ASP.NET Core Razor component virtualization

This article explains how to use component virtualization in ASP.NET Core Blazor apps.

Improve the perceived performance of component rendering using the Blazor framework's built-in virtualization support with the Virtualize component. Virtualization is a technique for limiting UI rendering to just the parts that are currently visible. For example, virtualization is helpful when the app must render a long list of items and only a subset of items is required to be visible at any given time.

Use the Virtualize component when:

  • Rendering a set of data items in a loop.
  • Most of the items aren't visible due to scrolling.
  • The rendered items are the same size.

When the user scrolls to an arbitrary point in the Virtualize component's list of items, the component calculates the visible items to show. Unseen items aren't rendered.

Without virtualization, a typical list might use a C# foreach loop to render each item in a list. In the following example:

  • allFlights is a collection of airplane flights.
  • The FlightSummary component displays details about each flight.
  • The @key directive attribute preserves the relationship of each FlightSummary component to its rendered flight by the flight's FlightId.
<div style="height:500px;overflow-y:scroll">
    @foreach (var flight in allFlights)
    {
        <FlightSummary @key="flight.FlightId" Details="@flight.Summary" />
    }
</div>

If the collection contains thousands of flights, rendering the flights takes a long time and users experience a noticeable UI lag. Most of the flights aren't seen because they fall outside of the height of the <div> element.

Instead of rendering the entire list of flights at once, replace the foreach loop in the preceding example with the Virtualize component:

  • Specify allFlights as a fixed item source to Virtualize<TItem>.Items. Only the currently visible flights are rendered by the Virtualize component.
  • Specify a context for each flight with the Context parameter. In the following example, flight is used as the context, which provides access to each flight's members.
<div style="height:500px;overflow-y:scroll">
    <Virtualize Items="@allFlights" Context="flight">
        <FlightSummary @key="flight.FlightId" Details="@flight.Summary" />
    </Virtualize>
</div>

If a context isn't specified with the Context parameter, use the value of context in the item content template to access each flight's members:

<div style="height:500px;overflow-y:scroll">
    <Virtualize Items="@allFlights">
        <FlightSummary @key="context.FlightId" Details="@context.Summary" />
    </Virtualize>
</div>

The Virtualize component:

  • Calculates the number of items to render based on the height of the container and the size of the rendered items.
  • Recalculates and rerenders the items as the user scrolls.
  • Only fetches the slice of records from an external API that correspond to the current visible region, instead of downloading all of the data from the collection.

The item content for the Virtualize component can include:

  • Plain HTML and Razor code, as the preceding example shows.
  • One or more Razor components.
  • A mix of HTML/Razor and Razor components.

Item provider delegate

If you don't want to load all of the items into memory, you can specify an items provider delegate method to the component's Virtualize<TItem>.ItemsProvider parameter that asynchronously retrieves the requested items on demand. In the following example, the LoadEmployees method provides the items to the Virtualize component:

<Virtualize Context="employee" ItemsProvider="@LoadEmployees">
    <p>
        @employee.FirstName @employee.LastName has the 
        job title of @employee.JobTitle.
    </p>
</Virtualize>

The items provider receives an ItemsProviderRequest, which specifies the required number of items starting at a specific start index. The items provider then retrieves the requested items from a database or other service and returns them as an ItemsProviderResult<TItem> along with a count of the total items. The items provider can choose to retrieve the items with each request or cache them so that they're readily available.

A Virtualize component can only accept one item source from its parameters, so don't attempt to simultaneously use an items provider and assign a collection to Items. If both are assigned, an InvalidOperationException is thrown when the component's parameters are set at runtime.

