Factor out RawView, make View listen to engine generated view foc…

…us events (flutter#143259)

## Description

This factors out a separate `RawView` that doesn't add a `MediaQuery` or a `FocusScope`. This PR also adds a new method `WidgetsBindingObserver.didChangeViewFocus` which allows the observer to know when the `FlutterView` that has focus has changed.

It also makes the `View` widget a stateful widget that contains a `FocusScope` and ` FocusTraversalGroup` so that it can respond to changes in the focus of the view.

I've also added a new function to `FocusScopeNode` that will allow the scope node itself to be focused, without looking for descendants that could take the focus. This lets the focus be "parked" at the `FocusManager.instance.rootScope` so that nothing else appears to have focus.

## Tests
 - Added tests for the new functionality.

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

@@ -47,6 +47,7 @@ mixin ServicesBinding on BindingBase, SchedulerBinding {
     SystemChannels.accessibility.setMessageHandler((dynamic message) => _handleAccessibilityMessage(message as Object));
     SystemChannels.lifecycle.setMessageHandler(_handleLifecycleMessage);
     SystemChannels.platform.setMethodCallHandler(_handlePlatformMessage);
+    platformDispatcher.onViewFocusChange = handleViewFocusChanged;
     TextInput.ensureInitialized();
     readInitialLifecycleStateFromNativeWindow();
     initializationComplete();
@@ -355,6 +356,19 @@ mixin ServicesBinding on BindingBase, SchedulerBinding {
     return;
   }
 
+  /// Called whenever the [PlatformDispatcher] receives a notification that the
+  /// focus state on a view has changed.
+  ///
+  /// The [event] contains the view ID for the view that changed its focus
+  /// state.
+  ///
+  /// See also:
+  ///
+  /// * [PlatformDispatcher.onViewFocusChange], which calls this method.
+  @protected
+  @mustCallSuper
+  void handleViewFocusChanged(ui.ViewFocusEvent event) {}
+
   Future<dynamic> _handlePlatformMessage(MethodCall methodCall) async {
     final String method = methodCall.method;
     switch (method) {

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

@@ -4,7 +4,8 @@
 
 import 'dart:async';
 import 'dart:developer' as developer;
-import 'dart:ui' show AccessibilityFeatures, AppExitResponse, AppLifecycleState, FrameTiming, Locale, PlatformDispatcher, TimingsCallback;
+import 'dart:ui' show AccessibilityFeatures, AppExitResponse, AppLifecycleState,
+  FrameTiming, Locale, PlatformDispatcher, TimingsCallback, ViewFocusEvent;
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
@@ -321,6 +322,18 @@ abstract mixin class WidgetsBindingObserver {
   ///    application lifecycle changes.
   void didChangeAppLifecycleState(AppLifecycleState state) { }
 
+  /// Called whenever the [PlatformDispatcher] receives a notification that the
+  /// focus state on a view has changed.
+  ///
+  /// The [event] contains the view ID for the view that changed its focus
+  /// state.
+  ///
+  /// The view ID of the [FlutterView] in which a particular [BuildContext]
+  /// resides can be retrieved with `View.of(context).viewId`, so that it may be
+  /// compared with the view ID in the `event` to see if the event pertains to
+  /// the given context.
+  void didChangeViewFocus(ViewFocusEvent event) { }
+
   /// Called when a request is received from the system to exit the application.
   ///
   /// If any observer responds with [AppExitResponse.cancel], it will cancel the
@@ -951,6 +964,14 @@ mixin WidgetsBinding on BindingBase, ServicesBinding, SchedulerBinding, GestureB
     }
   }
 
+  @override
+  void handleViewFocusChanged(ViewFocusEvent event) {
+    super.handleViewFocusChanged(event);
+    for (final WidgetsBindingObserver observer in List<WidgetsBindingObserver>.of(_observers)) {
+      observer.didChangeViewFocus(event);
+    }
+  }
+
   @override
   void handleMemoryPressure() {
     super.handleMemoryPressure();

