action_sheet tests: Cover "Share" button

test/test_share_plus.dart

@@ -0,0 +1,21 @@
+import 'package:flutter/services.dart';
+
+class MockSharePlus {
+  MockSharePlus();
+
+  /// The last string that `shareWithResult` was called with.
+  String? sharedString;
+
+  Future<Object?> handleMethodCall(MethodCall methodCall) async {
+    switch (methodCall.method) {
+      case 'shareWithResult':
+        // The method channel doesn't preserve Map<String, dynamic> as
+        // `arguments`'s type; logging runtimeType gives _Map<Object?, Object?>.
+        final arguments = methodCall.arguments as Map;
+        sharedString = arguments['text'] as String;
+        return 'some-success-response';
+      default:
+        throw UnimplementedError();
+    }
+  }
+}

test/widgets/action_sheet_test.dart

@@ -12,13 +12,15 @@ import 'package:zulip/widgets/compose_box.dart';
 import 'package:zulip/widgets/content.dart';
 import 'package:zulip/widgets/message_list.dart';
 import 'package:zulip/widgets/store.dart';
+import 'package:share_plus_platform_interface/method_channel/method_channel_share.dart';
 import '../api/fake_api.dart';
 
 import '../example_data.dart' as eg;
 import '../flutter_checks.dart';
 import '../model/binding.dart';
 import '../model/test_store.dart';
 import '../test_clipboard.dart';
+import '../test_share_plus.dart';
 import 'compose_box_checks.dart';
 import 'dialog_checks.dart';
 
@@ -88,6 +90,63 @@ void main() {
     (store.connection as FakeApiConnection).prepare(httpStatus: 400, json: fakeResponseJson);
   }
 
+  group('ShareButton', () {
+    // Tests should call setupMockSharePlus.
+    setUp(() async {
+      TestZulipBinding.ensureInitialized();
+      TestWidgetsFlutterBinding.ensureInitialized();
+    });
+
+    MockSharePlus setupMockSharePlus() {
+      final mock = MockSharePlus();
+      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger.setMockMethodCallHandler(
+        MethodChannelShare.channel,
+        mock.handleMethodCall,
+      );
+      return mock;
+    }
+
+    tearDown(() async {
+      TestZulipBinding.instance.reset();
+    });
+
+    Future<void> tapShareButton(WidgetTester tester) async {
+      await tester.ensureVisible(find.byIcon(Icons.adaptive.share, skipOffstage: false));
+      await tester.tap(find.byIcon(Icons.adaptive.share));
+      await tester.pump(); // [MenuItemButton.onPressed] called in a post-frame callback: flutter/flutter@e4a39fa2e
+    }
+
+    testWidgets('success', (WidgetTester tester) async {
+      final mockSharePlus = setupMockSharePlus();
+      final message = eg.streamMessage();
+      await setupToMessageActionSheet(tester, message: message, narrow: TopicNarrow.ofMessage(message));
+      final store = await testBinding.globalStore.perAccount(eg.selfAccount.id);
+
+      prepareRawContentResponseSuccess(store, message: message, rawContent: 'Hello world');
+      await tapShareButton(tester);
+      await tester.pump(Duration.zero);
+      check(mockSharePlus.sharedString).equals('Hello world');
+    });
+
+    testWidgets('request has an error', (WidgetTester tester) async {
+      final mockSharePlus = setupMockSharePlus();
+      final message = eg.streamMessage();
+      await setupToMessageActionSheet(tester, message: message, narrow: TopicNarrow.ofMessage(message));
+      final store = await testBinding.globalStore.perAccount(eg.selfAccount.id);
+
+      prepareRawContentResponseError(store);
+      await tapShareButton(tester);
+      await tester.pump(Duration.zero); // error arrives; error dialog shows
+
+      await tester.tap(find.byWidget(checkErrorDialog(tester,
+        expectedTitle: 'Sharing failed',
+        expectedMessage: 'That message does not seem to exist.',
+      )));
+
+      check(mockSharePlus.sharedString).isNull();
+    });
+  });
+
   group('QuoteAndReplyButton', () {
     ComposeBoxController? findComposeBoxController(WidgetTester tester) {
       return tester.widget<ComposeBox>(find.byType(ComposeBox))