The following LoadEmployees method example loads employees from an EmployeeService (not shown):

private async ValueTask<ItemsProviderResult<Employee>> LoadEmployees(
    ItemsProviderRequest request)
{
    var numEmployees = Math.Min(request.Count, totalEmployees - request.StartIndex);
    var employees = await EmployeesService.GetEmployeesAsync(request.StartIndex, 
        numEmployees, request.CancellationToken);

    return new ItemsProviderResult<Employee>(employees, totalEmployees);
}

Virtualize<TItem>.RefreshDataAsync instructs the component to rerequest data from its ItemsProvider. This is useful when external data changes. There's usually no need to call RefreshDataAsync when using Items.

RefreshDataAsync updates a Virtualize component's data without causing a rerender. If RefreshDataAsync is invoked from a Blazor event handler or component lifecycle method, triggering a render isn't required because a render is automatically triggered at the end of the event handler or lifecycle method. If RefreshDataAsync is triggered separately from a background task or event, such as in the following ForcecastUpdated delegate, call StateHasChanged to update the UI at the end of the background task or event:

private Virtualize<FetchData>? virtualizeComponent;

protected override void OnInitialized()
{
    WeatherForecastSource.ForcecastUpdated += async () => 
    {
        await InvokeAsync(() =>
        {
            await virtualizeComponent?.RefreshDataAsync();
            StateHasChanged();
        });
    });
}

Placeholder

Because requesting items from a remote data source might take some time, you have the option to render a placeholder with item content:

<Virtualize Context="employee" ItemsProvider="@LoadEmployees">
    <ItemContent>
        <p>
            @employee.FirstName @employee.LastName has the 
            job title of @employee.JobTitle.
        </p>
    </ItemContent>
    <Placeholder>
        <p>
            Loading&hellip;
        </p>
    </Placeholder>
</Virtualize>

Item size

The height of each item in pixels can be set with Virtualize<TItem>.ItemSize (default: 50). The following example changes the height of each item from the default of 50 pixels to 25 pixels:

<Virtualize Context="employee" Items="@employees" ItemSize="25">
    ...
</Virtualize>

By default, the Virtualize component measures the rendering size (height) of individual items after the initial render occurs. Use ItemSize to provide an exact item size in advance to assist with accurate initial render performance and to ensure the correct scroll position for page reloads. If the default ItemSize causes some items to render outside of the currently visible view, a second re-render is triggered. To correctly maintain the browser's scroll position in a virtualized list, the initial render must be correct. If not, users might view the wrong items.

Overscan count

Virtualize<TItem>.OverscanCount determines how many additional items are rendered before and after the visible region. This setting helps to reduce the frequency of rendering during scrolling. However, higher values result in more elements rendered in the page (default: 3). The following example changes the overscan count from the default of three items to four items:

<Virtualize Context="employee" Items="@employees" OverscanCount="4">
    ...
</Virtualize>

State changes

When making changes to items rendered by the Virtualize component, call StateHasChanged to force re-evaluation and rerendering of the component. For more information, see ASP.NET Core Razor component rendering.

Keyboard scroll support

To allow users to scroll virtualized content using their keyboard, ensure that the virtualized elements or scroll container itself is focusable. If you fail to take this step, keyboard scrolling doesn't work in Chromium-based browsers.

For example, you can use a tabindex attribute on the scroll container:

<div style="height:500px; overflow-y:scroll" tabindex="-1">
    <Virtualize Items="@allFlights">
        <div class="flight-info">...</div>
    </Virtualize>
</div>

To learn more about the meaning of tabindex value -1, 0, or other values, see tabindex (MDN documentation).

Advanced styles and scroll detection

The Virtualize component is only designed to support specific element layout mechanisms. To understand which element layouts work correctly, the following explains how Virtualize detects which elements should be visible for display in the correct place.

