Closing search with system back button works (#18174)

Fixes #18145.

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

@@ -26,8 +26,9 @@ import 'theme.dart';
 /// to the empty string. When `query` is set to null, `delegate.query` will
 /// be used as the initial query.
 ///
-/// The method returns the selected search result, which can be set in the
+/// This method returns the selected search result, which can be set in the
-/// [SearchDelegate.close] call.
+/// [SearchDelegate.close] call. If the search page is closed with the system
+/// back button, it returns null.
 ///
 /// A given [SearchDelegate] can only be associated with one active [showSearch]
 /// call. Call [SearchDelegate.close] before re-using the same delegate instance
@@ -52,14 +53,11 @@ Future<T> showSearch<T>({
 }) {
   assert(delegate != null);
   assert(context != null);
-  assert(delegate._result == null || delegate._result.isCompleted);
-  delegate._result = new Completer<T>();
   delegate.query = query ?? delegate.query;
   delegate._currentBody = _SearchBody.suggestions;
-  Navigator.of(context).push(new _SearchPageRoute<T>(
+  return Navigator.of(context).push(new _SearchPageRoute<T>(
     delegate: delegate,
   ));
-  return delegate._result.future;
 }
 
 /// Delegate for [showSearch] to define the content of the search page.
@@ -215,7 +213,6 @@ abstract class SearchDelegate<T> {
   void close(BuildContext context, T result) {
     _currentBody = null;
     _focusNode.unfocus();
-    _result.complete(result);
     Navigator.of(context)
       ..popUntil((Route<dynamic> route) => route == _route)
       ..pop(result);
@@ -242,8 +239,6 @@ abstract class SearchDelegate<T> {
     _currentBodyNotifier.value = value;
   }
 
-  Completer<T> _result;
-
   _SearchPageRoute<T> _route;
 
 }
@@ -263,7 +258,7 @@ enum _SearchBody {
 }
 
 
-class _SearchPageRoute<T> extends PageRoute<void> {
+class _SearchPageRoute<T> extends PageRoute<T> {
   _SearchPageRoute({
     @required this.delegate,
   }) : assert(delegate != null) {
@@ -323,10 +318,11 @@ class _SearchPageRoute<T> extends PageRoute<void> {
   }
 
   @override
-  void didComplete(void result) {
+  void didComplete(T result) {
     super.didComplete(result);
     assert(delegate._route == this);
     delegate._route = null;
+    delegate._currentBody = null;
   }
 }
 

packages/flutter/test/material/search_test.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
@@ -42,6 +43,51 @@ void main() {
     expect(selectedResults, <String>['Result']);
   });
 
+  testWidgets('Can close search with system back button to return null', (WidgetTester tester) async {
+    // regression test for https://github.com/flutter/flutter/issues/18145
+
+    final _TestSearchDelegate delegate = new _TestSearchDelegate();
+    final List<String> selectedResults = <String>[];
+
+    await tester.pumpWidget(new TestHomePage(
+      delegate: delegate,
+      results: selectedResults,
+    ));
+
+    // We are on the homepage
+    expect(find.text('HomeBody'), findsOneWidget);
+    expect(find.text('HomeTitle'), findsOneWidget);
+    expect(find.text('Suggestions'), findsNothing);
+
+    // Open search
+    await tester.tap(find.byTooltip('Search'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('HomeBody'), findsNothing);
+    expect(find.text('HomeTitle'), findsNothing);
+    expect(find.text('Suggestions'), findsOneWidget);
+
+    // Simulate system back button
+    final ByteData message = const JSONMethodCodec().encodeMethodCall(const MethodCall('popRoute'));
+    await BinaryMessages.handlePlatformMessage('flutter/navigation', message, (_) {});
+    await tester.pumpAndSettle();
+
+    expect(selectedResults, <void>[null]);
+
+    // We are on the homepage again
+    expect(find.text('HomeBody'), findsOneWidget);
+    expect(find.text('HomeTitle'), findsOneWidget);
+    expect(find.text('Suggestions'), findsNothing);
+
+    // Open search again
+    await tester.tap(find.byTooltip('Search'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('HomeBody'), findsNothing);
+    expect(find.text('HomeTitle'), findsNothing);
+    expect(find.text('Suggestions'), findsOneWidget);
+  });
+
   testWidgets('Requests suggestions', (WidgetTester tester) async {
     final _TestSearchDelegate delegate = new _TestSearchDelegate();
 
@@ -414,7 +460,7 @@ void main() {
     expect(find.text('HomeBody'), findsOneWidget);
     expect(find.text('Suggestions'), findsNothing);
     expect(find.text('Nested Suggestions'), findsNothing);
-    expect(nestedSearchResults, hasLength(0));
+    expect(nestedSearchResults, <String>[null]);
     expect(selectedResults, <String>['Result Foo']);
   });
 }