[flutter_test] Adds method to mock EventChannels (#123726)

[flutter_test] Adds method to mock EventChannels

packages/flutter/test/services/platform_channel_test.dart

@@ -265,69 +265,50 @@ void main() {
   });
 
   group('EventChannel', () {
-    const MessageCodec<dynamic> jsonMessage = JSONMessageCodec();
     const MethodCodec jsonMethod = JSONMethodCodec();
     const EventChannel channel = EventChannel('ch', jsonMethod);
-    void emitEvent(ByteData? event) {
-      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
-        'ch',
-        event,
-        (ByteData? reply) {},
-      );
-    }
+
     test('can receive event stream', () async {
       bool canceled = false;
-      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger.setMockMessageHandler(
-        'ch',
-        (ByteData? message) async {
-          final Map<dynamic, dynamic> methodCall = jsonMessage.decodeMessage(message) as Map<dynamic, dynamic>;
-          if (methodCall['method'] == 'listen') {
-            final String argument = methodCall['args'] as String;
-            emitEvent(jsonMethod.encodeSuccessEnvelope('${argument}1'));
-            emitEvent(jsonMethod.encodeSuccessEnvelope('${argument}2'));
-            emitEvent(null);
-            return jsonMethod.encodeSuccessEnvelope(null);
-          } else if (methodCall['method'] == 'cancel') {
+      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger.setMockStreamHandler(
+        channel,
+        MockStreamHandler.inline(
+          onListen: (Object? arguments, MockStreamHandlerEventSink events) {
+            events.success('${arguments}1');
+            events.success('${arguments}2');
+            events.endOfStream();
+          },
+          onCancel: (Object? arguments) {
             canceled = true;
-            return jsonMethod.encodeSuccessEnvelope(null);
-          } else {
-            fail('Expected listen or cancel');
-          }
-        },
+          },
+        ),
       );
-      final List<dynamic> events = await channel.receiveBroadcastStream('hello').toList();
+      final List<Object?> events = await channel.receiveBroadcastStream('hello').toList();
       expect(events, orderedEquals(<String>['hello1', 'hello2']));
       await Future<void>.delayed(Duration.zero);
       expect(canceled, isTrue);
     });
 
     test('can receive error event', () async {
-      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger.setMockMessageHandler(
-        'ch',
-        (ByteData? message) async {
-          final Map<dynamic, dynamic> methodCall = jsonMessage.decodeMessage(message) as Map<dynamic, dynamic>;
-          if (methodCall['method'] == 'listen') {
-            final String argument = methodCall['args'] as String;
-            emitEvent(jsonMethod.encodeErrorEnvelope(code: '404', message: 'Not Found.', details: argument));
-            return jsonMethod.encodeSuccessEnvelope(null);
-          } else if (methodCall['method'] == 'cancel') {
-            return jsonMethod.encodeSuccessEnvelope(null);
-          } else {
-            fail('Expected listen or cancel');
-          }
-        },
+      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger.setMockStreamHandler(
+        channel,
+        MockStreamHandler.inline(
+          onListen: (Object? arguments, MockStreamHandlerEventSink events) {
+            events.error(code: '404', message: 'Not Found.', details: arguments);
+          },
+        ),
       );
-      final List<dynamic> events = <dynamic>[];
-      final List<dynamic> errors = <dynamic>[];
+      final List<Object?> events = <Object?>[];
+      final List<Object?> errors = <Object?>[];
       channel.receiveBroadcastStream('hello').listen(events.add, onError: errors.add);
       await Future<void>.delayed(Duration.zero);
       expect(events, isEmpty);
       expect(errors, hasLength(1));
       expect(errors[0], isA<PlatformException>());
-      final PlatformException error = errors[0] as PlatformException;
-      expect(error.code, '404');
-      expect(error.message, 'Not Found.');
-      expect(error.details, 'hello');
+      final PlatformException? error = errors[0] as PlatformException?;
+      expect(error?.code, '404');
+      expect(error?.message, 'Not Found.');
+      expect(error?.details, 'hello');
     });
   });
 }

packages/flutter_test/lib/flutter_test.dart

@@ -69,6 +69,7 @@ export 'src/frame_timing_summarizer.dart';
 export 'src/goldens.dart';
 export 'src/image.dart';
 export 'src/matchers.dart';
+export 'src/mock_event_channel.dart';
 export 'src/nonconst.dart';
 export 'src/platform.dart';
 export 'src/restoration.dart';

