Inject current FlutterView into tree and make available via View.of(context) (#116924)

* enable View.of

* tests

* ++

* greg review

* rewording

* hide view from public

Author: goderbauer

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

@@ -19,6 +19,7 @@ import 'framework.dart';
 import 'platform_menu_bar.dart';
 import 'router.dart';
 import 'service_extensions.dart';
+import 'view.dart';
 import 'widget_inspector.dart';
 
 export 'dart:ui' show AppLifecycleState, Locale;
@@ -896,6 +897,22 @@ mixin WidgetsBinding on BindingBase, ServicesBinding, SchedulerBinding, GestureB
   @override
   bool get framesEnabled => super.framesEnabled && _readyToProduceFrames;
 
+  /// Used by [runApp] to wrap the provided `rootWidget` in the default [View].
+  ///
+  /// The [View] determines into what [FlutterView] the app is rendered into.
+  /// For backwards-compatibility reasons, this method currently chooses
+  /// [window] (which is a [FlutterView]) as the rendering target. This will
+  /// change in a future version of Flutter.
+  ///
+  /// The `rootWidget` widget provided to this method must not already be
+  /// wrapped in a [View].
+  Widget wrapWithDefaultView(Widget rootWidget) {
+    return View(
+      view: window,
+      child: rootWidget,
+    );
+  }
+
   /// Schedules a [Timer] for attaching the root widget.
   ///
   /// This is called by [runApp] to configure the widget tree. Consider using
@@ -1014,8 +1031,9 @@ mixin WidgetsBinding on BindingBase, ServicesBinding, SchedulerBinding, GestureB
 ///  * [WidgetsBinding.handleBeginFrame], which pumps the widget pipeline to
 ///    ensure the widget, element, and render trees are all built.
 void runApp(Widget app) {
-  WidgetsFlutterBinding.ensureInitialized()
-    ..scheduleAttachRootWidget(app)
+  final WidgetsBinding binding = WidgetsFlutterBinding.ensureInitialized();
+  binding
+    ..scheduleAttachRootWidget(binding.wrapWithDefaultView(app))
     ..scheduleWarmUpFrame();
 }
 

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

@@ -0,0 +1,94 @@
+// 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' show FlutterView;
+
+import 'framework.dart';
+import 'lookup_boundary.dart';
+
+/// Injects a [FlutterView] into the tree and makes it available to descendants
+/// within the same [LookupBoundary] via [View.of] and [View.maybeOf].
+///
+/// In a future version of Flutter, the functionality of this widget will be
+/// extended to actually bootstrap the render tree that is going to be rendered
+/// into the provided [view]. This will enable rendering content into multiple
+/// [FlutterView]s from a single widget tree.
+///
+/// Each [FlutterView] can be associated with at most one [View] widget in the
+/// widget tree. Two or more [View] widgets configured with the same
+/// [FlutterView] must never exist within the same widget tree at the same time.
+/// Internally, this limitation is enforced by a [GlobalObjectKey] that derives
+/// its identity from the [view] provided to this widget.
+class View extends InheritedWidget {
+  /// Injects the provided [view] into the widget tree.
+  View({required this.view, required super.child}) : super(key: GlobalObjectKey(view));
+
+  /// The [FlutterView] to be injected into the tree.
+  final FlutterView view;
+
+  @override
+  bool updateShouldNotify(View oldWidget) => view != oldWidget.view;
+
+  /// Returns the [FlutterView] that the provided `context` will render into.
+  ///
+  /// Returns null if the `context` is not associated with a [FlutterView].
+  ///
+  /// The method creates a dependency on the `context`, which will be informed
+  /// when the identity of the [FlutterView] changes (i.e. the `context` is
+  /// moved to render into a different [FlutterView] then before). The context
+  /// will not be informed when the properties on the [FlutterView] itself
+  /// change their values. To access the property values of a [FlutterView] it
+  /// is best practise to use [MediaQuery.maybeOf] instead, which will ensure
+  /// that the `context` is informed when the view properties change.
+  ///
+  /// See also:
+  ///
+  ///  * [View.of], which throws instead of returning null if no [FlutterView]
+  ///    is found.
+  static FlutterView? maybeOf(BuildContext context) {
+    return LookupBoundary.dependOnInheritedWidgetOfExactType<View>(context)?.view;
+  }
+
+  /// Returns the [FlutterView] that the provided `context` will render into.
+  ///
+  /// Throws if the `context` is not associated with a [FlutterView].
+  ///
+  /// The method creates a dependency on the `context`, which will be informed
+  /// when the identity of the [FlutterView] changes (i.e. the `context` is
+  /// moved to render into a different [FlutterView] then before). The context
+  /// will not be informed when the properties on the [FlutterView] itself
+  /// change their values. To access the property values of a [FlutterView] it
+  /// is best practise to use [MediaQuery.of] instead, which will ensure that
+  /// the `context` is informed when the view properties change.
+  ///
+  /// See also:
+  ///
+  ///  * [View.maybeOf], which throws instead of returning null if no
+  ///    [FlutterView] is found.
+  static FlutterView of(BuildContext context) {
+    final FlutterView? result = maybeOf(context);
+    assert(() {
+      if (result == null) {
+        final bool hiddenByBoundary = LookupBoundary.debugIsHidingAncestorWidgetOfExactType<View>(context);
+        final List<DiagnosticsNode> information = <DiagnosticsNode>[
+          if (hiddenByBoundary) ...<DiagnosticsNode>[
+            ErrorSummary('View.of() was called with a context that does not have access to a View widget.'),
+            ErrorDescription('The context provided to View.of() does have a View widget ancestor, but it is hidden by a LookupBoundary.'),
+          ] else ...<DiagnosticsNode>[
+            ErrorSummary('View.of() was called with a context that does not contain a View widget.'),
+            ErrorDescription('No View widget ancestor could be found starting from the context that was passed to View.of().'),
+          ],
+          ErrorDescription(
+            'The context used was:\n'
+            '  $context',
+          ),
+          ErrorHint('This usually means that the provided context is not associated with a View.'),
+        ];
+        throw FlutterError.fromParts(information);
+      }
+      return true;
+    }());
+    return result!;
+  }
+}