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

@@ -219,8 +219,8 @@ class FocusAttachment {
       _node._manager?._markDetached(_node);
       _node._parent?._removeChild(_node);
       _node._attachment = null;
-      assert(!_node.hasPrimaryFocus);
-      assert(_node._manager?._markedForFocus != _node);
+      assert(!_node.hasPrimaryFocus, 'Node ${_node.debugLabel ?? _node} still has primary focus while being detached.');
+      assert(_node._manager?._markedForFocus != _node, 'Node ${_node.debugLabel ?? _node} still marked for focus while being detached.');
     }
     assert(!isAttached);
   }
@@ -1296,8 +1296,10 @@ class FocusScopeNode extends FocusNode {
   ///
   /// Returns null if there is no currently focused child.
   FocusNode? get focusedChild {
-    assert(_focusedChildren.isEmpty || _focusedChildren.last.enclosingScope == this, 'Focused child does not have the same idea of its enclosing scope as the scope does.');
-    return _focusedChildren.isNotEmpty ? _focusedChildren.last : null;
+    assert(_focusedChildren.isEmpty || _focusedChildren.last.enclosingScope == this,
+      '$debugLabel: Focused child does not have the same idea of its enclosing scope '
+      '(${_focusedChildren.lastOrNull?.enclosingScope}) as the scope does.');
+    return _focusedChildren.lastOrNull;
   }
 
   // A stack of the children that have been set as the focusedChild, most recent
@@ -1377,11 +1379,20 @@ class FocusScopeNode extends FocusNode {
     _manager?._markNeedsUpdate();
   }
 
+  /// Requests that the scope itself receive focus, without trying to find
+  /// a descendant that should receive focus.
+  ///
+  /// This is used only if you want to park the focus on a scope itself.
+  void requestScopeFocus() {
+    _doRequestFocus(findFirstFocus: false);
+  }
+
   @override
   void _doRequestFocus({required bool findFirstFocus}) {
-
-    // It is possible that a previously focused child is no longer focusable.
-    while (this.focusedChild != null && !this.focusedChild!.canRequestFocus) {
+    // It is possible that a previously focused child is no longer focusable, so
+    // clean out the list if so.
+    while (_focusedChildren.isNotEmpty &&
+           (!_focusedChildren.last.canRequestFocus || _focusedChildren.last.enclosingScope == null)) {
       _focusedChildren.removeLast();
     }
 

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

@@ -757,6 +757,9 @@ class FocusScope extends Focus {
     super.onKeyEvent,
     super.onKey,
     super.debugLabel,
+    super.includeSemantics,
+    super.descendantsAreFocusable,
+    super.descendantsAreTraversable,
   })  : super(
           focusNode: node,
         );
@@ -770,6 +773,7 @@ class FocusScope extends Focus {
     required FocusScopeNode focusScopeNode,
     FocusNode? parentNode,
     bool autofocus,
+    bool includeSemantics,
     ValueChanged<bool>? onFocusChange,
   })  = _FocusScopeWithExternalFocusNode;
 
@@ -798,6 +802,7 @@ class _FocusScopeWithExternalFocusNode extends FocusScope {
     required FocusScopeNode focusScopeNode,
     super.parentNode,
     super.autofocus,
+    super.includeSemantics,
     super.onFocusChange,
   }) : super(
     node: focusScopeNode,
@@ -834,13 +839,17 @@ class _FocusScopeState extends _FocusState {
   @override
   Widget build(BuildContext context) {
     _focusAttachment!.reparent(parent: widget.parentNode);
-    return Semantics(
-      explicitChildNodes: true,
-      child: _FocusInheritedScope(
-        node: focusNode,
-        child: widget.child,
-      ),
+    Widget result = _FocusInheritedScope(
+      node: focusNode,
+      child: widget.child,
     );
+    if (widget.includeSemantics) {
+      result = Semantics(
+        explicitChildNodes: true,
+        child: result,
+      );
+    }
+    return result;
   }
 }