Stand-alone widget tree with multiple render trees to enable multi-view rendering (#125003)

This change enables Flutter to generate multiple Scenes to be rendered into separate FlutterViews from a single widget tree. Each Scene is described by a separate render tree, which are all associated with the single widget tree.

This PR implements the framework-side mechanisms to describe the content to be rendered into multiple views. Separate engine-side changes are necessary to provide these views to the framework and to draw the framework-generated Scene into them.

## Summary of changes

The details of this change are described in [flutter.dev/go/multiple-views](https://flutter.dev/go/multiple-views). Below is a high-level summary organized by layers.

### Rendering layer changes

* The `RendererBinding` no longer owns a single `renderView`. In fact, it doesn't OWN any `RenderView`s at all anymore. Instead, it offers an API (`addRenderView`/`removeRenderView`) to add and remove `RenderView`s that then will be MANAGED by the binding. The `RenderView` itself is now owned by a higher-level abstraction (e.g. the `RawView` Element of the widgets layer, see below), who is also in charge of adding it to the binding. When added, the binding will interact with the `RenderView` to produce a frame (e.g. by calling `compositeFrame` on it) and to perform hit tests for incoming pointer events. Multiple `RenderView`s can be added to the binding (typically one per `FlutterView`) to produce multiple Scenes.
* Instead of owning a single `pipelineOwner`, the `RendererBinding` now owns the root of the `PipelineOwner` tree (exposed as `rootPipelineOwner` on the binding). Each `PipelineOwner` in that tree (except for the root) typically manages its own render tree typically rooted in one of the `RenderView`s mentioned in the previous bullet. During frame production, the binding will instruct each `PipelineOwner` of that tree to flush layout, paint, semantics etc. A higher-level abstraction (e.g. the widgets layer, see below) is in charge of adding `PipelineOwner`s to this tree.
* Backwards compatibility: The old `renderView` and `pipelineOwner` properties of the `RendererBinding` are retained, but marked as deprecated. Care has been taken to keep their original behavior for the deprecation period, i.e. if you just call `runApp`, the render tree bootstrapped by this call is rooted in the deprecated `RendererBinding.renderView` and managed by the deprecated `RendererBinding.pipelineOwner`.

### Widgets layer changes

* The `WidgetsBinding` no longer attaches the widget tree to an existing render tree. Instead, it bootstraps a stand-alone widget tree that is not backed by a render tree. For this, `RenderObjectToWidgetAdapter` has been replaced by `RootWidget`.
* Multiple render trees can be bootstrapped and attached to the widget tree with the help of the `View` widget, which internally is backed by a `RawView` widget. Configured with a `FlutterView` to render into, the `RawView` creates a new `PipelineOwner` and a new `RenderView` for the new render tree. It adds the new `RenderView` to the `RendererBinding` and its `PipelineOwner` to the pipeline owner tree.
* The `View` widget can only appear in certain well-defined locations in the widget tree since it bootstraps a new render tree and does not insert a `RenderObject` into an ancestor. However, almost all Elements expect that their children insert `RenderObject`s, otherwise they will not function properly. To produce a good error message when the `View` widget is used in an illegal location, the `debugMustInsertRenderObjectIntoSlot` method has been added to Element, where a child can ask whether a given slot must insert a RenderObject into its ancestor or not. In practice, the `View` widget can be used as a child of the `RootWidget`, inside the `view` slot of the `ViewAnchor` (see below) and inside a `ViewCollection` (see below). In those locations, the `View` widget may be wrapped in other non-RenderObjectWidgets (e.g. InheritedWidgets).
* The new `ViewAnchor` can be used to create a side-view inside a parent `View`. The `child` of the `ViewAnchor` widget renders into the parent `View` as usual, but the `view` slot can take on another `View` widget, which has access to all inherited widgets above the `ViewAnchor`. Metaphorically speaking, the view is anchored to the location of the `ViewAnchor` in the widget tree.
* The new `ViewCollection` widget allows for multiple sibling views as it takes a list of `View`s as children. It can be used in all the places that accept a `View` widget.

## Google3

As of July 5, 2023 this change passed a TAP global presubmit (TGP) in google3: tap/OCL:544707016:BASE:545809771:1688597935864:e43dd651

## Note to reviewers

This change is big (sorry). I suggest focusing the initial review on the changes inside of `packages/flutter` first. The majority of the changes describe above are implemented in (listed in suggested review order):

* `rendering/binding.dart`
* `widgets/binding.dart`
* `widgets/view.dart`
* `widgets/framework.dart`

All other changes included in the PR are basically the fallout of what's implemented in those files. Also note that a lot of the lines added in this PR are documentation and tests.

