Supply "IsConnected" state info to components

src/Components/Blazor/Blazor/src/Hosting/WebAssemblyHostBuilder.cs

@@ -88,7 +88,7 @@ private void CreateServiceProvider()
             services.AddSingleton(_BrowserHostBuilderContext);
             services.AddSingleton<IWebAssemblyHost, WebAssemblyHost>();
             services.AddSingleton<IJSRuntime>(WebAssemblyJSRuntime.Instance);
-
+            services.AddSingleton<IComponentContext, WebAssemblyComponentContext>();
             services.AddSingleton<IUriHelper>(WebAssemblyUriHelper.Instance);
             services.AddSingleton<HttpClient>(s =>
             {

src/Components/Blazor/Blazor/src/Services/WebAssemblyComponentContext.cs

@@ -0,0 +1,12 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.AspNetCore.Components.Services;
+
+namespace Microsoft.AspNetCore.Blazor.Services
+{
+    internal class WebAssemblyComponentContext : IComponentContext
+    {
+        public bool IsConnected => true;
+    }
+}

src/Components/Blazor/Blazor/test/WebAssemblyComponentContextTest.cs

@@ -0,0 +1,16 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Xunit;
+
+namespace Microsoft.AspNetCore.Blazor.Services.Test
+{
+    public class WebAssemblyComponentContextTest
+    {
+        [Fact]
+        public void IsConnected()
+        {
+            Assert.True(new WebAssemblyComponentContext().IsConnected);
+        }
+    }
+}

src/Components/Components/ref/Microsoft.AspNetCore.Components.netstandard2.0.cs

@@ -836,6 +836,10 @@ public enum NavLinkMatch
 }
 namespace Microsoft.AspNetCore.Components.Services
 {
+    public partial interface IComponentContext
+    {
+        bool IsConnected { get; }
+    }
     public partial interface IUriHelper
     {
         event System.EventHandler<string> OnLocationChanged;

src/Components/Components/src/Services/IComponentContext.cs

@@ -0,0 +1,19 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+namespace Microsoft.AspNetCore.Components.Services
+{
+    /// <summary>
+    /// Provides information about the environment in which components are executing.
+    /// </summary>
+    public interface IComponentContext
+    {
+        /// <summary>
+        /// Gets a flag to indicate whether there is an active connection to the user's display.
+        /// </summary>
+        /// <example>During prerendering, the value will always be false.</example>
+        /// <example>During server-side execution, the value can be true or false depending on whether there is an active SignalR connection.</example>
+        /// <example>During client-side execution, the value will always be true.</example>
+        bool IsConnected { get; }
+    }
+}

src/Components/Server/src/Circuits/DefaultCircuitFactory.cs

@@ -41,7 +41,9 @@ public override CircuitHost CreateCircuitHost(
             var scope = _scopeFactory.CreateScope();
             var encoder = scope.ServiceProvider.GetRequiredService<HtmlEncoder>();
             var jsRuntime = (RemoteJSRuntime)scope.ServiceProvider.GetRequiredService<IJSRuntime>();
+            var componentContext = (RemoteComponentContext)scope.ServiceProvider.GetRequiredService<IComponentContext>();
             jsRuntime.Initialize(client);
+            componentContext.Initialize(client);
 
             var uriHelper = (RemoteUriHelper)scope.ServiceProvider.GetRequiredService<IUriHelper>();
             if (client != CircuitClientProxy.OfflineClient)

src/Components/Server/src/Circuits/RemoteComponentContext.cs

@@ -0,0 +1,20 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using Microsoft.AspNetCore.Components.Services;
+
+namespace Microsoft.AspNetCore.Components.Server.Circuits
+{
+    internal class RemoteComponentContext : IComponentContext
+    {
+        private CircuitClientProxy _clientProxy;
+
+        public bool IsConnected => _clientProxy != null && _clientProxy.Connected;
+
+        internal void Initialize(CircuitClientProxy clientProxy)
+        {
+            _clientProxy = clientProxy ?? throw new ArgumentNullException(nameof(clientProxy));
+        }
+    }
+}

src/Components/Server/src/DependencyInjection/ComponentServiceCollectionExtensions.cs

@@ -44,6 +44,7 @@ public static IServiceCollection AddRazorComponents(this IServiceCollection serv
             // Standard razor component services implementations
             services.AddScoped<IUriHelper, RemoteUriHelper>();
             services.AddScoped<IJSRuntime, RemoteJSRuntime>();
+            services.AddScoped<IComponentContext, RemoteComponentContext>();
 
             return services;
         }

src/Components/Server/test/Circuits/RemoteComponentContextTest.cs

@@ -0,0 +1,46 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Components.Server.Circuits;
+using Microsoft.AspNetCore.SignalR;
+using Xunit;
+
+namespace Microsoft.AspNetCore.Components.Browser.Rendering
+{
+    public class RemoteComponentContextTest
+    {
+        [Fact]
+        public void IfNotInitialized_IsConnectedReturnsFalse()
+        {
+            Assert.False(new RemoteComponentContext().IsConnected);
+        }
+
+        [Fact]
+        public void IfInitialized_IsConnectedValueDeterminedByCircuitProxy()
+        {
+            // Arrange
+            var clientProxy = new FakeClientProxy();
+            var circuitProxy = new CircuitClientProxy(clientProxy, "test connection");
+            var remoteComponentContext = new RemoteComponentContext();
+
+            // Act/Assert: Can observe connected state
+            remoteComponentContext.Initialize(circuitProxy);
+            Assert.True(remoteComponentContext.IsConnected);
+
+            // Act/Assert: Can observe disconnected state
+            circuitProxy.SetDisconnected();
+            Assert.False(remoteComponentContext.IsConnected);
+        }
+
+        private class FakeClientProxy : IClientProxy
+        {
+            public Task SendCoreAsync(string method, object[] args, CancellationToken cancellationToken = default)
+            { 
+                throw new NotImplementedException();
+            }
+        }
+    }
+}