If your source code looks like the following:

<div style="height:500px; overflow-y:scroll" tabindex="-1">
    <Virtualize Items="@allFlights" ItemSize="100">
        <div class="flight-info">Flight @context.Id</div>
    </Virtualize>
</div>

At runtime, the Virtualize component renders a DOM structure similar to the following:

<div style="height:500px; overflow-y:scroll" tabindex="-1">
    <div style="height:1100px"><!-- Placeholder --></div>
    <div class="flight-info">Flight 12</div>
    <div class="flight-info">Flight 13</div>
    <div class="flight-info">Flight 14</div>
    <div class="flight-info">Flight 15</div>
    <div class="flight-info">Flight 16</div>
    <div style="height:3400px"><!-- Placeholder --></div>
</div>

The actual number of rows rendered and the size of the placeholders vary according to your styling and Items collection size. However, notice that there are placeholder div elements injected before and after your content. These serve two purposes:

  • To provide an offset before and after your content, causing currently-visible items to appear at the correct location in the scroll range and the scroll range itself to represent the total size of all content.
  • To detect when the user is scrolling beyond the current visible range, meaning that different content must be rendered.

The placeholder elements internally use an Intersection Observer to receive notification when they're becoming visible. Virtualize depends on receiving these events. Virtualize works under the following conditions:

  • All content items are of identical height. This makes it possible to calculate which content corresponds to a given scroll position without first fetching every data item and rendering the data into a DOM element.

  • Both the placeholders and the content rows are rendered in a single vertical stack with every item filling the whole horizontal width. This is generally the default. In typical cases with div elements, Virtualize works by default. If you're using CSS to create a more advanced layout, bear in mind the following requirements:

    • Scroll container styling requires a display with any of the following values:
      • block (the default for a div).
      • table-row-group (the default for a tbody).
      • flex with flex-direction set to column. Ensure that immediate children of the Virtualize component don't shrink under flex rules. For example, add .mycontainer > div { flex-shrink: 0 }.
    • Content row styling requires a display with either of the following values:
      • block (the default for a div).
      • table-row (the default for a tr).
    • Don't use CSS to interfere with the layout for the placeholder elements. By default, the placeholder elements have a display value of block, except if the parent is a table row group, in which case they default to table-row. Don't try to influence placeholder element width or height, including by causing them to have a border or content pseudo-elements.

Any approach that stops the placeholders and content elements from rendering as a single vertical stack, or causes the content items to vary in height, prevents correct functioning of the Virtualize component.

Improve the perceived performance of component rendering using the Blazor framework's built-in virtualization support with the Virtualize component. Virtualization is a technique for limiting UI rendering to just the parts that are currently visible. For example, virtualization is helpful when the app must render a long list of items and only a subset of items is required to be visible at any given time.

Use the Virtualize component when:

  • Rendering a set of data items in a loop.
  • Most of the items aren't visible due to scrolling.
  • The rendered items are the same size.

When the user scrolls to an arbitrary point in the Virtualize component's list of items, the component calculates the visible items to show. Unseen items aren't rendered.

Without virtualization, a typical list might use a C# foreach loop to render each item in a list. In the following example:

  • allFlights is a collection of airplane flights.
  • The FlightSummary component displays details about each flight.
  • The @key directive attribute preserves the relationship of each FlightSummary component to its rendered flight by the flight's FlightId.
<div style="height:500px;overflow-y:scroll">
    @foreach (var flight in allFlights)
    {
        <FlightSummary @key="flight.FlightId" Details="@flight.Summary" />
    }
</div>

If the collection contains thousands of flights, rendering the flights takes a long time and users experience a noticeable UI lag. Most of the flights aren't seen because they fall outside of the height of the <div> element.

Instead of rendering the entire list of flights at once, replace the foreach loop in the preceding example with the Virtualize component:

  • Specify allFlights as a fixed item source to Virtualize<TItem>.Items. Only the currently visible flights are rendered by the Virtualize component.
  • Specify a context for each flight with the Context parameter. In the following example, flight is used as the context, which provides access to each flight's members.
<div style="height:500px;overflow-y:scroll">
    <Virtualize Items="@allFlights" Context="flight">
        <FlightSummary @key="flight.FlightId" Details="@flight.Summary" />
    </Virtualize>
</div>

If a context isn't specified with the Context parameter, use the value of context in the item content template to access each flight's members:

<div style="height:500px;overflow-y:scroll">
    <Virtualize Items="@allFlights">
        <FlightSummary @key="context.FlightId" Details="@context.Summary" />
    </Virtualize>
</div>

The Virtualize component:

  • Calculates the number of items to render based on the height of the container and the size of the rendered items.
  • Recalculates and rerenders the items as the user scrolls.
  • Only fetches the slice of records from an external API that correspond to the current visible region, instead of downloading all of the data from the collection.