I am also very happy to walk reviewers through the code in person or via video call, if that is helpful.

I appreciate any feedback.

## Feedback to address before submitting ("TODO")

Author: goderbauer

dev/benchmarks/microbenchmarks/lib/stocks/layout_bench.dart

@@ -39,7 +39,7 @@ Future<void> main() async {
       size: const Size(355.0, 635.0),
       view: tester.view,
     );
-    final RenderView renderView = WidgetsBinding.instance.renderView;
+    final RenderView renderView = WidgetsBinding.instance.renderViews.single;
     binding.framePolicy = LiveTestWidgetsFlutterBindingFramePolicy.benchmark;
 
     watch.start();

dev/bots/test.dart

@@ -1361,7 +1361,7 @@ Future<void> _runWebTreeshakeTest() async {
   final String javaScript = mainDartJs.readAsStringSync();
 
   // Check that we're not looking at minified JS. Otherwise this test would result in false positive.
-  expect(javaScript.contains('RenderObjectToWidgetElement'), true);
+  expect(javaScript.contains('RootElement'), true);
 
   const String word = 'debugFillProperties';
   int count = 0;

dev/integration_tests/flutter_gallery/test/smoke_test.dart

@@ -79,8 +79,8 @@ Future<void> smokeDemo(WidgetTester tester, GalleryDemo demo) async {
   // Verify that the dumps are pretty.
   final String routeName = demo.routeName;
   verifyToStringOutput('debugDumpApp', routeName, WidgetsBinding.instance.rootElement!.toStringDeep());
-  verifyToStringOutput('debugDumpRenderTree', routeName, RendererBinding.instance.renderView.toStringDeep());
-  verifyToStringOutput('debugDumpLayerTree', routeName, RendererBinding.instance.renderView.debugLayer?.toStringDeep() ?? '');
+  verifyToStringOutput('debugDumpRenderTree', routeName, RendererBinding.instance.renderViews.single.toStringDeep());
+  verifyToStringOutput('debugDumpLayerTree', routeName, RendererBinding.instance.renderViews.single.debugLayer?.toStringDeep() ?? '');
   verifyToStringOutput('debugDumpFocusTree', routeName, WidgetsBinding.instance.focusManager.toStringDeep());
 
   // Scroll the demo around a bit more.

examples/layers/rendering/custom_coordinate_systems.dart

@@ -6,6 +6,7 @@
 // system. Most of the guts of this examples are in src/sector_layout.dart.
 
 import 'package:flutter/rendering.dart';
+import 'src/binding.dart';
 import 'src/sector_layout.dart';
 
 RenderBox buildSectorExample() {
@@ -21,5 +22,5 @@ RenderBox buildSectorExample() {
 }
 
 void main() {
-  RenderingFlutterBinding(root: buildSectorExample()).scheduleFrame();
+  ViewRenderingFlutterBinding(root: buildSectorExample()).scheduleFrame();
 }

examples/layers/rendering/flex_layout.dart

@@ -7,6 +7,7 @@
 
 import 'package:flutter/rendering.dart';
 
+import 'src/binding.dart';
 import 'src/solid_color_box.dart';
 
 void main() {
@@ -86,5 +87,5 @@ void main() {
     child: RenderPadding(child: table, padding: const EdgeInsets.symmetric(vertical: 50.0)),
   );
 
-  RenderingFlutterBinding(root: root).scheduleFrame();
+  ViewRenderingFlutterBinding(root: root).scheduleFrame();
 }

examples/layers/rendering/hello_world.dart

@@ -7,9 +7,11 @@
 
 import 'package:flutter/rendering.dart';
 
+import 'src/binding.dart';
+
 void main() {
-  // We use RenderingFlutterBinding to attach the render tree to the window.
-  RenderingFlutterBinding(
+  // We use ViewRenderingFlutterBinding to attach the render tree to the window.
+  ViewRenderingFlutterBinding(
     // The root of our render tree is a RenderPositionedBox, which centers its
     // child both vertically and horizontally.
     root: RenderPositionedBox(

examples/layers/rendering/spinning_square.dart

@@ -11,6 +11,8 @@ import 'package:flutter/animation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/scheduler.dart';
 
+import 'src/binding.dart';
+
 class NonStopVSync implements TickerProvider {
   const NonStopVSync();
   @override
@@ -42,7 +44,7 @@ void main() {
     child: spin,
   );
   // and attach it to the window.
-  RenderingFlutterBinding(root: root);
+  ViewRenderingFlutterBinding(root: root);
 
