[Blazor] Prevent HtmlRenderer from calling OnAfterRender by default

Author: javiercn

src/Components/Components/src/Rendering/HtmlRenderer.cs

@@ -4,8 +4,8 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
-using System.Runtime.CompilerServices;
 using System.Runtime.ExceptionServices;
+using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Components.RenderTree;
 using Microsoft.Extensions.Logging;
@@ -22,6 +22,8 @@ public class HtmlRenderer : Renderer
             "area", "base", "br", "col", "embed", "hr", "img", "input", "link", "meta", "param", "source", "track", "wbr"
         };
 
+        private static readonly Task CancelledRenderTask = Task.FromCanceled(new CancellationToken(canceled: true));
+
         private readonly Func<string, string> _htmlEncoder;
 
         /// <summary>
@@ -41,7 +43,13 @@ public HtmlRenderer(IServiceProvider serviceProvider, ILoggerFactory loggerFacto
         /// <inheritdoc />
         protected override Task UpdateDisplayAsync(in RenderBatch renderBatch)
         {
-            return Task.CompletedTask;
+            // By default we return a cancelled task. This has the effect of making it so that the
+            // OnAfterRenderAsync callbacks on components don't run by default.
+            // This way, by default prerendering gets the correct behavior and other renderers
+            // override the UpdateDisplayAsync method already, so those components can cancel
+            // either complete a task when the client acknowledges the render, or a cancelled task
+            // when the renderer gets disposed.
+            return CancelledRenderTask;
         }
 
         /// <summary>

src/Components/Web.JS/dist/Release/blazor.server.js

@@ -59,6 +59,26 @@ public async Task CanRender_ComponentWithParametersObject()
             Assert.Equal("<p>Hello Steve!</p>", content);
         }
 
+        [Fact]
+        public async Task RenderComponent_DoesNotInvokeOnAfterRenderInComponent()
+        {
+            // Arrange
+            var helper = CreateHelper();
+            var writer = new StringWriter();
+
+            // Act
+            var state = new State();
+            var result = await helper.RenderStaticComponentAsync<OnAfterRenderComponent>(new
+            {
+                State = state
+            });
+            result.WriteTo(writer, HtmlEncoder.Default);
+
+            // Assert
+            Assert.Equal("<p>Hello</p>", writer.ToString());
+            Assert.False(state.Value);
+        }
+
         [Fact]
         public async Task CanCatch_ComponentWithSynchronousException()
         {
@@ -310,6 +330,26 @@ protected override async Task OnParametersSetAsync()
             }
         }
 
+        private class OnAfterRenderComponent : ComponentBase
+        {
+            [Parameter] public State State { get; set; }
+
+            protected override void OnAfterRender()
+            {
+                State.Value = true;
+            }
+
+            protected override void BuildRenderTree(RenderTreeBuilder builder)
+            {
+                builder.AddMarkupContent(0, "<p>Hello</p>");
+            }
+        }
+
+        private class State
+        {
+            public bool Value { get; set; }
+        }
+
         private class GreetingComponent : ComponentBase
         {
             [Parameter] public string Name { get; set; }