AspNetCore.Docs/aspnetcore/blazor/components/rendering.md

title	author	description	monikerRange	ms.author	ms.custom	ms.date	uid
ASP.NET Core Razor component rendering	guardrex	Learn about Razor component rendering in ASP.NET Core Blazor apps, including when to manually trigger a component to render.	>= aspnetcore-3.1	riande	mvc	02/09/2024	blazor/components/rendering

ASP.NET Core Razor component rendering

[!INCLUDE]

This article explains Razor component rendering in ASP.NET Core Blazor apps, including when to call xref:Microsoft.AspNetCore.Components.ComponentBase.StateHasChanged%2A to manually trigger a component to render.

Rendering conventions for ComponentBase

Components must render when they're first added to the component hierarchy by a parent component. This is the only time that a component must render. Components may render at other times according to their own logic and conventions.

By default, Razor components inherit from the xref:Microsoft.AspNetCore.Components.ComponentBase base class, which contains logic to trigger rerendering at the following times:

• After applying an updated set of parameters from a parent component.
• After applying an updated value for a cascading parameter.
• After notification of an event and invoking one of its own event handlers.
• After a call to its own xref:Microsoft.AspNetCore.Components.ComponentBase.StateHasChanged%2A method (see xref:blazor/components/lifecycle#state-changes-statehaschanged). For guidance on how to prevent overwriting child component parameters when xref:Microsoft.AspNetCore.Components.ComponentBase.StateHasChanged%2A is called in a parent component, see xref:blazor/components/overwriting-parameters.

Components inherited from xref:Microsoft.AspNetCore.Components.ComponentBase skip rerenders due to parameter updates if either of the following are true:

• All of the parameters are from a set of known types† or any primitive type that hasn't changed since the previous set of parameters were set.

  †The Blazor framework uses a set of built-in rules and explicit parameter type checks for change detection. These rules and the types are subject to change at any time. For more information, see the ChangeDetection API in the ASP.NET Core reference source.

  [!INCLUDE]

• The component's ShouldRender method returns false.

Control the rendering flow

In most cases, xref:Microsoft.AspNetCore.Components.ComponentBase conventions result in the correct subset of component rerenders after an event occurs. Developers aren't usually required to provide manual logic to tell the framework which components to rerender and when to rerender them. The overall effect of the framework's conventions is that the component receiving an event rerenders itself, which recursively triggers rerendering of descendant components whose parameter values may have changed.

For more information on the performance implications of the framework's conventions and how to optimize an app's component hierarchy for rendering, see xref:blazor/performance#optimize-rendering-speed.

::: moniker range=">= aspnetcore-8.0"

Streaming rendering

Use streaming rendering with static server-side rendering (static SSR) or prerendering to stream content updates on the response stream and improve the user experience for components that perform long-running asynchronous tasks to fully render.

For example, consider a component that makes a long-running database query or web API call to render data when the page loads. Normally, asynchronous tasks executed as part of rendering a server-side component must complete before the rendered response is sent, which can delay loading the page. Any significant delay in rendering the page harms the user experience. To improve the user experience, streaming rendering initially renders the entire page quickly with placeholder content while asynchronous operations execute. After the operations are complete, the updated content is sent to the client on the same response connection and patched into the DOM.

Streaming rendering requires the server to avoid buffering the output. The response data must flow to the client as the data is generated. For hosts that enforce buffering, streaming rendering degrades gracefully, and the page loads without streaming rendering.

To stream content updates when using static server-side rendering (static SSR) or prerendering, apply the [StreamRendering(true)] attribute to the component. Streaming rendering must be explicitly enabled because streamed updates may cause content on the page to shift. Components without the attribute automatically adopt streaming rendering if the parent component uses the feature. Pass false to the attribute in a child component to disable the feature at that point and further down the component subtree. The attribute is functional when applied to components supplied by a Razor class library.

The following example is based on the Weather component in an app created from the Blazor Web App project template. The call to xref:System.Threading.Tasks.Task.Delay%2A?displayProperty=nameWithType simulates retrieving weather data asynchronously. The component initially renders placeholder content ("Loading...") without waiting for the asynchronous delay to complete. When the asynchronous delay completes and the weather data content is generated, the content is streamed to the response and patched into the weather forecast table.

Weather.razor:

@page "/weather"
@attribute [StreamRendering(true)]

...

