViewModel> GetProducts()
       {
           var products = Enumerable.Range(0, 2000).Select(i => new ProductViewModel
           {
               ProductID = i,
               ProductName = "ProductName" + i
           });
   
           return products;
       }
   

2. Add a DropDownList to the view and configure it to use virtualization.

       @model MvcApplication1.Models.ProductViewModel
   
       @(Html.Kendo().DropDownListFor(m => m.ProductID)
           .Filter("contains")
           .DataTextField("ProductName")
           .DataValueField("ProductID")
           .OptionLabel("Select product...")
           .DataSource(source =>
           {
               source.Custom()
                   .ServerFiltering(true)
                   .ServerPaging(true)
                   .PageSize(80)
                   .Type("aspnetmvc-ajax")
                   .Transport(transport =>
                   {
                       transport.Read("ProductsVirtualization_Read", "Home");
                   })
                   .Schema(schema =>
                   {
                       schema.Data("Data")
                               .Total("Total");
                   });
           })
           .Virtual(v => v.ItemHeight(26).ValueMapper("valueMapper"))
       )
   
       <script>
           function valueMapper(options) {
               $.ajax({
                   url: "@Url.Action("Products_ValueMapper", "Home")",
                   data: convertValues(options.value),
                   success: function (data) {
                       options.success(data);
                   }
               });
           }
   
           function convertValues(value) {
               var data = {};
   
               value = $.isArray(value) ? value : [value];
   
               for (var idx = 0; idx < value.length; idx++) {
                   data["values[" + idx + "]"] = value[idx];
               }
   
               return data;
           }
       </script>
   

3. If the AutoBind option of the DropDownList is set to false and you need the widget to display the model value as selected, set the Text configuration option by passing the field set as DataTextField to the Text option.

       @model MvcApplication1.Models.ProductViewModel
   
       @(Html.Kendo().DropDownListFor(m => m.ProductID)
           .AutoBind(false)
           .Text(Model.ProductName)
           .DataTextField("ProductName")
           // Additional configuration.
       )
   

   See Also

• Virtualization by the DropDownList HtmlHelper for ASP.NET MVC (Demo)
• Server-Side API