ModalRoute resumes previous focus on didPopNext (#33152)

This PR changes ModalRoute so that, when the route on top of it is popped, it requests to focus on the last focus.

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

@@ -1086,6 +1086,17 @@ abstract class ModalRoute<T> extends TransitionRoute<T> with LocalHistoryRoute<T
     return await super.willPop();
   }
 
+  @override
+  void didPopNext(Route<dynamic> nextRoute) {
+    super.didPopNext(nextRoute);
+    // When the route on top of this route is popped, resume focus onto the
+    // widget that received the last focus.
+    final _ModalScopeState<T> modalScopeState = _scopeKey.currentState;
+    if (modalScopeState != null) {
+      modalScopeState.focusScopeNode.requestFocus();
+    }
+  }
+
   /// Enables this route to veto attempts by the user to dismiss it.
   ///
   /// This callback is typically added using a [WillPopScope] widget. That

packages/flutter/test/material/text_field_focus_test.dart

@@ -40,11 +40,6 @@ void main() {
     Navigator.of(tester.element(find.text('Dialog'))).pop();
     await tester.pump();
 
-    expect(tester.testTextInput.isVisible, isFalse);
-
-    await tester.tap(find.byType(TextField));
-    await tester.idle();
-
     expect(tester.testTextInput.isVisible, isTrue);
 
     await tester.pumpWidget(Container());

packages/flutter/test/widgets/routes_test.dart

@@ -8,6 +8,7 @@ import 'package:flutter/foundation.dart';
 import 'package:mockito/mockito.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
+import 'package:flutter/material.dart';
 
 final List<String> results = <String>[];
 
@@ -503,6 +504,52 @@ void main() {
       verifyNoMoreInteractions(pageRouteAware);
     });
   });
+
+  group('ModalRoute', () {
+    testWidgets('should resume focus when the next route is popped', (WidgetTester tester) async {
+      // TODO(tongmu): It's currently using PageRoute (from MaterialApp)
+      // instead of ModalRoute, which might leave a risk but avoids building
+      // everything from scratch by using the existing setup by MaterialApp and
+      // TextField as a focusable widget. We might want to write a better test.
+
+      final FocusNode node = FocusNode();
+      BuildContext pageContext;
+      final Widget widget = MaterialApp(
+        theme: ThemeData(),
+        home: Material(
+          child: Builder(
+            builder: (BuildContext context) {
+              pageContext = context;
+              return Column(
+                children: <Widget>[
+                  const TextField(),
+                  TextField(focusNode: node),
+                ],
+              );
+            }
+          ),
+        ),
+      );
+
+      await tester.pumpWidget(widget);
+      expect(node.hasFocus, false);
+
+      node.requestFocus();
+      await tester.pumpAndSettle();
+      expect(node.hasFocus, true);
+
+      showDialog<void>(
+        context: pageContext,
+        builder: (BuildContext _) => const Center(),
+      );
+      await tester.pumpAndSettle();
+      expect(node.hasFocus, false);
+
+      Navigator.of(pageContext).pop();
+      await tester.pumpAndSettle();
+      expect(node.hasFocus, true);
+    });
+  });
 }
 
 class MockPageRoute extends Mock implements PageRoute<dynamic> { }