@if (forecasts == null)
{
    <p><em>Loading...</em></p>
}
else
{
    <table class="table">
        ...
        <tbody>
            @foreach (var forecast in forecasts)
            {
                <tr>
                    <td>@forecast.Date.ToShortDateString()</td>
                    <td>@forecast.TemperatureC</td>
                    <td>@forecast.TemperatureF</td>
                    <td>@forecast.Summary</td>
                </tr>
            }
        </tbody>
    </table>
}

@code {
    ...

    private WeatherForecast[]? forecasts;

    protected override async Task OnInitializedAsync()
    {
        await Task.Delay(500);

        ...

        forecasts = ...
    }
}

:::moniker-end

Suppress UI refreshing (ShouldRender)

xref:Microsoft.AspNetCore.Components.ComponentBase.ShouldRender%2A is called each time a component is rendered. Override xref:Microsoft.AspNetCore.Components.ComponentBase.ShouldRender%2A to manage UI refreshing. If the implementation returns true, the UI is refreshed.

Even if xref:Microsoft.AspNetCore.Components.ComponentBase.ShouldRender%2A is overridden, the component is always initially rendered.

ControlRender.razor:

:::moniker range=">= aspnetcore-8.0"

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/Pages/ControlRender.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-7.0 < aspnetcore-8.0"

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/rendering/ControlRender.razor":::

::: moniker-end

::: moniker range=">= aspnetcore-6.0 < aspnetcore-7.0"

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/rendering/ControlRender.razor":::

::: moniker-end

::: moniker range=">= aspnetcore-5.0 < aspnetcore-6.0"

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/rendering/ControlRender.razor":::

::: moniker-end

::: moniker range="< aspnetcore-5.0"

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/rendering/ControlRender.razor":::

::: moniker-end

For more information on performance best practices pertaining to xref:Microsoft.AspNetCore.Components.ComponentBase.ShouldRender%2A, see xref:blazor/performance#avoid-unnecessary-rendering-of-component-subtrees.

When to call StateHasChanged

Calling xref:Microsoft.AspNetCore.Components.ComponentBase.StateHasChanged%2A allows you to trigger a render at any time. However, be careful not to call xref:Microsoft.AspNetCore.Components.ComponentBase.StateHasChanged%2A unnecessarily, which is a common mistake that imposes unnecessary rendering costs.

Code shouldn't need to call xref:Microsoft.AspNetCore.Components.ComponentBase.StateHasChanged%2A when:

• Routinely handling events, whether synchronously or asynchronously, since xref:Microsoft.AspNetCore.Components.ComponentBase triggers a render for most routine event handlers.
• Implementing typical lifecycle logic, such as OnInitialized or OnParametersSetAsync, whether synchronously or asynchronously, since xref:Microsoft.AspNetCore.Components.ComponentBase triggers a render for typical lifecycle events.

However, it might make sense to call xref:Microsoft.AspNetCore.Components.ComponentBase.StateHasChanged%2A in the cases described in the following sections of this article:

• An asynchronous handler involves multiple asynchronous phases
• Receiving a call from something external to the Blazor rendering and event handling system
• To render component outside the subtree that is rerendered by a particular event

An asynchronous handler involves multiple asynchronous phases

Due to the way that tasks are defined in .NET, a receiver of a xref:System.Threading.Tasks.Task can only observe its final completion, not intermediate asynchronous states. Therefore, xref:Microsoft.AspNetCore.Components.ComponentBase can only trigger rerendering when the xref:System.Threading.Tasks.Task is first returned and when the xref:System.Threading.Tasks.Task finally completes. The framework can't know to rerender a component at other intermediate points, such as when an xref:System.Collections.Generic.IAsyncEnumerable%601 returns data in a series of intermediate Tasks. If you want to rerender at intermediate points, call xref:Microsoft.AspNetCore.Components.ComponentBase.StateHasChanged%2A at those points.

Consider the following CounterState1 component, which updates the count four times each time the IncrementCount method executes:

• Automatic renders occur after the first and last increments of currentCount.
• Manual renders are triggered by calls to xref:Microsoft.AspNetCore.Components.ComponentBase.StateHasChanged%2A when the framework doesn't automatically trigger rerenders at intermediate processing points where currentCount is incremented.

CounterState1.razor:

:::moniker range=">= aspnetcore-8.0"

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/Pages/CounterState1.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-7.0 < aspnetcore-8.0"

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/rendering/CounterState1.razor" highlight="17,21,25,29":::

::: moniker-end

::: moniker range=">= aspnetcore-6.0 < aspnetcore-7.0"

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/rendering/CounterState1.razor" highlight="17,21,25,29":::

::: moniker-end

::: moniker range=">= aspnetcore-5.0 < aspnetcore-6.0"

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/rendering/CounterState1.razor" highlight="17,21,25,29":::

::: moniker-end

::: moniker range="< aspnetcore-5.0"

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/rendering/CounterState1.razor" highlight="17,21,25,29":::

::: moniker-end

Receiving a call from something external to the Blazor rendering and event handling system

xref:Microsoft.AspNetCore.Components.ComponentBase only knows about its own lifecycle methods and Blazor-triggered events. xref:Microsoft.AspNetCore.Components.ComponentBase doesn't know about other events that may occur in code. For example, any C# events raised by a custom data store are unknown to Blazor. In order for such events to trigger rerendering to display updated values in the UI, call xref:Microsoft.AspNetCore.Components.ComponentBase.StateHasChanged%2A.

