Revert "delete fast reassemble code (#102842)" (#102856)

This reverts commit ec8693e80b.

packages/flutter/lib/src/foundation/binding.dart

@@ -157,6 +157,13 @@ abstract class BindingBase {
   static Type? _debugInitializedType;
   static bool _debugServiceExtensionsRegistered = false;
 
+  /// Additional configuration used by the framework during hot reload.
+  ///
+  /// See also:
+  ///
+  ///  * [DebugReassembleConfig], which describes the configuration.
+  static DebugReassembleConfig? debugReassembleConfig;
+
   /// The main window to which this binding is bound.
   ///
   /// A number of additional bindings are defined as extensions of
@@ -871,3 +878,23 @@ abstract class BindingBase {
 Future<void> _exitApplication() async {
   exit(0);
 }
+
+/// Additional configuration used for hot reload reassemble optimizations.
+///
+/// Do not extend, implement, or mixin this class. This may only be instantiated
+/// in debug mode.
+class DebugReassembleConfig {
+  /// Create a new [DebugReassembleConfig].
+  ///
+  /// Throws a [FlutterError] if this is called in profile or release mode.
+  DebugReassembleConfig({
+    this.widgetName,
+  }) {
+    if (!kDebugMode) {
+      throw FlutterError('Cannot instantiate DebugReassembleConfig in profile or release mode.');
+    }
+  }
+
+  /// The name of the widget that was modified, or `null` if the change was elsewhere.
+  final String? widgetName;
+}

packages/flutter/lib/src/rendering/binding.dart

@@ -514,14 +514,16 @@ mixin RendererBinding on BindingBase, ServicesBinding, SchedulerBinding, Gesture
   @override
   Future<void> performReassemble() async {
     await super.performReassemble();
-    if (!kReleaseMode) {
-      Timeline.startSync('Preparing Hot Reload (layout)');
-    }
-    try {
-      renderView.reassemble();
-    } finally {
+    if (BindingBase.debugReassembleConfig?.widgetName == null) {
       if (!kReleaseMode) {
-        Timeline.finishSync();
+        Timeline.startSync('Preparing Hot Reload (layout)');
+      }
+      try {
+        renderView.reassemble();
+      } finally {
+        if (!kReleaseMode) {
+          Timeline.finishSync();
+        }
       }
     }
     scheduleWarmUpFrame();

packages/flutter/lib/src/widgets/binding.dart

@@ -412,7 +412,16 @@ mixin WidgetsBinding on BindingBase, ServicesBinding, SchedulerBinding, GestureB
       registerServiceExtension(
         name: 'fastReassemble',
         callback: (Map<String, Object> params) async {
-          await reassembleApplication();
+          // This mirrors the implementation of the 'reassemble' callback registration
+          // in lib/src/foundation/binding.dart, but with the extra binding config used
+          // to skip some reassemble work.
+          final String? className = params['className'] as String?;
+          BindingBase.debugReassembleConfig = DebugReassembleConfig(widgetName: className);
+          try {
+            await reassembleApplication();
+          } finally {
+            BindingBase.debugReassembleConfig = null;
+          }
           return <String, String>{'type': 'Success'};
         },
       );
@@ -469,7 +478,7 @@ mixin WidgetsBinding on BindingBase, ServicesBinding, SchedulerBinding, GestureB
 
   Future<void> _forceRebuild() {
     if (renderViewElement != null) {
-      buildOwner!.reassemble(renderViewElement!);
+      buildOwner!.reassemble(renderViewElement!, null);
       return endOfFrame;
     }
     return Future<void>.value();
@@ -936,7 +945,7 @@ mixin WidgetsBinding on BindingBase, ServicesBinding, SchedulerBinding, GestureB
     }());
 
     if (renderViewElement != null) {
-      buildOwner!.reassemble(renderViewElement!);
+      buildOwner!.reassemble(renderViewElement!, BindingBase.debugReassembleConfig);
     }
     return super.performReassemble();
   }

packages/flutter/lib/src/widgets/framework.dart

@@ -3028,13 +3028,14 @@ class BuildOwner {
   /// changed implementations.
   ///
   /// This is expensive and should not be called except during development.
-  void reassemble(Element root) {
+  void reassemble(Element root, DebugReassembleConfig? reassembleConfig) {
     if (!kReleaseMode) {
       Timeline.startSync('Preparing Hot Reload (widgets)');
     }
     try {
       assert(root._parent == null);
       assert(root.owner == this);
+      root._debugReassembleConfig = reassembleConfig;
       root.reassemble();
     } finally {
       if (!kReleaseMode) {
@@ -3142,6 +3143,7 @@ abstract class Element extends DiagnosticableTree implements BuildContext {
       _widget = widget;
 
   Element? _parent;
+  DebugReassembleConfig? _debugReassembleConfig;
   _NotificationNode? _notificationTree;
 
