Mechanism for components running logic when parents change their properties

src/Microsoft.AspNetCore.Blazor/Components/BlazorComponent.cs

@@ -11,7 +11,7 @@ namespace Microsoft.AspNetCore.Blazor.Components
     /// Optional base class for Blazor components. Alternatively, Blazor components may
     /// implement <see cref="IComponent"/> directly.
     /// </summary>
-    public abstract class BlazorComponent : IComponent
+    public abstract class BlazorComponent : IComponent, IHandlePropertiesChanged
     {
         /// <inheritdoc />
         public virtual void BuildRenderTree(RenderTreeBuilder builder)
@@ -35,6 +35,11 @@ namespace Microsoft.AspNetCore.Blazor.Components
         public virtual Task ExecuteAsync()
             => throw new NotImplementedException($"Blazor components do not implement {nameof(ExecuteAsync)}.");
 
+        /// <inheritdoc />
+        public virtual void OnPropertiesChanged()
+        {
+        }
+
         /// <summary>
         /// Handles click events by invoking <paramref name="handler"/>.
         /// </summary>

src/Microsoft.AspNetCore.Blazor/Components/IHandlePropertiesChanged.cs

@@ -0,0 +1,17 @@
+// 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.Blazor.Components
+{
+    /// <summary>
+    /// Interface implemented by components that receive notifications when their
+    /// properties are changed by their parent component.
+    /// </summary>
+    public interface IHandlePropertiesChanged
+    {
+        /// <summary>
+        /// Notifies the component that its properties have changed.
+        /// </summary>
+        void OnPropertiesChanged();
+    }
+}

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

@@ -271,9 +271,7 @@ namespace Microsoft.AspNetCore.Blazor.RenderTree
 
             if (hasSetAnyProperty)
             {
-                // TODO: Instead, call some OnPropertiesUpdated method on IComponent,
+                TriggerChildComponentRender(batchBuilder, newComponentNode);
-                // whose default implementation causes itself to be rerendered
-                _renderer.RenderInExistingBatch(batchBuilder, componentId);
             }
         }
 
@@ -508,11 +506,28 @@ namespace Microsoft.AspNetCore.Blazor.RenderTree
                             attributeNode.AttributeValue);
                     }
 
-                    _renderer.RenderInExistingBatch(batchBuilder, node.ComponentId);
+                    TriggerChildComponentRender(batchBuilder, node);
                 }
             }
         }
 
+        private void TriggerChildComponentRender(RenderBatchBuilder batchBuilder, in RenderTreeNode node)
+        {
+            if (node.Component is IHandlePropertiesChanged notifyableComponent)
+            {
+                // TODO: Ensure any exceptions thrown here are handled equivalently to
+                // unhandled exceptions during rendering.
+                notifyableComponent.OnPropertiesChanged();
+            }
+
+            // TODO: Consider moving the responsibility for triggering re-rendering
+            // into the OnPropertiesChanged handler (if implemented) so that components
+            // can control whether any given set of property changes cause re-rendering.
+            // Not doing so yet because it's unclear that the usage patterns would be
+            // good to use.
+            _renderer.RenderInExistingBatch(batchBuilder, node.ComponentId);
+        }
+
         private void DisposeChildComponents(RenderBatchBuilder batchBuilder, RenderTreeNode[] nodes, int elementOrComponentIndex)
         {
             var endIndex = nodes[elementOrComponentIndex].ElementDescendantsEndIndex;

test/Microsoft.AspNetCore.Blazor.E2ETest/Tests/ComponentRenderingTest.cs

@@ -148,6 +148,23 @@ namespace Microsoft.AspNetCore.Blazor.E2ETest.Tests
                 elem => Assert.Equal("Child 3", elem.FindElement(By.ClassName("message")).Text));
         }
 
+        [Fact]
+        public void ChildComponentsNotifiedWhenPropertiesChanged()
+        {
+            // Child component receives notification that lets it compute a property before first render
+            var appElement = MountTestComponent<PropertiesChangedHandlerParent>();
+            var suppliedValueElement = appElement.FindElement(By.ClassName("supplied"));
+            var computedValueElement = appElement.FindElement(By.ClassName("computed"));
+            var incrementButton = appElement.FindElement(By.TagName("button"));
+            Assert.Equal("You supplied: 100", suppliedValueElement.Text);
+            Assert.Equal("I computed: 200", computedValueElement.Text);
+
+            // When property changes, child is renotified before rerender
+            incrementButton.Click();
+            Assert.Equal("You supplied: 101", suppliedValueElement.Text);
+            Assert.Equal("I computed: 202", computedValueElement.Text);
+        }
+
         private IWebElement MountTestComponent<TComponent>() where TComponent: IComponent
         {
             var componentTypeName = typeof(TComponent).FullName;

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

@@ -762,6 +762,59 @@ namespace Microsoft.AspNetCore.Blazor.Test
             Assert.Null(newComponentInstance.ObjectProperty); // To observe that the property wasn't even written, we nulled it out on the original
         }
 
