In tree diffs, omit trailing Continue entries and skip over unmodified subtrees

src/Microsoft.Blazor/RenderTree/RenderTreeDiff.cs

@@ -9,7 +9,7 @@ namespace Microsoft.Blazor.RenderTree
     {
         private const int MinBufferLength = 10;
         private RenderTreeDiffEntry[] _entries = new RenderTreeDiffEntry[10];
-        private int _entriesInUse = 0;
+        private int _entriesInUse;
 
         public ArraySegment<RenderTreeDiffEntry> ComputeDifference(
             ArraySegment<RenderTreeNode> oldTree,
@@ -17,6 +17,7 @@ namespace Microsoft.Blazor.RenderTree
         {
             _entriesInUse = 0;
             AppendDiffEntriesForRange(oldTree.Array, 0, oldTree.Count, newTree.Array, 0, newTree.Count);
+            TrimTrailingContinueNodes();
 
             // If the previous usage of the buffer showed that we have allocated
             // much more space than needed, free up the excess memory
@@ -292,6 +293,19 @@ namespace Microsoft.Blazor.RenderTree
 
         private void Append(RenderTreeDiffEntry entry)
         {
+            if (entry.Type == RenderTreeDiffEntryType.StepOut)
+            {
+                TrimTrailingContinueNodes();
+
+                // If the preceding node is now a StepIn, then we can coalesce the StepIn+StepOut
+                // down to a single Continue
+                if (_entriesInUse > 0 && _entries[_entriesInUse - 1].Type == RenderTreeDiffEntryType.StepIn)
+                {
+                    _entriesInUse--;
+                    entry = RenderTreeDiffEntry.Continue();
+                }
+            }
+
             if (_entriesInUse == _entries.Length)
             {
                 Array.Resize(ref _entries, _entries.Length * 2);
@@ -299,5 +313,13 @@ namespace Microsoft.Blazor.RenderTree
 
             _entries[_entriesInUse++] = entry;
         }
+
+        private void TrimTrailingContinueNodes()
+        {
+            while (_entriesInUse > 0 && _entries[_entriesInUse - 1].Type == RenderTreeDiffEntryType.Continue)
+            {
+                _entriesInUse--;
+            }
+        }
     }
 }

test/Microsoft.Blazor.Test/RenderTreeDiffTest.cs

@@ -28,8 +28,7 @@ namespace Microsoft.Blazor.Test
             var result = diff.ComputeDifference(oldTree.GetNodes(), newTree.GetNodes());
 
             // Assert
-            Assert.Collection(result,
+            Assert.Empty(result);
-                entry => Assert.Equal(RenderTreeDiffEntryType.Continue, entry.Type));
         }
 
         public static IEnumerable<object[]> RecognizesEquivalentNodesAsSameCases()
@@ -69,8 +68,7 @@ namespace Microsoft.Blazor.Test
                 {
                     Assert.Equal(RenderTreeDiffEntryType.PrependNode, entry.Type);
                     Assert.Equal(1, entry.NewTreeIndex);
-                },
+                });
-                entry => Assert.Equal(RenderTreeDiffEntryType.Continue, entry.Type));
         }
 
         [Fact]
@@ -92,8 +90,7 @@ namespace Microsoft.Blazor.Test
             // Assert
             Assert.Collection(result,
                 entry => Assert.Equal(RenderTreeDiffEntryType.Continue, entry.Type),
-                entry => Assert.Equal(RenderTreeDiffEntryType.RemoveNode, entry.Type),
+                entry => Assert.Equal(RenderTreeDiffEntryType.RemoveNode, entry.Type));
-                entry => Assert.Equal(RenderTreeDiffEntryType.Continue, entry.Type));
         }
 
         [Fact]
@@ -117,8 +114,7 @@ namespace Microsoft.Blazor.Test
             Assert.Collection(result,
                 entry => Assert.Equal(RenderTreeDiffEntryType.Continue, entry.Type),
                 entry => Assert.Equal(RenderTreeDiffEntryType.RemoveNode, entry.Type),
-                entry => Assert.Equal(RenderTreeDiffEntryType.RemoveNode, entry.Type),
+                entry => Assert.Equal(RenderTreeDiffEntryType.RemoveNode, entry.Type));
-                entry => Assert.Equal(RenderTreeDiffEntryType.Continue, entry.Type));
         }
 
         [Fact]
@@ -150,8 +146,7 @@ namespace Microsoft.Blazor.Test
                 {
                     Assert.Equal(RenderTreeDiffEntryType.PrependNode, entry.Type);
                     Assert.Equal(2, entry.NewTreeIndex);
-                },
+                });
-                entry => Assert.Equal(RenderTreeDiffEntryType.Continue, entry.Type));
         }
 
         [Fact]
@@ -241,8 +236,7 @@ namespace Microsoft.Blazor.Test
                 {
                     Assert.Equal(RenderTreeDiffEntryType.PrependNode, entry.Type);
                     Assert.Equal(2, entry.NewTreeIndex);
-                },
+                });
-                entry => Assert.Equal(RenderTreeDiffEntryType.Continue, entry.Type));
         }
 
         [Fact]
@@ -266,8 +260,7 @@ namespace Microsoft.Blazor.Test
             Assert.Collection(result,
                 entry => Assert.Equal(RenderTreeDiffEntryType.Continue, entry.Type),
                 entry => Assert.Equal(RenderTreeDiffEntryType.RemoveNode, entry.Type),
-                entry => Assert.Equal(RenderTreeDiffEntryType.RemoveNode, entry.Type),
+                entry => Assert.Equal(RenderTreeDiffEntryType.RemoveNode, entry.Type));
-                entry => Assert.Equal(RenderTreeDiffEntryType.Continue, entry.Type));
         }
 