The item content for the Virtualize component can include:

  • Plain HTML and Razor code, as the preceding example shows.
  • One or more Razor components.
  • A mix of HTML/Razor and Razor components.

Item provider delegate

If you don't want to load all of the items into memory, you can specify an items provider delegate method to the component's Virtualize<TItem>.ItemsProvider parameter that asynchronously retrieves the requested items on demand. In the following example, the LoadEmployees method provides the items to the Virtualize component:

<Virtualize Context="employee" ItemsProvider="@LoadEmployees">
    <p>
        @employee.FirstName @employee.LastName has the 
        job title of @employee.JobTitle.
    </p>
</Virtualize>

The items provider receives an ItemsProviderRequest, which specifies the required number of items starting at a specific start index. The items provider then retrieves the requested items from a database or other service and returns them as an ItemsProviderResult<TItem> along with a count of the total items. The items provider can choose to retrieve the items with each request or cache them so that they're readily available.

A Virtualize component can only accept one item source from its parameters, so don't attempt to simultaneously use an items provider and assign a collection to Items. If both are assigned, an InvalidOperationException is thrown when the component's parameters are set at runtime.

The following LoadEmployees method example loads employees from an EmployeeService (not shown):

private async ValueTask<ItemsProviderResult<Employee>> LoadEmployees(
    ItemsProviderRequest request)
{
    var numEmployees = Math.Min(request.Count, totalEmployees - request.StartIndex);
    var employees = await EmployeesService.GetEmployeesAsync(request.StartIndex, 
        numEmployees, request.CancellationToken);

    return new ItemsProviderResult<Employee>(employees, totalEmployees);
}

Virtualize<TItem>.RefreshDataAsync instructs the component to rerequest data from its ItemsProvider. This is useful when external data changes. There's no need to call RefreshDataAsync when using Items.

Placeholder

Because requesting items from a remote data source might take some time, you have the option to render a placeholder with item content:

<Virtualize Context="employee" ItemsProvider="@LoadEmployees">
    <ItemContent>
        <p>
            @employee.FirstName @employee.LastName has the 
            job title of @employee.JobTitle.
        </p>
    </ItemContent>
    <Placeholder>
        <p>
            Loading&hellip;
        </p>
    </Placeholder>
</Virtualize>

Item size

The height of each item in pixels can be set with Virtualize<TItem>.ItemSize (default: 50). The following example changes the height of each item from the default of 50 pixels to 25 pixels:

<Virtualize Context="employee" Items="@employees" ItemSize="25">
    ...
</Virtualize>

By default, the Virtualize component measures the rendering size (height) of individual items after the initial render occurs. Use ItemSize to provide an exact item size in advance to assist with accurate initial render performance and to ensure the correct scroll position for page reloads. If the default ItemSize causes some items to render outside of the currently visible view, a second re-render is triggered. To correctly maintain the browser's scroll position in a virtualized list, the initial render must be correct. If not, users might view the wrong items.

Overscan count

Virtualize<TItem>.OverscanCount determines how many additional items are rendered before and after the visible region. This setting helps to reduce the frequency of rendering during scrolling. However, higher values result in more elements rendered in the page (default: 3). The following example changes the overscan count from the default of three items to four items:

<Virtualize Context="employee" Items="@employees" OverscanCount="4">
    ...
</Virtualize>

State changes

When making changes to items rendered by the Virtualize component, call StateHasChanged to force re-evaluation and rerendering of the component. For more information, see ASP.NET Core Razor component rendering.