Custom Blazor WASM loading progress indicator

aspnetcore/blazor/components/rendering.md

@@ -245,3 +245,10 @@ For approaches to manage state, see the following resources:
 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](#receiving-a-call-from-something-external-to-the-blazor-rendering-and-event-handling-system). 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)`](xref:System.Threading.Tasks.Task.ConfigureAwait%2A). For more information, see the [Receiving a call from something external to the Blazor rendering and event handling system](#receiving-a-call-from-something-external-to-the-blazor-rendering-and-event-handling-system) section.
+
+## WebAssembly loading progress indicator for Blazor Web Apps
+
+<!-- UPDATE 9.0 Will be removed for a new feature in this area. 
+                Tracked by: https://github.com/dotnet/aspnetcore/issues/49056 -->
+
+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>.

aspnetcore/blazor/fundamentals/startup.md

@@ -520,7 +520,128 @@ For more information on CSPs, see <xref:blazor/security/content-security-policy>
 
 ## Client-side loading progress indicators
 
-*This section only applies to Blazor WebAssembly apps.*
+A loading progress indicator shows the loading progress of the app to users, indicating that the app is loading normally and that the user should wait until loading is finished.
+
+:::moniker-end
+
+:::moniker range=">= aspnetcore-8.0"
+
+### Blazor Web App loading progress
+
+The loading progress indicator used in Blazor WebAssembly apps isn't present in an app created from the Blazor Web App project template. Usually, a loading progress indicator isn't desirable for interactive WebAssembly components because Blazor Web Apps prerender client-side components on the server for fast initial load times. For mixed-render-mode situations, the framework or developer code must also be careful to avoid the following problems:
+
+* Showing multiple loading indicators on the same rendered page.
+* Inadvertently discarding prerendered content while the WebAssembly runtime is loading.
+
+<!-- UPDATE 9.0 Will be removed for a new feature in this area. 
+                Tracked by: https://github.com/dotnet/aspnetcore/issues/49056 -->
+
+A future release of .NET might provide a framework-based loading progress indicator. In the meantime, you can add a custom loading progress indicator to a Blazor Web App.
+
+Create a `LoadingProgress` component in the `.Client` app that calls <xref:System.OperatingSystem.IsBrowser%2A?displayProperty=nameWithType>:
+
+* When `false`, display a loading progress indicator while the Blazor bundle is downloaded and before the Blazor runtime activates on the client.
+* When `true`, render the requested component's content.
+
+The following demonstration uses the loading progress indicator found in apps created from the Blazor WebAssembly template, including a modification of the styles that the template provides. The styles are loaded into the app's `<head>` content by the <xref:Microsoft.AspNetCore.Components.Web.HeadContent> component. For more information, see <xref:blazor/components/control-head-content>.
+
+`LoadingProgress.razor`:
+
+```razor
+@if (!OperatingSystem.IsBrowser())
+{
+    <HeadContent>
+        <style>
+            .loading-progress {
+                position: relative;
+                display: block;
+                width: 8rem;
+                height: 8rem;
+                margin: 20vh auto 1rem auto;
+            }
+
+                .loading-progress circle {
+                    fill: none;
+                    stroke: #e0e0e0;
+                    stroke-width: 0.6rem;
+                    transform-origin: 50% 50%;
+                    transform: rotate(-90deg);
+                }
+
+                    .loading-progress circle:last-child {
+                        stroke: #1b6ec2;
+                        stroke-dasharray: 
+                            calc(3.141 * var(--blazor-load-percentage, 0%) * 0.8), 
+                            500%;
+                        transition: stroke-dasharray 0.05s ease-in-out;
+                    }
+
+            .loading-progress-text {
+                position: relative;
+                text-align: center;
+                font-weight: bold;
+                top: -90px;
+            }
+
+                .loading-progress-text:after {
+                    content: var(--blazor-load-percentage-text, "Loading");
+                }
+
+            code {
+                color: #c02d76;
+            }
+        </style>
+    </HeadContent>
+    <svg class="loading-progress">
+        <circle r="40%" cx="50%" cy="50%" />
+        <circle r="40%" cx="50%" cy="50%" />
+    </svg>
+    <div class="loading-progress-text"></div>
+}
+else
+{
+    @ChildContent
+}
+
+@code {
+    [Parameter]
+    public RenderFragment? ChildContent { get; set; }
+}
+```
+
+In a component that adopts Interactive WebAssembly rendering, wrap the component's Razor markup with the `LoadingProgress` component. The following example demonstrates the approach with the `Counter` component of an app created from the Blazor Web App project template.
+
+`Pages/Counter.razor`:
+
+```razor
+@page "/counter"
+@rendermode InteractiveWebAssembly
+
+<PageTitle>Counter</PageTitle>
+
+<LoadingProgress>
+    <h1>Counter</h1>
+
+    <p role="status">Current count: @currentCount</p>
+
+    <button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
+</LoadingProgress>
+
+@code {
+    private int currentCount = 0;
+
+    private void IncrementCount()
+    {
+        currentCount++;
+    }
+}
+```
+
+### Blazor WebAssembly app loading progress
+
+:::moniker-end
+
+:::moniker range=">= aspnetcore-7.0"
 
 The project template contains Scalable Vector Graphics (SVG) and text indicators that show the loading progress of the app.