Make FocusManager configurable in BuildOwner (#71872)

packages/flutter/lib/src/gestures/pointer_router.dart

@@ -71,6 +71,22 @@ class PointerRouter {
     _globalRoutes.remove(route);
   }
 
+  /// The number of global routes that have been registered.
+  ///
+  /// This is valid in debug builds only. In release builds, this will throw an
+  /// [UnsupportedError].
+  int get debugGlobalRouteCount {
+    int? count;
+    assert(() {
+      count = _globalRoutes.length;
+      return true;
+    }());
+    if (count != null) {
+      return count!;
+    }
+    throw UnsupportedError('debugGlobalRouteCount is not supported in release builds');
+  }
+
   void _dispatch(PointerEvent event, PointerRoute route, Matrix4? transform) {
     try {
       event = event.transformed(transform);

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

@@ -2554,7 +2554,8 @@ abstract class BuildContext {
 /// widget tree.
 class BuildOwner {
   /// Creates an object that manages widgets.
-  BuildOwner({ this.onBuildScheduled });
+  BuildOwner({ this.onBuildScheduled, FocusManager? focusManager }) :
+      focusManager = focusManager ?? FocusManager();
 
   /// Called on each build pass when the first buildable element is marked
   /// dirty.
@@ -2585,7 +2586,7 @@ class BuildOwner {
   /// the [FocusScopeNode] for a given [BuildContext].
   ///
   /// See [FocusManager] for more details.
-  FocusManager focusManager = FocusManager();
+  FocusManager focusManager;
 
   /// Adds an element to the dirty elements list so that it will be rebuilt
   /// when [WidgetsBinding.drawFrame] calls [buildScope].

packages/flutter/test/widgets/framework_test.dart

@@ -2,7 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
@@ -1483,6 +1485,27 @@ void main() {
       )
     );
   });
+
+  testWidgets('Can create BuildOwner that does not interfere with pointer router or raw key event handler', (WidgetTester tester) async {
+    final int pointerRouterCount = GestureBinding.instance!.pointerRouter.debugGlobalRouteCount;
+    final RawKeyEventHandler? rawKeyEventHandler = RawKeyboard.instance.keyEventHandler;
+    expect(rawKeyEventHandler, isNotNull);
+    BuildOwner(focusManager: _FakeFocusManager());
+    expect(GestureBinding.instance!.pointerRouter.debugGlobalRouteCount, pointerRouterCount);
+    expect(RawKeyboard.instance.keyEventHandler, same(rawKeyEventHandler));
+  });
+}
+
+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 {