Skip rerendering child components if their params are definitely unchanged

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

@@ -1,6 +1,7 @@
 // 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.Blazor.RenderTree;
 
 namespace Microsoft.AspNetCore.Blazor.Components
@@ -39,5 +40,99 @@ namespace Microsoft.AspNetCore.Blazor.Components
         /// <returns>The enumerator.</returns>
         public ParameterEnumerator GetEnumerator()
             => new ParameterEnumerator(_frames, _ownerIndex);
+
+        // It's internal because there isn't a known use case for user code comparing
+        // ParameterCollection instances, and even if there was, it's unlikely it should
+        // use these equality rules which are designed for their effect on rendering.
+        internal bool DefinitelyEquals(ParameterCollection oldParameters)
+        {
+            // In general we can't detect mutations on arbitrary objects. We can't trust
+            // things like .Equals or .GetHashCode because they usually only tell us about
+            // shallow changes, not deep mutations. So we return false if both:
+            //  [1] All the parameters are known to be immutable (i.e., Type.IsPrimitive
+            //      or is in a known set of common immutable types)
+            //  [2] And all the parameter values are equal to their previous values
+            // Otherwise be conservative and return false.
+            // To make this check cheaper, since parameters are virtually always generated in
+            // a deterministic order, we don't bother to account for reordering, so if any
+            // of the names don't match sequentially we just return false too.
+            //
+            // The logic here may look kind of epic, and would certainly be simpler if we
+            // used ParameterEnumerator.GetEnumerator(), but it's perf-critical and this
+            // implementation requires a lot fewer instructions than a GetEnumerator-based one.
+
+            var oldIndex = oldParameters._ownerIndex;
+            var newIndex = _ownerIndex;
+            var oldEndIndexExcl = oldIndex + oldParameters._frames[oldIndex].ComponentSubtreeLength;
+            var newEndIndexExcl = newIndex + _frames[newIndex].ComponentSubtreeLength;
+            while (true)
+            {
+                // First, stop if we've reached the end of either subtree
+                oldIndex++;
+                newIndex++;
+                var oldFinished = oldIndex == oldEndIndexExcl;
+                var newFinished = newIndex == newEndIndexExcl;
+                if (oldFinished || newFinished)
+                {
+                    return oldFinished == newFinished; // Same only if we have same number of parameters
+                }
+                else
+                {
+                    // Since neither subtree has finished, it's safe to read the next frame from both
+                    ref var oldFrame = ref oldParameters._frames[oldIndex];
+                    ref var newFrame = ref _frames[newIndex];
+
+                    // Stop if we've reached the end of either subtree's sequence of attributes
+                    oldFinished = oldFrame.FrameType != RenderTreeFrameType.Attribute;
+                    newFinished = newFrame.FrameType != RenderTreeFrameType.Attribute;
+                    if (oldFinished || newFinished)
+                    {
+                        return oldFinished == newFinished; // Same only if we have same number of parameters
+                    }
+                    else
+                    {
+                        if (!string.Equals(oldFrame.AttributeName, newFrame.AttributeName, StringComparison.Ordinal))
+                        {
+                            return false; // Different names
+                        }
+
+                        var oldValue = oldFrame.AttributeValue;
+                        var newValue = newFrame.AttributeValue;
+                        var oldIsNotNull = oldValue != null;
+                        var newIsNotNull = newValue != null;
+                        if (oldIsNotNull != newIsNotNull)
+                        {
+                            return false; // One's null and the other isn't, so different
+                        }
+                        else if (oldIsNotNull) // i.e., both are not null (considering previous check)
+                        {
+                            var oldValueType = oldValue.GetType();
+                            var newValueType = newValue.GetType();
+                            if (oldValueType != newValueType            // Definitely different
+                                || !IsKnownImmutableType(oldValueType)  // Maybe different
+                                || !oldValue.Equals(newValue))          // Somebody says they are different
+                            {
+                                return false;
+                            }
+                        }
+                        else
+                        {
+                            // Both null, hence equal, so continue
+                        }
+                    }
+                }
+            }
+        }
+
+        // The contents of this list need to trade off false negatives against computation
+        // time. So we don't want a huge list of types to check (or would have to move to
+        // a hashtable lookup, which is differently expensive). It's better not to include
+        // uncommon types here even if they are known to be immutable.
+        private bool IsKnownImmutableType(Type type)
+            => type.IsPrimitive
+            || type == typeof(string)
+            || type == typeof(DateTime)
+            || type == typeof(RenderFragment)
+            || type == typeof(decimal);
     }
 }

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

@@ -153,13 +153,22 @@ namespace Microsoft.AspNetCore.Blazor.RenderTree
             // Preserve the actual componentInstance
             newComponentFrame = newComponentFrame.WithComponentInstance(componentId, componentInstance);
 