+        [Fact]
+        public void NotifiesIHandlePropertiesChangedBeforeFirstRender()
+        {
+            // Arrange
+            newTree.OpenComponentElement<HandlePropertiesChangedComponent>(0);
+            newTree.CloseElement();
+
+            // Act
+            var batch = GetRenderedBatch();
+            var diffForChildComponent = batch.UpdatedComponents.Array[1];
+
+            // Assert
+            Assert.Collection(diffForChildComponent.CurrentState,
+                node => AssertNode.Text(node, "Notifications: 1", 0));
+        }
+
+        [Fact]
+        public void NotifiesIHandlePropertiesChangedWhenChanged()
+        {
+            // Arrange
+            var newTree1 = new RenderTreeBuilder(renderer);
+            var newTree2 = new RenderTreeBuilder(renderer);
+            oldTree.OpenComponentElement<HandlePropertiesChangedComponent>(0);
+            oldTree.AddAttribute(1, nameof(HandlePropertiesChangedComponent.IntProperty), 123);
+            oldTree.CloseElement();
+            newTree1.OpenComponentElement<HandlePropertiesChangedComponent>(0);
+            newTree1.AddAttribute(1, nameof(HandlePropertiesChangedComponent.IntProperty), 123);
+            newTree1.CloseElement();
+            newTree2.OpenComponentElement<HandlePropertiesChangedComponent>(0);
+            newTree2.AddAttribute(1, nameof(HandlePropertiesChangedComponent.IntProperty), 456);
+            newTree2.CloseElement();
+
+            // Act/Assert 0: Initial render
+            var batch0 = GetRenderedBatch(new RenderTreeBuilder(renderer), oldTree);
+            var diffForChildComponent0 = batch0.UpdatedComponents.Array[1];
+            var childComponentNode = batch0.UpdatedComponents.Array[0].CurrentState.Array[0];
+            var childComponentInstance = (HandlePropertiesChangedComponent)childComponentNode.Component;
+            Assert.Equal(1, childComponentInstance.NotificationsCount);
+            Assert.Collection(diffForChildComponent0.CurrentState,
+                node => AssertNode.Text(node, "Notifications: 1", 0));
+
+            // Act/Assert 1: If properties didn't change, we don't notify
+            GetRenderedBatch(oldTree, newTree1);
+            Assert.Equal(1, childComponentInstance.NotificationsCount);
+
+            // Act/Assert 2: If properties did change, we do notify
+            var batch2 = GetRenderedBatch(newTree1, newTree2);
+            var diffForChildComponent2 = batch2.UpdatedComponents.Array[1];
+            Assert.Equal(2, childComponentInstance.NotificationsCount);
+            Assert.Collection(diffForChildComponent2.CurrentState,
+                node => AssertNode.Text(node, "Notifications: 2", 0));
+        }
+
         [Fact]
         public void CallsDisposeOnlyOnRemovedChildComponents()
         {
@@ -801,9 +854,12 @@ namespace Microsoft.AspNetCore.Blazor.Test
         }
 
         private RenderBatch GetRenderedBatch()
+            => GetRenderedBatch(oldTree, newTree);
+
+        private RenderBatch GetRenderedBatch(RenderTreeBuilder from, RenderTreeBuilder to)
         {
             var batchBuilder = new RenderBatchBuilder();
-            diff.ApplyNewRenderTreeVersion(batchBuilder, 0, oldTree.GetNodes(), newTree.GetNodes());
+            diff.ApplyNewRenderTreeVersion(batchBuilder, 0, from.GetNodes(), to.GetNodes());
             return batchBuilder.ToBatch();
         }
 
@@ -835,6 +891,23 @@ namespace Microsoft.AspNetCore.Blazor.Test
             }
         }
 
+        private class HandlePropertiesChangedComponent : IComponent, IHandlePropertiesChanged
+        {
+            public int NotificationsCount { get; private set; }
+
+            public int IntProperty { get; set; }
+
+            public void BuildRenderTree(RenderTreeBuilder builder)
+            {
+                builder.AddText(0, $"Notifications: {NotificationsCount}");
+            }
+
+            public void OnPropertiesChanged()
+            {
+                NotificationsCount++;
+            }
+        }
+
         private class DisposableComponent : IComponent, IDisposable
         {
             public int DisposalCount { get; private set; }

test/testapps/BasicTestApp/PropertiesChangedHandlerChild.cshtml

@@ -0,0 +1,12 @@
+<div class="supplied">You supplied: @SuppliedValue</div>
+<div class="computed">I computed: @ComputedValue</div>
+
+@functions {
+    public int SuppliedValue { get; set; }
+    private int ComputedValue { get; set; }
+
+    public override void OnPropertiesChanged()
+    {
+        ComputedValue = SuppliedValue * 2;
+    }
+}

test/testapps/BasicTestApp/PropertiesChangedHandlerParent.cshtml

@@ -0,0 +1,6 @@
+<c:PropertiesChangedHandlerChild SuppliedValue="@valueToSupply" />
+<button onclick=@{ valueToSupply++; }>Increment</button>
+
+@functions {
+    private int valueToSupply = 100;
+}