[Blazor] Allow cascading value subscribers to get added and removed during change notification

src/Components/Components/src/CascadingValueSource.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Concurrent;
+using System.Runtime.CompilerServices;
 using Microsoft.AspNetCore.Components.Rendering;
 
 namespace Microsoft.AspNetCore.Components;
@@ -96,7 +97,16 @@ public Task NotifyChangedAsync()
             {
                 tasks.Add(dispatcher.InvokeAsync(() =>
                 {
-                    foreach (var subscriber in subscribers)
+                    var subscribersBuffer = new ComponentStateBuffer();
+                    var subscribersCount = subscribers.Count;
+                    var subscribersCopy = subscribersCount <= ComponentStateBuffer.Capacity
+                        ? subscribersBuffer[..subscribersCount]
+                        : new ComponentState[subscribersCount];
+                    subscribers.CopyTo(subscribersCopy);
+
+                    // We iterate over a copy of the list because new subscribers might get
+                    // added or removed during change notification
+                    foreach (var subscriber in subscribersCopy)
                     {
                         subscriber.NotifyCascadingValueChanged(ParameterViewLifetime.Unbound);
                     }
@@ -174,4 +184,15 @@ void ICascadingValueSupplier.Unsubscribe(ComponentState subscriber, in Cascading
             }
         }
     }
+
+    [InlineArray(Capacity)]
+    internal struct ComponentStateBuffer
+    {
+        public const int Capacity = 64;
+#pragma warning disable IDE0051 // Remove unused private members
+#pragma warning disable IDE0044 // Add readonly modifier
+        private ComponentState _values;
+#pragma warning restore IDE0044 // Add readonly modifier
+#pragma warning restore IDE0051 // Remove unused private members
+    }
 }

src/Components/Components/test/CascadingParameterTest.cs

@@ -634,6 +634,61 @@ public async Task CanTriggerUpdatesOnCascadingValuesFromServiceProvider()
         await cascadingValueSource.NotifyChangedAsync(new MyParamType("Nobody is listening, but this shouldn't be an error"));
     }
 
+    [Fact]
+    public async Task CanAddSubscriberDuringChangeNotification()
+    {
+        // Arrange
+        var services = new ServiceCollection();
+        var paramValue = new MyParamType("Initial value");
+        var cascadingValueSource = new CascadingValueSource<MyParamType>(paramValue, isFixed: false);
+        services.AddCascadingValue(_ => cascadingValueSource);
+        var renderer = new TestRenderer(services.BuildServiceProvider());
+        var component = new ConditionallyRenderSubscriberComponent()
+        {
+            RenderWhenEqualTo = "Final value",
+        };
+
+        // Act/Assert: Initial render
+        var componentId = await renderer.Dispatcher.InvokeAsync(() => renderer.AssignRootComponentId(component));
+        renderer.RenderRootComponent(componentId);
+        var firstBatch = renderer.Batches.Single();
+        var diff = firstBatch.DiffsByComponentId[componentId].Single();
+        Assert.Collection(diff.Edits,
+            edit =>
+            {
+                Assert.Equal(RenderTreeEditType.PrependFrame, edit.Type);
+                AssertFrame.Text(
+                    firstBatch.ReferenceFrames[edit.ReferenceFrameIndex],
+                    "CascadingParameter=Initial value");
+            });
+        Assert.Equal(1, component.NumRenders);
+
+        // Act: Second render
+        paramValue.ChangeValue("Final value");
+        await cascadingValueSource.NotifyChangedAsync();
+        var secondBatch = renderer.Batches[1];
+        var diff2 = secondBatch.DiffsByComponentId[componentId].Single();
+
+        // Assert: Subscriber can get added during change notification and receive the cascading value
+        AssertFrame.Text(
+            secondBatch.ReferenceFrames[diff2.Edits[0].ReferenceFrameIndex],
+            "CascadingParameter=Final value");
+        Assert.Equal(2, component.NumRenders);
+
+        // Assert: Subscriber can get added during change notification and receive the cascading value
+        var nestedComponent = FindComponent<SimpleSubscriberComponent>(secondBatch, out var nestedComponentId);
+        var nestedComponentDiff = secondBatch.DiffsByComponentId[nestedComponentId].Single();
+        Assert.Collection(nestedComponentDiff.Edits,
+            edit =>
+            {
+                Assert.Equal(RenderTreeEditType.PrependFrame, edit.Type);
+                AssertFrame.Text(
+                    secondBatch.ReferenceFrames[edit.ReferenceFrameIndex],
+                    "CascadingParameter=Final value");
+            });
+        Assert.Equal(1, nestedComponent.NumRenders);
+    }
+
     [Fact]
     public async Task AfterSupplyingValueThroughNotifyChanged_InitialValueFactoryIsNotUsed()
     {
@@ -772,6 +827,40 @@ public void CanUseTryAddPatternForCascadingValuesInServiceCollection_CascadingVa
         Assert.Equal(2, services.Count());
     }
 