-            // Supply latest parameters. They might not have changed, but it's up to the
-            // recipient to decide what "changed" means. Currently we only supply the new
+            // As an important rendering optimization, we want to skip parameter update
+            // notifications if we know for sure they haven't changed/mutated. The
+            // "MayHaveChangedSince" logic is conservative, in that it returns true if
+            // any parameter is of a type we don't know is immutable. In this case
+            // we call SetParameters and it's up to the recipient to implement
+            // whatever change-detection logic they want. Currently we only supply the new
             // set of parameters and assume the recipient has enough info to do whatever
             // comparisons it wants with the old values. Later we could choose to pass the
-            // old parameter values if we wanted.
+            // old parameter values if we wanted. By default, components always rerender
+            // after any SetParameters call, which is safe but now always optimal for perf.
+            var oldParameters = new ParameterCollection(oldTree, oldComponentIndex);
             var newParameters = new ParameterCollection(newTree, newComponentIndex);
-            componentInstance.SetParameters(newParameters);
+            if (!newParameters.DefinitelyEquals(oldParameters))
+            {
+                componentInstance.SetParameters(newParameters);
+            }
         }
 
         private static int NextSiblingIndex(RenderTreeFrame frame, int frameIndex)

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

@@ -832,6 +832,44 @@ namespace Microsoft.AspNetCore.Blazor.Test
             Assert.Same(objectWillNotChange, newComponentInstance.ObjectProperty);
         }
 
+        [Fact]
+        public void SkipsUpdatingParametersOnChildComponentsIfAllAreDefinitelyImmutableAndUnchanged()
+        {
+            // We only know that types are immutable if either Type.IsPrimitive, or it's one of
+            // a known set of common immutable types.
+
+            // Arrange: Populate old and new with equivalent content
+            RenderFragment fragmentWillNotChange = builder => throw new NotImplementedException();
+            var dateTimeWillNotChange = DateTime.Now;
+            foreach (var tree in new[] { oldTree, newTree })
+            {
+                tree.OpenComponent<CaptureSetParametersComponent>(0);
+                tree.AddAttribute(1, "MyString", "Some fixed string");
+                tree.AddAttribute(1, "MyByte", (byte)123);
+                tree.AddAttribute(1, "MyInt", int.MaxValue);
+                tree.AddAttribute(1, "MyLong", long.MaxValue);
+                tree.AddAttribute(1, "MyBool", true);
+                tree.AddAttribute(1, "MyFloat", float.MaxValue);
+                tree.AddAttribute(1, "MyDouble", double.MaxValue);
+                tree.AddAttribute(1, "MyDecimal", decimal.MinusOne);
+                tree.AddAttribute(1, "MyDate", dateTimeWillNotChange);
+                tree.AddAttribute(1, "MyFragment", fragmentWillNotChange); // Treat fragments as primitive
+                tree.CloseComponent();
+            }
+
+            RenderTreeDiffBuilder.ComputeDiff(renderer, new RenderBatchBuilder(), 0, new RenderTreeBuilder(renderer).GetFrames(), oldTree.GetFrames());
+            var originalComponentInstance = (CaptureSetParametersComponent)oldTree.GetFrames().Array[0].Component;
+            Assert.Equal(1, originalComponentInstance.SetParametersCallCount);
+
+            // Act
+            var renderBatch = GetRenderedBatch();
+            var newComponentInstance = (CaptureSetParametersComponent)oldTree.GetFrames().Array[0].Component;
+
+            // Assert
+            Assert.Same(originalComponentInstance, newComponentInstance);
+            Assert.Equal(1, originalComponentInstance.SetParametersCallCount); // Received no further parameter change notification
+        }
+
         [Fact]
         public void QueuesRemovedChildComponentsForDisposal()
         {
@@ -915,6 +953,20 @@ namespace Microsoft.AspNetCore.Blazor.Test
             }
         }
 
+        private class CaptureSetParametersComponent : IComponent
+        {
+            public int SetParametersCallCount { get; private set; }
+
+            public void Init(RenderHandle renderHandle)
+            {
+            }
+
+            public void SetParameters(ParameterCollection parameters)
+            {
+                SetParametersCallCount++;
+            }
+        }
+
         private class DisposableComponent : IComponent, IDisposable
         {
             public int DisposalCount { get; private set; }

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

@@ -341,7 +341,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
                     Assert.Equal(0, edit.ReferenceFrameIndex);
                 });
             AssertFrame.Text(batch.ReferenceFrames[0], "Modified message");
-            Assert.Empty(batch.DiffsByComponentId[nestedComponentFrame.ComponentId].Single().Edits);
+            Assert.False(batch.DiffsByComponentId.ContainsKey(nestedComponentFrame.ComponentId));
         }
 
         [Fact]