Consider the following CounterState2 component that uses xref:System.Timers.Timer?displayProperty=fullName to update a count at a regular interval and calls xref:Microsoft.AspNetCore.Components.ComponentBase.StateHasChanged%2A to update the UI:

• OnTimerCallback runs outside of any Blazor-managed rendering flow or event notification. Therefore, OnTimerCallback must call xref:Microsoft.AspNetCore.Components.ComponentBase.StateHasChanged%2A because Blazor isn't aware of the changes to currentCount in the callback.
• The component implements xref:System.IDisposable, where the xref:System.Timers.Timer is disposed when the framework calls the Dispose method. For more information, see xref:blazor/components/lifecycle#component-disposal-with-idisposable-and-iasyncdisposable.

Because the callback is invoked outside of Blazor's synchronization context, the component must wrap the logic of OnTimerCallback in xref:Microsoft.AspNetCore.Components.ComponentBase.InvokeAsync%2A?displayProperty=nameWithType to move it onto the renderer's synchronization context. This is equivalent to marshalling to the UI thread in other UI frameworks. xref:Microsoft.AspNetCore.Components.ComponentBase.StateHasChanged%2A can only be called from the renderer's synchronization context and throws an exception otherwise:

:::no-loc text="System.InvalidOperationException: 'The current thread is not associated with the Dispatcher. Use InvokeAsync() to switch execution to the Dispatcher when triggering rendering or component state.'":::

CounterState2.razor:

:::moniker range=">= aspnetcore-8.0"

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/Pages/CounterState2.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-7.0 < aspnetcore-8.0"

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/rendering/CounterState2.razor" highlight="23-27":::

::: moniker-end

::: moniker range=">= aspnetcore-6.0 < aspnetcore-7.0"

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/rendering/CounterState2.razor" highlight="23-27":::

::: moniker-end

::: moniker range=">= aspnetcore-5.0 < aspnetcore-6.0"

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/rendering/CounterState2.razor" highlight="23-27":::

::: moniker-end

::: moniker range="< aspnetcore-5.0"

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/rendering/CounterState2.razor" highlight="23-27":::

::: moniker-end

To render a component outside the subtree that's rerendered by a particular event

The UI might involve:

1. Dispatching an event to one component.
2. Changing some state.
3. Rerendering a completely different component that isn't a descendant of the component receiving the event.

One way to deal with this scenario is to provide a state management class, often as a dependency injection (DI) service, injected into multiple components. When one component calls a method on the state manager, the state manager raises a C# event that's then received by an independent component.

For approaches to manage state, see the following resources:

• Server-side in-memory state container service (client-side equivalent) section of the State management article.
• Pass data across a component hierarchy using cascading values and parameters.
• Bind across more than two components using data bindings.

For the state manager approach, C# events are outside the Blazor rendering pipeline. Call xref:Microsoft.AspNetCore.Components.ComponentBase.StateHasChanged%2A on other components you wish to rerender in response to the state manager's events.

The state manager approach is similar to the earlier case with xref:System.Timers.Timer?displayProperty=fullName in the previous section. Since the execution call stack typically remains on the renderer's synchronization context, calling xref:Microsoft.AspNetCore.Components.ComponentBase.InvokeAsync%2A isn't normally required. Calling xref:Microsoft.AspNetCore.Components.ComponentBase.InvokeAsync%2A is only required if the logic escapes the synchronization context, such as calling xref:System.Threading.Tasks.Task.ContinueWith%2A on a xref:System.Threading.Tasks.Task or awaiting a xref:System.Threading.Tasks.Task with ConfigureAwait(false). For more information, see the Receiving a call from something external to the Blazor rendering and event handling system section.

WebAssembly loading progress indicator for Blazor Web Apps

A loading progress indicator isn't present in an app created from the Blazor Web App project template. A new loading progress indicator feature is planned for a future release of .NET. In the meantime, an app can adopt custom code to create a loading progress indicator. For more information, see xref:blazor/fundamentals/startup#client-side-loading-progress-indicators.