+    [Theory]
+    [InlineData(0)]
+    [InlineData(1)]
+    [InlineData(CascadingValueSource<MyParamType>.ComponentStateBuffer.Capacity - 1)]
+    [InlineData(CascadingValueSource<MyParamType>.ComponentStateBuffer.Capacity)]
+    [InlineData(CascadingValueSource<MyParamType>.ComponentStateBuffer.Capacity + 1)]
+    [InlineData(CascadingValueSource<MyParamType>.ComponentStateBuffer.Capacity * 2)]
+    public async Task CanHaveManySubscribers(int numSubscribers)
+    {
+        // Arrange
+        var services = new ServiceCollection();
+        var paramValue = new MyParamType("Initial value");
+        var cascadingValueSource = new CascadingValueSource<MyParamType>(paramValue, isFixed: false);
+        services.AddCascadingValue(_ => cascadingValueSource);
+        var renderer = new TestRenderer(services.BuildServiceProvider());
+        var components = Enumerable.Range(0, numSubscribers).Select(_ => new SimpleSubscriberComponent()).ToArray();
+
+        // Act/Assert: Initial render
+        foreach (var component in components)
+        {
+            await renderer.Dispatcher.InvokeAsync(() => renderer.AssignRootComponentId(component));
+            component.TriggerRender();
+            Assert.Equal(1, component.NumRenders);
+        }
+
+        // Act/Assert: All components re-render when the cascading value changes
+        paramValue.ChangeValue("Final value");
+        await cascadingValueSource.NotifyChangedAsync();
+        foreach (var component in components)
+        {
+            Assert.Equal(2, component.NumRenders);
+        }
+    }
+
     private class SingleDeliveryValue(string text)
     {
         public string Text => text;
@@ -861,6 +950,43 @@ public void AttemptIllegalAccessToLastParameterView()
         }
     }
 
+    class ConditionallyRenderSubscriberComponent : AutoRenderComponent
+    {
+        public int NumRenders { get; private set; }
+
+        public SimpleSubscriberComponent NestedSubscriber { get; private set; }
+
+        [Parameter] public string RenderWhenEqualTo { get; set; }
+
+        [CascadingParameter] MyParamType CascadingParameter { get; set; }
+
+        protected override void BuildRenderTree(RenderTreeBuilder builder)
+        {
+            NumRenders++;
+            builder.AddContent(0, $"CascadingParameter={CascadingParameter}");
+
+            if (string.Equals(RenderWhenEqualTo, CascadingParameter.ToString(), StringComparison.OrdinalIgnoreCase))
+            {
+                builder.OpenComponent<SimpleSubscriberComponent>(1);
+                builder.AddComponentReferenceCapture(2, component => NestedSubscriber = component as SimpleSubscriberComponent);
+                builder.CloseComponent();
+            }
+        }
+    }
+
+    class SimpleSubscriberComponent : AutoRenderComponent
+    {
+        public int NumRenders { get; private set; }
+
+        [CascadingParameter] MyParamType CascadingParameter { get; set; }
+
+        protected override void BuildRenderTree(RenderTreeBuilder builder)
+        {
+            NumRenders++;
+            builder.AddContent(0, $"CascadingParameter={CascadingParameter}");
+        }
+    }
+
     class SingleDeliveryParameterConsumerComponent : AutoRenderComponent
     {
         public int NumSetParametersCalls { get; private set; }