Rerender child components when their parameters change

src/Microsoft.AspNetCore.Blazor/RenderTree/RenderTreeDiffComputer.cs

@@ -179,6 +179,7 @@ namespace Microsoft.AspNetCore.Blazor.RenderTree
             ref var newComponentNode = ref newTree[newComponentIndex];
             var componentId = oldComponentNode.ComponentId;
             var componentInstance = oldComponentNode.Component;
+            var hasSetAnyProperty = false;
 
             // Preserve the actual componentInstance
             newComponentNode.SetChildComponentInstance(componentId, componentInstance);
@@ -207,6 +208,7 @@ namespace Microsoft.AspNetCore.Blazor.RenderTree
                         if (!Equals(oldPropertyValue, newPropertyValue))
                         {
                             SetChildComponentProperty(componentInstance, newName, newPropertyValue);
+                            hasSetAnyProperty = true;
                         }
                     }
                     else
@@ -216,6 +218,7 @@ namespace Microsoft.AspNetCore.Blazor.RenderTree
                         // could consider removing the 'name equality' check entirely for perf
                         SetChildComponentProperty(componentInstance, newName, newPropertyValue);
                         RemoveChildComponentProperty(componentInstance, oldName);
+                        hasSetAnyProperty = true;
                     }
 
                     oldStartIndex++;
@@ -235,6 +238,7 @@ namespace Microsoft.AspNetCore.Blazor.RenderTree
                         SetChildComponentProperty(componentInstance,
                             newTree[newStartIndex].AttributeName,
                             newTree[newStartIndex].AttributeValue);
+                        hasSetAnyProperty = true;
                         newStartIndex++;
                         hasMoreNew = newEndIndexIncl >= newStartIndex;
                     }
@@ -242,11 +246,19 @@ namespace Microsoft.AspNetCore.Blazor.RenderTree
                     {
                         RemoveChildComponentProperty(componentInstance,
                             oldTree[oldStartIndex].AttributeName);
+                        hasSetAnyProperty = true;
                         oldStartIndex++;
                         hasMoreOld = oldEndIndexIncl >= oldStartIndex;
                     }
                 }
             }
+
+            if (hasSetAnyProperty)
+            {
+                // TODO: Instead, call some OnPropertiesUpdated method on IComponent,
+                // whose default implementation causes itself to be rerendered
+                _renderer.RenderComponent(componentId);
+            }
         }
 
         private static void RemoveChildComponentProperty(IComponent component, string componentPropertyName)

src/Microsoft.AspNetCore.Blazor/Rendering/Renderer.cs

@@ -55,7 +55,7 @@ namespace Microsoft.AspNetCore.Blazor.Rendering
         /// Updates the rendered state of the specified <see cref="IComponent"/>.
         /// </summary>
         /// <param name="componentId">The identifier of the <see cref="IComponent"/> to render.</param>
-        protected void RenderComponent(int componentId)
+        protected internal void RenderComponent(int componentId)
             => GetRequiredComponentState(componentId).Render();
 
         /// <summary>

test/Microsoft.AspNetCore.Blazor.Test/RenderTreeDiffComputerTest.cs

@@ -819,7 +819,8 @@ namespace Microsoft.AspNetCore.Blazor.Test
         private class FakeRenderer : Renderer
         {
             internal protected override void UpdateDisplay(int componentId, RenderTreeDiff renderTreeDiff)
-                => throw new NotImplementedException();
+            {
+            }
         }
 
         private class FakeComponent : IComponent
@@ -831,7 +832,8 @@ namespace Microsoft.AspNetCore.Blazor.Test
             private string PrivateProperty { get; set; }
 
             public void BuildRenderTree(RenderTreeBuilder builder)
-                => throw new NotImplementedException();
+            {
+            }
         }
 
         private class FakeComponent2 : IComponent

test/Microsoft.AspNetCore.Blazor.Test/RendererTest.cs

@@ -374,6 +374,42 @@ namespace Microsoft.AspNetCore.Blazor.Test
             Assert.Same(objectThatWillNotChange, updatedComponentInstance.ObjectProperty);
         }
 
+        [Fact]
+        public void ReRendersChildComponentsWhenPropertiesChange()
+        {
+            // Arrange: First render
+            var renderer = new TestRenderer();
+            var firstRender = true;
+            var component = new TestComponent(builder =>
+            {
+                builder.OpenComponentElement<MessageComponent>(1);
+                builder.AddAttribute(2, nameof(MessageComponent.Message), firstRender ? "first" : "second");
+                builder.CloseElement();
+            });
+
+            var rootComponentId = renderer.AssignComponentId(component);
+            renderer.RenderComponent(rootComponentId);
+
+            var childComponentId = renderer.RenderTreesByComponentId[rootComponentId]
+                .Single(node => node.NodeType == RenderTreeNodeType.Component)
+                .ComponentId;
+
+            // This isn't strictly necessary for the test, but it's more common for components
+            // to be updated after their first render than before it
+            renderer.RenderComponent(childComponentId);
+
+            // Act: Second render
+            firstRender = false;
+            renderer.RenderComponent(rootComponentId);
+
+            var updatedComponentNode = renderer.RenderTreesByComponentId[rootComponentId]
+                .Single(node => node.NodeType == RenderTreeNodeType.Component);
+
+            // Assert
+            Assert.Collection(renderer.RenderTreesByComponentId[updatedComponentNode.ComponentId],
+                node => AssertNode.Text(node, "second"));
+        }
+
         private class NoOpRenderer : Renderer
         {
             public new int AssignComponentId(IComponent component)