Fix FocusManager constructor (#75894)

The FocusManager constructor was registering global event handlers
on the shared RawKeyboard instance and the global pointer router.
This posed a few problems: (1) there was no way to unregister these
handlers, and (2) instantiating a second FocusManager would overwrite
the existing focus manager's RawKeyboard handler. This was manifesting
in unexpected ways, such as the fact that constructing a second
BuildOwner (for a parallel tree, for instance) was obliterating the
event handler for the main BuildOwner's focus manager, thus messing
with focus.

This change separates those global event registrations into a
dedicated method, registerGlobalHandlers(), and overrides dispose()
to properly unregister those handlers.

examples/layers/widgets/spinning_mixed.dart

@@ -33,7 +33,6 @@ class Rectangle extends StatelessWidget {
 
 double? value;
 RenderObjectToWidgetElement<RenderBox>? element;
-BuildOwner owner = BuildOwner();
 void attachWidgetTreeToRenderTree(RenderProxyBox container) {
   element = RenderObjectToWidgetAdapter<RenderBox>(
     container: container,
@@ -74,7 +73,7 @@ void attachWidgetTreeToRenderTree(RenderProxyBox container) {
         ),
       ),
     ),
-  ).attachToRenderTree(owner, element);
+  ).attachToRenderTree(WidgetsBinding.instance!.buildOwner!, element);
 }
 
 Duration? timeBase;
@@ -87,7 +86,7 @@ void rotate(Duration timeStamp) {
   transformBox.setIdentity();
   transformBox.rotateZ(delta);
 
-  owner.buildScope(element!);
+  WidgetsBinding.instance!.buildOwner!.buildScope(element!);
 }
 
 void main() {

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

@@ -1439,12 +1439,39 @@ class FocusManager with DiagnosticableTreeMixin, ChangeNotifier {
   /// This constructor is rarely called directly. To access the [FocusManager],
   /// consider using the [FocusManager.instance] accessor instead (which gets it
   /// from the [WidgetsBinding] singleton).
+  ///
+  /// This newly constructed focus manager does not have the necessary event
+  /// handlers registered to allow it to manage focus. To register those event
+  /// handlers, callers must call [registerGlobalHandlers]. See the
+  /// documentation in that method for caveats to watch out for.
   FocusManager() {
     rootScope._manager = this;
+  }
+
+  /// Registers global input event handlers that are needed to manage focus.
+  ///
+  /// This sets the [RawKeyboard.keyEventHandler] for the shared instance of
+  /// [RawKeyboard] and adds a route to the global entry in the gesture routing
+  /// table. As such, only one [FocusManager] instance should register its
+  /// global handlers.
+  ///
+  /// When this focus manager is no longer needed, calling [dispose] on it will
+  /// unregister these handlers.
+  void registerGlobalHandlers() {
+    assert(RawKeyboard.instance.keyEventHandler == null);
     RawKeyboard.instance.keyEventHandler = _handleRawKeyEvent;
     GestureBinding.instance!.pointerRouter.addGlobalRoute(_handlePointerEvent);
   }
 
+  @override
+  void dispose() {
+    if (RawKeyboard.instance.keyEventHandler == _handleRawKeyEvent) {
+      RawKeyboard.instance.keyEventHandler = null;
+      GestureBinding.instance!.pointerRouter.removeGlobalRoute(_handlePointerEvent);
+    }
+    super.dispose();
+  }
+
   /// Provides convenient access to the current [FocusManager] singleton from
   /// the [WidgetsBinding] instance.
   static FocusManager get instance => WidgetsBinding.instance!.focusManager;

