Virtualization

Virtualization is useful for displaying large data sets. The UI virtualization technique uses a fixed amount of list items in the pop-up 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.

Detailed information on the feature could be found in ComboBox virtualization article.

Important

The value type to which the DropDownList can be bound on the server can only be a primitive type or an enum value.

  1. Create the Read and ValueMapper actions.

    Example
        public IActionResult Index()
        {
            return View(new ProductViewModel
            {
                ProductID = 4,
                ProductName = "ProductName4"
            });
        }
    
        [HttpPost]
        public IActionResult ProductsVirtualization_Read([DataSourceRequest] DataSourceRequest request)
        {
            return Json(GetProducts().ToDataSourceResult(request));
        }
    
        public IActionResult 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);
        }
    
        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.

    Example
        @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.

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