   // To make the square spin, we use an animation that repeats every 1800
   // milliseconds.

examples/layers/rendering/src/binding.dart

@@ -0,0 +1,69 @@
+// 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 'dart:ui';
+
+import 'package:flutter/rendering.dart';
+
+/// An extension of [RenderingFlutterBinding] that owns and manages a
+/// [renderView].
+///
+/// Unlike [RenderingFlutterBinding], this binding also creates and owns a
+/// [renderView] to simplify bootstrapping for apps that have a dedicated main
+/// view.
+class ViewRenderingFlutterBinding extends RenderingFlutterBinding {
+  /// Creates a binding for the rendering layer.
+  ///
+  /// The `root` render box is attached directly to the [renderView] and is
+  /// given constraints that require it to fill the window. The [renderView]
+  /// itself is attached to the [rootPipelineOwner].
+  ///
+  /// This binding does not automatically schedule any frames. Callers are
+  /// responsible for deciding when to first call [scheduleFrame].
+  ViewRenderingFlutterBinding({ RenderBox? root }) : _root = root;
+
+  @override
+  void initInstances() {
+    super.initInstances();
+    // TODO(goderbauer): Create window if embedder doesn't provide an implicit view.
+    assert(PlatformDispatcher.instance.implicitView != null);
+    _renderView = initRenderView(PlatformDispatcher.instance.implicitView!);
+    _renderView.child = _root;
+    _root = null;
+  }
+
+  RenderBox? _root;
+
+  @override
+  RenderView get renderView => _renderView;
+  late RenderView _renderView;
+
+  /// Creates a [RenderView] object to be the root of the
+  /// [RenderObject] rendering tree, and initializes it so that it
+  /// will be rendered when the next frame is requested.
+  ///
+  /// Called automatically when the binding is created.
+  RenderView initRenderView(FlutterView view) {
+    final RenderView renderView = RenderView(view: view);
+    rootPipelineOwner.rootNode = renderView;
+    addRenderView(renderView);
+    renderView.prepareInitialFrame();
+    return renderView;
+  }
+
+  @override
+  PipelineOwner createRootPipelineOwner() {
+    return PipelineOwner(
+      onSemanticsOwnerCreated: () {
+        renderView.scheduleInitialSemantics();
+      },
+      onSemanticsUpdate: (SemanticsUpdate update) {
+        renderView.updateSemantics(update);
+      },
+      onSemanticsOwnerDisposed: () {
+        renderView.clearSemantics();
+      },
+    );
+  }
+}

examples/layers/rendering/touch_input.dart

@@ -8,6 +8,8 @@
 import 'package:flutter/material.dart'; // Imported just for its color palette.
 import 'package:flutter/rendering.dart';
 
+import 'src/binding.dart';
+
 // Material design colors. :p
 List<Color> _kColors = <Color>[
   Colors.teal,
@@ -133,5 +135,5 @@ void main() {
     ..left = 20.0;
 
   // Finally, we attach the render tree we've built to the screen.
-  RenderingFlutterBinding(root: stack).scheduleFrame();
+  ViewRenderingFlutterBinding(root: stack).scheduleFrame();
 }

examples/layers/widgets/spinning_mixed.dart

@@ -52,10 +52,10 @@ void attachWidgetTreeToRenderTree(RenderProxyBox container) {
                   mainAxisAlignment: MainAxisAlignment.spaceAround,
                   children: <Widget>[
                     ElevatedButton(
-                      child: Row(
+                      child: const Row(
                         children: <Widget>[
-                          Image.network('https://flutter.dev/images/favicon.png'),
-                          const Text('PRESS ME'),
+                          FlutterLogo(),
+                          Text('PRESS ME'),
                         ],
                       ),
                       onPressed: () {
@@ -102,6 +102,16 @@ void main() {
   transformBox = RenderTransform(child: flexRoot, transform: Matrix4.identity(), alignment: Alignment.center);
   final RenderPadding root = RenderPadding(padding: const EdgeInsets.all(80.0), child: transformBox);
 
-  binding.renderView.child = root;
+  // TODO(goderbauer): Create a window if embedder doesn't provide an implicit view to draw into.
+  assert(binding.platformDispatcher.implicitView != null);
+  final RenderView view = RenderView(
+    view: binding.platformDispatcher.implicitView!,
+    child: root,
+  );
+  final PipelineOwner pipelineOwner = PipelineOwner()..rootNode = view;
+  binding.rootPipelineOwner.adoptChild(pipelineOwner);
+  binding.addRenderView(view);
+  view.prepareInitialFrame();
+
   binding.addPersistentFrameCallback(rotate);
 }