src/Components/test/E2ETest/ServerExecutionTests/PrerenderingTest.cs

@@ -0,0 +1,47 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.AspNetCore.Components.E2ETest.Infrastructure;
+using Microsoft.AspNetCore.Components.E2ETest.Infrastructure.ServerFixtures;
+using Microsoft.AspNetCore.E2ETesting;
+using OpenQA.Selenium;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace Microsoft.AspNetCore.Components.E2ETests.ServerExecutionTests
+{
+    public class PrerenderingTest : ServerTestBase<AspNetSiteServerFixture>
+    {
+        public PrerenderingTest(
+            BrowserFixture browserFixture,
+            AspNetSiteServerFixture serverFixture,
+            ITestOutputHelper output)
+            : base(browserFixture, serverFixture, output)
+        {
+            _serverFixture.Environment = AspNetEnvironment.Development;
+            _serverFixture.BuildWebHostMethod = TestServer.Program.BuildWebHost;
+        }
+
+        [Fact]
+        public void CanTransitionFromPrerenderedToInteractiveMode()
+        {
+            Navigate("/prerendered/prerendered-transition");
+
+            // Prerendered output shows "not connected"
+            Browser.Equal("not connected", () => Browser.FindElement(By.Id("connected-state")).Text);
+
+            // Once connected, output changes
+            BeginInteractivity();
+            Browser.Equal("connected", () => Browser.FindElement(By.Id("connected-state")).Text);
+
+            // ... and now the counter works
+            Browser.FindElement(By.Id("increment-count")).Click();
+            Browser.Equal("1", () => Browser.FindElement(By.Id("count")).Text);
+        }
+
+        private void BeginInteractivity()
+        {
+            Browser.FindElement(By.Id("load-boot-script")).Click();
+        }
+    }
+}

src/Components/test/testassets/BasicTestApp/PrerenderedToInteractiveTransition.razor

@@ -0,0 +1,20 @@
+@page "/prerendered-transition"
+@using Microsoft.AspNetCore.Components.Services
+@inject IComponentContext ComponentContext
+
+<h1>Hello</h1>
+
+<p>
+    Current state:
+    <strong id="connected-state">@(ComponentContext.IsConnected ? "connected" : "not connected")</strong>
+</p>
+
+<p>
+    Clicks:
+    <strong id="count">@count</strong>
+    <button id="increment-count" onclick="@(() => count++)">Click me</button>
+</p>
+
+@functions {
+    int count;
+}

src/Components/test/testassets/TestServer/Pages/PrerenderedHost.cshtml

@@ -0,0 +1,27 @@
+@page
+@using BasicTestApp.RouterTest
+<!DOCTYPE html>
+<html>
+<head>
+    <title>Prerendering tests</title>
+    <base href="~/" />
+</head>
+<body>
+    <app>@(await Html.RenderComponentAsync<TestRouter>())</app>
+
+    @*
+        So that E2E tests can make assertions about both the prerendered and
+        interactive states, we only load the .js file when told to.
+    *@
+    <hr />
+    <button id="load-boot-script" onclick="loadBootScript(event)">Load boot script</button>
+    <script>
+    function loadBootScript(event) {
+        event.srcElement.disabled = true;
+        var scriptElem = document.createElement('script');
+        scriptElem.src = '_framework/components.server.js';
+        document.body.appendChild(scriptElem);
+    }
+    </script>
+</body>
+</html>

src/Components/test/testassets/TestServer/Startup.cs

@@ -1,4 +1,5 @@
 using BasicTestApp;
+using BasicTestApp.RouterTest;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Components.Server;
 using Microsoft.AspNetCore.Hosting;
@@ -68,6 +69,19 @@ public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
             {
                 endpoints.MapControllers();
             });
+
+            // Separately, mount a prerendered server-side Blazor app on /prerendered
+            app.Map("/prerendered", subdirApp =>
+            {
+                subdirApp.UsePathBase("/prerendered");
+                subdirApp.UseStaticFiles();
+                subdirApp.UseRouting();
+                subdirApp.UseEndpoints(endpoints =>
+                {
+                    endpoints.MapFallbackToPage("/PrerenderedHost");
+                    endpoints.MapComponentHub<TestRouter>("app");
+                });
+            });
         }
 
         private static void AllowCorsForAnyLocalhostPort(IApplicationBuilder app)

src/Components/test/testassets/TestServer/TestServer.csproj

@@ -1,7 +1,8 @@
-<Project Sdk="Microsoft.NET.Sdk.Web">
+<Project Sdk="Microsoft.NET.Sdk.Web">
 
   <PropertyGroup>
     <TargetFramework>netcoreapp3.0</TargetFramework>
+    <AddRazorSupportForMvc>true</AddRazorSupportForMvc>
   </PropertyGroup>
 
   <ItemGroup>