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

@@ -2327,7 +2327,7 @@ abstract class BuildContext {
 /// Size measureWidget(Widget widget) {
 ///   final PipelineOwner pipelineOwner = PipelineOwner();
 ///   final MeasurementView rootView = pipelineOwner.rootNode = MeasurementView();
-///   final BuildOwner buildOwner = BuildOwner(focusManager: FailingFocusManager());
+///   final BuildOwner buildOwner = BuildOwner(focusManager: FocusManager());
 ///   final RenderObjectToWidgetElement<RenderBox> element = RenderObjectToWidgetAdapter<RenderBox>(
 ///     container: rootView,
 ///     debugShortDescription: '[root]',
@@ -2344,17 +2344,6 @@ abstract class BuildContext {
 ///   }
 /// }
 ///
-/// // The default FocusManager, when created, modifies some static properties
-/// // that we don't want to modify, which is why we use a failing implementation
-/// // here.
-/// class FailingFocusManager implements FocusManager {
-///   @override
-///   dynamic noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
-///
-///   @override
-///   String toString({ DiagnosticLevel minLevel = DiagnosticLevel.info }) => 'FailingFocusManager';
-/// }
-///
 /// class MeasurementView extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
 ///   @override
 ///   void performLayout() {
@@ -2370,8 +2359,14 @@ abstract class BuildContext {
 /// {@end-tool}
 class BuildOwner {
   /// Creates an object that manages widgets.
+  ///
+  /// If the `focusManager` argument is not specified or is null, this will
+  /// construct a new [FocusManager] and register its global input handlers
+  /// via [FocusManager.registerGlobalHandlers], which will modify static
+  /// state. Callers wishing to avoid altering this state can explicitly pass
+  /// a focus manager here.
   BuildOwner({ this.onBuildScheduled, FocusManager? focusManager }) :
-      focusManager = focusManager ?? FocusManager();
+      focusManager = focusManager ?? (FocusManager()..registerGlobalHandlers());
 
   /// Called on each build pass when the first buildable element is marked
   /// dirty.
@@ -2402,6 +2397,12 @@ class BuildOwner {
   /// the [FocusScopeNode] for a given [BuildContext].
   ///
   /// See [FocusManager] for more details.
+  ///
+  /// This field will default to a [FocusManager] that has registered its
+  /// global input handlers via [FocusManager.registerGlobalHandlers]. Callers
+  /// wishing to avoid registering those handlers (and modifying the associated
+  /// static state) can explicitly pass a focus manager to the [new BuildOwner]
+  /// constructor.
   FocusManager focusManager;
 
   /// Adds an element to the dirty elements list so that it will be rebuilt

packages/flutter/test/widgets/framework_test.dart

@@ -1490,7 +1490,7 @@ void main() {
     final int pointerRouterCount = GestureBinding.instance!.pointerRouter.debugGlobalRouteCount;
     final RawKeyEventHandler? rawKeyEventHandler = RawKeyboard.instance.keyEventHandler;
     expect(rawKeyEventHandler, isNotNull);
-    BuildOwner(focusManager: _FakeFocusManager());
+    BuildOwner(focusManager: FocusManager());
     expect(GestureBinding.instance!.pointerRouter.debugGlobalRouteCount, pointerRouterCount);
     expect(RawKeyboard.instance.keyEventHandler, same(rawKeyEventHandler));
   });
@@ -1516,18 +1516,6 @@ void main() {
   });
 }
 
-class _FakeFocusManager implements FocusManager {
-  @override
-  dynamic noSuchMethod(Invocation invocation) {
-    return super.noSuchMethod(invocation);
-  }
-
-  @override
-  String toString({ DiagnosticLevel minLevel = DiagnosticLevel.info }) {
-    return '_FakeFocusManager';
-  }
-}
-
 class _WidgetWithNoVisitChildren extends StatelessWidget {
   const _WidgetWithNoVisitChildren(this.child, { Key? key }) :
     super(key: key);

packages/flutter/test/widgets/independent_widget_layout_test.dart

@@ -54,7 +54,7 @@ class OffscreenWidgetTree {
   }
 
   final RenderView renderView = OffscreenRenderView();
-  final BuildOwner buildOwner = BuildOwner();
+  final BuildOwner buildOwner = BuildOwner(focusManager: FocusManager());
   final PipelineOwner pipelineOwner = PipelineOwner();
   RenderObjectToWidgetElement<RenderBox>? root;
 

packages/flutter_test/lib/src/binding.dart

@@ -897,7 +897,8 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
     FlutterError.demangleStackTrace = _oldStackTraceDemangler;
     _pendingExceptionDetails = null;
     _parentZone = null;
-    buildOwner!.focusManager = FocusManager();
+    buildOwner!.focusManager.dispose();
+    buildOwner!.focusManager = FocusManager()..registerGlobalHandlers();
     // Disabling the warning because @visibleForTesting doesn't take the testing
     // framework itself into account, but we don't want it visible outside of
     // tests.