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

@@ -0,0 +1,8 @@
+// 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.
+
+/// Placeholder to be used in a future version of Flutter.
+abstract class Window {
+  const Window._();
+}

packages/flutter/lib/widgets.dart

@@ -148,8 +148,11 @@ export 'src/widgets/transitions.dart';
 export 'src/widgets/tween_animation_builder.dart';
 export 'src/widgets/unique_widget.dart';
 export 'src/widgets/value_listenable_builder.dart';
+// TODO(goderbauer): Enable once clean-up in google3 is done.
+// export 'src/widgets/view.dart';
 export 'src/widgets/viewport.dart';
 export 'src/widgets/visibility.dart';
 export 'src/widgets/widget_inspector.dart';
 export 'src/widgets/widget_span.dart';
 export 'src/widgets/will_pop_scope.dart';
+export 'src/widgets/window.dart';

packages/flutter/test/material/debug_test.dart

@@ -8,7 +8,7 @@ import 'package:flutter_test/flutter_test.dart';
 
 void main() {
   testWidgets('debugCheckHasMaterial control test', (WidgetTester tester) async {
-    await tester.pumpWidget(const Chip(label: Text('label')));
+    await tester.pumpWidget(const Center(child: Chip(label: Text('label'))));
     final dynamic exception = tester.takeException();
     expect(exception, isFlutterError);
     final FlutterError error = exception as FlutterError;
@@ -25,7 +25,7 @@ void main() {
     expect(error.diagnostics[3], isA<DiagnosticsProperty<Element>>());
     expect(error.diagnostics[4], isA<DiagnosticsBlock>());
     expect(
-      error.toStringDeep(),
+      error.toStringDeep(), startsWith(
       'FlutterError\n'
       '   No Material widget found.\n'
       '   Chip widgets require a Material widget ancestor within the\n'
@@ -42,12 +42,13 @@ void main() {
       '   The specific widget that could not find a Material ancestor was:\n'
       '     Chip\n'
       '   The ancestors of this widget were:\n'
-      '     [root]\n',
-    );
+      '     Center\n'
+      // End of ancestor chain omitted, not relevant for test.
+    ));
   });
 
   testWidgets('debugCheckHasMaterialLocalizations control test', (WidgetTester tester) async {
-    await tester.pumpWidget(const BackButton());
+    await tester.pumpWidget(const Center(child: BackButton()));
     final dynamic exception = tester.takeException();
     expect(exception, isFlutterError);
     final FlutterError error = exception as FlutterError;
@@ -64,7 +65,7 @@ void main() {
     expect(error.diagnostics[4], isA<DiagnosticsProperty<Element>>());
     expect(error.diagnostics[5], isA<DiagnosticsBlock>());
     expect(
-      error.toStringDeep(),
+      error.toStringDeep(), startsWith(
       'FlutterError\n'
       '   No MaterialLocalizations found.\n'
       '   BackButton widgets require MaterialLocalizations to be provided\n'
@@ -78,8 +79,9 @@ void main() {
       '   ancestor was:\n'
       '     BackButton\n'
       '   The ancestors of this widget were:\n'
-      '     [root]\n',
-    );
+      '     Center\n'
+      // End of ancestor chain omitted, not relevant for test.
+    ));
   });
 
   testWidgets('debugCheckHasScaffold control test', (WidgetTester tester) async {
@@ -233,6 +235,7 @@ void main() {
       '     HeroControllerScope\n'
       '     ScrollConfiguration\n'
       '     MaterialApp\n'
+      '     View-[GlobalObjectKey TestWindow#00000]\n'
       '     [root]\n'
       '   Typically, the Scaffold widget is introduced by the MaterialApp\n'
       '   or WidgetsApp widget at the top of your application widget tree.\n'
@@ -377,6 +380,7 @@ void main() {
       '     Scaffold-[LabeledGlobalKey<ScaffoldState>#00000]\n'
       '     MediaQuery\n'
       '     Directionality\n'
+      '     View-[GlobalObjectKey TestWindow#00000]\n'
       '     [root]\n'
       '   Typically, the ScaffoldMessenger widget is introduced by the\n'
       '   MaterialApp at the top of your application widget tree.\n'

packages/flutter/test/material/scaffold_test.dart

@@ -2458,6 +2458,7 @@ void main() {
       '     Scaffold\n'
       '     MediaQuery\n'
       '     Directionality\n'
+      '     View-[GlobalObjectKey TestWindow#e6136]\n'
       '     [root]\n'
       '   Typically, the ScaffoldMessenger widget is introduced by the\n'
       '   MaterialApp at the top of your application widget tree.\n',

packages/flutter/test/material/text_field_test.dart

@@ -7390,7 +7390,6 @@ void main() {
     final dynamic exception = tester.takeException();
     expect(exception, isFlutterError);
     expect(exception.toString(), startsWith('No Material widget found.'));
-    expect(exception.toString(), endsWith(':\n  $textField\nThe ancestors of this widget were:\n  [root]'));
   });
 
   testWidgets('TextField loses focus when disabled', (WidgetTester tester) async {