         [Fact]
@@ -387,8 +380,7 @@ namespace Microsoft.Blazor.Test
                 {
                     Assert.Equal(RenderTreeDiffEntryType.SetAttribute, entry.Type);
                     Assert.Equal(2, entry.NewTreeIndex);
-                },
+                });
-                entry => Assert.Equal(RenderTreeDiffEntryType.Continue, entry.Type));
         }
 
         [Fact]
@@ -415,8 +407,7 @@ namespace Microsoft.Blazor.Test
                 {
                     Assert.Equal(RenderTreeDiffEntryType.RemoveAttribute, entry.Type);
                     Assert.Equal("will be removed", entry.RemovedAttributeName);
-                },
+                });
-                entry => Assert.Equal(RenderTreeDiffEntryType.Continue, entry.Type));
         }
 
         [Fact]
@@ -444,8 +435,7 @@ namespace Microsoft.Blazor.Test
                 {
                     Assert.Equal(RenderTreeDiffEntryType.SetAttribute, entry.Type);
                     Assert.Equal(2, entry.NewTreeIndex);
-                },
+                });
-                entry => Assert.Equal(RenderTreeDiffEntryType.Continue, entry.Type));
         }
 
         [Fact]
@@ -476,8 +466,7 @@ namespace Microsoft.Blazor.Test
                 {
                     Assert.Equal(RenderTreeDiffEntryType.SetAttribute, entry.Type);
                     Assert.Equal(2, entry.NewTreeIndex);
-                },
+                });
-                entry => Assert.Equal(RenderTreeDiffEntryType.Continue, entry.Type));
         }
 
         [Fact]
@@ -508,8 +497,7 @@ namespace Microsoft.Blazor.Test
                 {
                     Assert.Equal(RenderTreeDiffEntryType.RemoveAttribute, entry.Type);
                     Assert.Equal("oldname", entry.RemovedAttributeName);
-                },
+                });
-                entry => Assert.Equal(RenderTreeDiffEntryType.Continue, entry.Type));
         }
 
         [Fact]
@@ -553,6 +541,74 @@ namespace Microsoft.Blazor.Test
                 entry => Assert.Equal(RenderTreeDiffEntryType.StepOut, entry.Type));
         }
 
+        [Fact]
+        public void SkipsUnmodifiedSubtrees()
+        {
+            // Arrange
+            var oldTree = new RenderTreeBuilder(new FakeRenderer());
+            var newTree = new RenderTreeBuilder(new FakeRenderer());
+            var diff = new RenderTreeDiff();
+            oldTree.OpenElement(10, "root");
+            oldTree.AddText(11, "Text that will change");
+            oldTree.OpenElement(12, "Subtree that will not change");
+            oldTree.OpenElement(13, "Another");
+            oldTree.AddText(14, "Text that will not change");
+            oldTree.CloseElement();
+            oldTree.CloseElement();
+            oldTree.CloseElement();
+
+            newTree.OpenElement(10, "root");
+            newTree.AddText(11, "Text that has changed");
+            newTree.OpenElement(12, "Subtree that will not change");
+            newTree.OpenElement(13, "Another");
+            newTree.AddText(14, "Text that will not change");
+            newTree.CloseElement();
+            newTree.CloseElement();
+            newTree.CloseElement();
+
+            // Act
+            var result = diff.ComputeDifference(oldTree.GetNodes(), newTree.GetNodes());
+
+            // Assert
+            Assert.Collection(result,
+                entry => Assert.Equal(RenderTreeDiffEntryType.StepIn, entry.Type),
+                entry =>
+                {
+                    Assert.Equal(RenderTreeDiffEntryType.UpdateText, entry.Type);
+                    Assert.Equal(1, entry.NewTreeIndex);
+                },
+                entry => Assert.Equal(RenderTreeDiffEntryType.StepOut, entry.Type));
+        }
+
+        [Fact]
+        public void SkipsUnmodifiedTrailingSiblings()
+        {
+            // Arrange
+            var oldTree = new RenderTreeBuilder(new FakeRenderer());
+            var newTree = new RenderTreeBuilder(new FakeRenderer());
+            var diff = new RenderTreeDiff();
+            oldTree.AddText(10, "text1");
+            oldTree.AddText(11, "text2");
+            oldTree.AddText(12, "text3");
+            oldTree.AddText(13, "text4");
+            newTree.AddText(10, "text1");
+            newTree.AddText(11, "text2modified");
+            newTree.AddText(12, "text3");
+            newTree.AddText(13, "text4");
+
+            // Act
+            var result = diff.ComputeDifference(oldTree.GetNodes(), newTree.GetNodes());
+
+            // Assert
+            Assert.Collection(result,
+                entry => Assert.Equal(RenderTreeDiffEntryType.Continue, entry.Type),
+                entry =>
+                {
+                    Assert.Equal(RenderTreeDiffEntryType.UpdateText, entry.Type);
+                    Assert.Equal(1, entry.NewTreeIndex);
+                });
+        }
+
         private class FakeRenderer : Renderer
         {
             internal protected override void UpdateDisplay(int componentId, ArraySegment<RenderTreeNode> renderTree)