fix: add parameter to maintainState of SearchDelegate (flutter#152444)

Add parameter to `showSearch`, which is passed to `_SearchPageRoute` to maintain the state of the SearchDelegate.

- fix flutter#43582

_Just contributed this change to start the conversation on how the flutter#43582 can be addressed. Have not added tests yet._

Author: gazal-k

packages/flutter/lib/src/material/search.dart

@@ -48,6 +48,10 @@ import 'theme.dart';
 /// route created by this method is pushed to the nearest navigator to the
 /// given `context`. It can not be `null`.
 ///
+/// The `maintainState` argument is used to determine if the route should remain
+/// in memory when it is inactive (see [ModalRoute.maintainState] for more details].
+/// By default, `maintainState` is `false`.
+///
 /// The transition to the search page triggered by this method looks best if the
 /// screen triggering the transition contains an [AppBar] at the top and the
 /// transition is called from an [IconButton] that's part of [AppBar.actions].
@@ -68,11 +72,13 @@ Future<T?> showSearch<T>({
   required SearchDelegate<T> delegate,
   String? query = '',
   bool useRootNavigator = false,
+  bool maintainState = false,
 }) {
   delegate.query = query ?? delegate.query;
   delegate._currentBody = _SearchBody.suggestions;
   return Navigator.of(context, rootNavigator: useRootNavigator).push(_SearchPageRoute<T>(
     delegate: delegate,
+    maintainState: maintainState
   ));
 }
 
@@ -417,6 +423,7 @@ enum _SearchBody {
 class _SearchPageRoute<T> extends PageRoute<T> {
   _SearchPageRoute({
     required this.delegate,
+    required this.maintainState,
   }) {
     assert(
       delegate._route == null,
@@ -429,6 +436,9 @@ class _SearchPageRoute<T> extends PageRoute<T> {
 
   final SearchDelegate<T> delegate;
 
+  @override
+  final bool maintainState;
+
   @override
   Color? get barrierColor => null;
 
@@ -438,9 +448,6 @@ class _SearchPageRoute<T> extends PageRoute<T> {
   @override
   Duration get transitionDuration => const Duration(milliseconds: 300);
 
-  @override
-  bool get maintainState => false;
-
   @override
   Widget buildTransitions(
     BuildContext context,

packages/flutter/test/material/search_test.dart

@@ -1175,6 +1175,63 @@ void main() {
     await tester.pump();
     expect(textField.controller!.text.length, 15);
   }, skip: kIsWeb); // [intended] We do not use Flutter-rendered context menu on the Web.
+
+  testWidgets('showSearch with maintainState on the route', (WidgetTester tester) async {
+    final _MyNavigatorObserver navigationObserver = _MyNavigatorObserver();
+
+    final _TestEmptySearchDelegate delegate = _TestEmptySearchDelegate();
+    addTearDown(delegate.dispose);
+
+    await tester.pumpWidget(MaterialApp(
+      navigatorObservers: <NavigatorObserver>[navigationObserver],
+      home: Builder(builder: (BuildContext context) => Column(
+        mainAxisAlignment: MainAxisAlignment.center,
+        children: <Widget>[
+          TextButton(
+            onPressed: () async {
+              await showSearch(
+                context: context,
+                delegate: delegate,
+              );
+            },
+            child: const Text('showSearch'),
+          ),
+          TextButton(
+            onPressed: () async {
+              await showSearch(
+                context: context,
+                delegate: delegate,
+                maintainState: true,
+              );
+            },
+            child: const Text('showSearchWithMaintainState'),
+          ),
+        ],
+      )),
+    ));
+
+    expect(navigationObserver.pushCount, 0);
+    expect(navigationObserver.maintainState, false);
+
+    // showSearch normal and back.
+    await tester.tap(find.text('showSearch'));
+    await tester.pumpAndSettle();
+    final Finder backButtonFinder = find.byType(BackButton);
+    expect(backButtonFinder, findsWidgets);
+    await tester.tap(find.byTooltip('Close'));
+    await tester.pumpAndSettle();
+    expect(navigationObserver.pushCount, 1);
+    expect(navigationObserver.maintainState, false);
+
+    // showSearch with maintainState.
+    await tester.tap(find.text('showSearchWithMaintainState'));
+    await tester.pumpAndSettle();
+    expect(backButtonFinder, findsWidgets);
+    await tester.tap(find.byTooltip('Close'));
+    await tester.pumpAndSettle();
+    expect(navigationObserver.pushCount, 2);
+    expect(navigationObserver.maintainState, true);
+  });
 }
 
 class TestHomePage extends StatelessWidget {
@@ -1358,6 +1415,7 @@ class _TestEmptySearchDelegate extends SearchDelegate<String> {
 }
 
 class _MyNavigatorObserver extends NavigatorObserver {
+  bool maintainState = false;
   int pushCount = 0;
 
   @override
@@ -1366,6 +1424,9 @@ class _MyNavigatorObserver extends NavigatorObserver {
     if (<String>['nested', '/'].contains(route.settings.name)) {
       return;
     }
+    if (route is PageRoute) {
+      maintainState = route.maintainState;
+    }
     pushCount++;
   }
 }