   /// Compare two widgets for equality.
@@ -3271,10 +3273,15 @@ abstract class Element extends DiagnosticableTree implements BuildContext {
   @mustCallSuper
   @protected
   void reassemble() {
-    markNeedsBuild();
+    if (_debugShouldReassemble(_debugReassembleConfig, _widget)) {
+      markNeedsBuild();
+      _debugReassembleConfig = null;
+    }
     visitChildren((Element child) {
+      child._debugReassembleConfig = _debugReassembleConfig;
       child.reassemble();
     });
+    _debugReassembleConfig = null;
   }
 
   bool _debugIsInScope(Element target) {
@@ -4918,7 +4925,9 @@ class StatefulElement extends ComponentElement {
 
   @override
   void reassemble() {
-    state.reassemble();
+    if (_debugShouldReassemble(_debugReassembleConfig, _widget)) {
+      state.reassemble();
+    }
     super.reassemble();
   }
 
@@ -6454,3 +6463,9 @@ class _NullWidget extends Widget {
   @override
   Element createElement() => throw UnimplementedError();
 }
+
+// Whether a [DebugReassembleConfig] indicates that an element holding [widget] can skip
+// a reassemble.
+bool _debugShouldReassemble(DebugReassembleConfig? config, Widget? widget) {
+  return config == null || config.widgetName == null || widget?.runtimeType.toString() == config.widgetName;
+}

packages/flutter/lib/src/widgets/widget_inspector.dart

@@ -911,7 +911,7 @@ mixin WidgetInspectorService {
   Future<void> forceRebuild() {
     final WidgetsBinding binding = WidgetsBinding.instance;
     if (binding.renderViewElement != null) {
-      binding.buildOwner!.reassemble(binding.renderViewElement!);
+      binding.buildOwner!.reassemble(binding.renderViewElement!, null);
       return binding.endOfFrame;
     }
     return Future<void>.value();

packages/flutter/test/widgets/fast_reassemble_test.dart

@@ -0,0 +1,123 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/foundation.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('reassemble with a className only marks subtrees from the first matching element as dirty', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const Foo(Bar(Fizz(SizedBox())))
+    );
+
+    expect(Foo.count, 0);
+    expect(Bar.count, 0);
+    expect(Fizz.count, 0);
+
+    DebugReassembleConfig config = DebugReassembleConfig(widgetName: 'Bar');
+    WidgetsBinding.instance.buildOwner!.reassemble(WidgetsBinding.instance.renderViewElement!, config);
+
+    expect(Foo.count, 0);
+    expect(Bar.count, 1);
+    expect(Fizz.count, 1);
+
+    config = DebugReassembleConfig(widgetName: 'Fizz');
+    WidgetsBinding.instance.buildOwner!.reassemble(WidgetsBinding.instance.renderViewElement!, config);
+
+    expect(Foo.count, 0);
+    expect(Bar.count, 1);
+    expect(Fizz.count, 2);
+
+    config = DebugReassembleConfig(widgetName: 'NoMatch');
+    WidgetsBinding.instance.buildOwner!.reassemble(WidgetsBinding.instance.renderViewElement!, config);
+
+    expect(Foo.count, 0);
+    expect(Bar.count, 1);
+    expect(Fizz.count, 2);
+
+    config = DebugReassembleConfig();
+    WidgetsBinding.instance.buildOwner!.reassemble(WidgetsBinding.instance.renderViewElement!, config);
+
+    expect(Foo.count, 1);
+    expect(Bar.count, 2);
+    expect(Fizz.count, 3);
+
+    WidgetsBinding.instance.buildOwner!.reassemble(WidgetsBinding.instance.renderViewElement!, null);
+
+    expect(Foo.count, 2);
+    expect(Bar.count, 3);
+    expect(Fizz.count, 4);
+  });
+}
+
+class Foo extends StatefulWidget {
+  const Foo(this.child, {super.key});
+
+  final Widget child;
+  static int count = 0;
+
+  @override
+  State<Foo> createState() => _FooState();
+}
+
+class _FooState extends State<Foo> {
+  @override
+  void reassemble() {
+    Foo.count += 1;
+    super.reassemble();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return widget.child;
+  }
+}
+
+
+class Bar extends StatefulWidget {
+  const Bar(this.child, {super.key});
+
+  final Widget child;
+  static int count = 0;
+
+  @override
+  State<Bar> createState() => _BarState();
+}
+
+class _BarState extends State<Bar> {
+  @override
+  void reassemble() {
+    Bar.count += 1;
+    super.reassemble();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return widget.child;
+  }
+}
+
+class Fizz extends StatefulWidget {
+  const Fizz(this.child, {super.key});
+
+  final Widget child;
+  static int count = 0;
+
+  @override
+  State<Fizz> createState() => _FizzState();
+}
+
+class _FizzState extends State<Fizz> {
+  @override
+  void reassemble() {
+    Fizz.count += 1;
+    super.reassemble();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return widget.child;
+  }
+}

packages/flutter/test/widgets/widget_inspector_test_utils.dart

@@ -59,7 +59,7 @@ class TestWidgetInspectorService extends Object with WidgetInspectorService {
     final WidgetsBinding binding = WidgetsBinding.instance;
 
     if (binding.renderViewElement != null) {
-      binding.buildOwner!.reassemble(binding.renderViewElement!);
+      binding.buildOwner!.reassemble(binding.renderViewElement!, null);
     }
   }