packages/flutter_test/lib/src/mock_event_channel.dart

@@ -0,0 +1,79 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:async';
+
+import 'package:flutter/services.dart';
+
+/// A mock stream handler for an [EventChannel] that mimics the native
+/// StreamHandler API.
+///
+/// The [onListen] callback is provided a [MockStreamHandlerEventSink] with
+/// the following API:
+/// - [MockStreamHandlerEventSink.success] sends a success event.
+/// - [MockStreamHandlerEventSink.error] sends an error event.
+/// - [MockStreamHandlerEventSink.endOfStream] sends an end of stream event.
+abstract class MockStreamHandler {
+  /// Create a new [MockStreamHandler].
+  MockStreamHandler();
+
+  /// Create a new inline [MockStreamHandler] with the given [onListen] and
+  /// [onCancel] handlers.
+  factory MockStreamHandler.inline({
+    required MockStreamHandlerOnListenCallback onListen,
+    MockStreamHandlerOnCancelCallback? onCancel,
+  }) => _InlineMockStreamHandler(onListen: onListen, onCancel: onCancel);
+
+  /// Handler for the listen event.
+  void onListen(Object? arguments, MockStreamHandlerEventSink events);
+
+  /// Handler for the cancel event.
+  void onCancel(Object? arguments);
+}
+
+/// Typedef for the inline onListen callback.
+typedef MockStreamHandlerOnListenCallback = void Function(Object? arguments, MockStreamHandlerEventSink events);
+
+/// Typedef for the inline onCancel callback.
+typedef MockStreamHandlerOnCancelCallback = void Function(Object? arguments);
+
+class _InlineMockStreamHandler extends MockStreamHandler {
+  _InlineMockStreamHandler({
+    required MockStreamHandlerOnListenCallback onListen,
+    MockStreamHandlerOnCancelCallback? onCancel,
+  })  : _onListenInline = onListen,
+        _onCancelInline = onCancel;
+
+  final MockStreamHandlerOnListenCallback _onListenInline;
+  final MockStreamHandlerOnCancelCallback? _onCancelInline;
+
+  @override
+  void onListen(Object? arguments, MockStreamHandlerEventSink events) => _onListenInline(arguments, events);
+
+  @override
+  void onCancel(Object? arguments) => _onCancelInline?.call(arguments);
+}
+
+/// A mock event sink for a [MockStreamHandler] that mimics the native
+/// [EventSink](https://api.flutter.dev/javadoc/io/flutter/plugin/common/EventChannel.EventSink.html)
+/// API.
+class MockStreamHandlerEventSink {
+  /// Create a new [MockStreamHandlerEventSink] with the given [sink].
+  MockStreamHandlerEventSink(EventSink<Object?> sink) : _sink = sink;
+
+  final EventSink<Object?> _sink;
+
+  /// Send a success event.
+  void success(Object? event) => _sink.add(event);
+
+  /// Send an error event.
+  void error({
+    required String code,
+    String? message,
+    Object? details,
+  }) => _sink.addError(PlatformException(code: code, message: message, details: details));
+
+  /// Send an end of stream event.
+  void endOfStream() => _sink.close();
+}

packages/flutter_test/lib/src/test_default_binary_messenger.dart

@@ -8,6 +8,9 @@ import 'dart:ui' as ui;
 import 'package:fake_async/fake_async.dart';
 import 'package:flutter/services.dart';
 
+import 'mock_event_channel.dart';
+import 'widget_tester.dart';
+
 /// A function which takes the name of the method channel, it's handler,
 /// platform message and asynchronously returns an encoded response.
 typedef AllMessagesHandler = Future<ByteData?>? Function(
@@ -197,6 +200,9 @@ class TestDefaultBinaryMessenger extends BinaryMessenger {
   ///
   ///  * [setMockMethodCallHandler], which wraps this method but decodes
   ///    the messages using a [MethodCodec].
+  ///
+  ///  * [setMockStreamHandler], which wraps [setMockMethodCallHandler] to
+  ///    handle [EventChannel] messages.
   void setMockMessageHandler(String channel, MessageHandler? handler, [ Object? identity ]) {
     if (handler == null) {
       _outboundHandlers.remove(channel);
@@ -237,6 +243,9 @@ class TestDefaultBinaryMessenger extends BinaryMessenger {
   ///
   ///  * [setMockMethodCallHandler], which is similar but decodes
   ///    the messages using a [MethodCodec].
+  ///
+  ///  * [setMockStreamHandler], which wraps [setMockMethodCallHandler] to
+  ///    handle [EventChannel] messages.
   void setMockDecodedMessageHandler<T>(BasicMessageChannel<T> channel, Future<T> Function(T? message)? handler) {
     if (handler == null) {
       setMockMessageHandler(channel.name, null);
@@ -302,6 +311,81 @@ class TestDefaultBinaryMessenger extends BinaryMessenger {
     }, handler);
   }
 
+  /// Set a handler for intercepting stream events sent to the
+  /// platform on the given channel.
+  ///
+  /// Intercepted method calls are not forwarded to the platform.
+  ///
+  /// The given handler will replace the currently registered
+  /// handler for that channel, if any. To stop intercepting messages
+  /// at all, pass null as the handler.
+  ///
+  /// Events are decoded using the codec of the channel.
+  ///
+  /// The handler's stream messages are used as a response, after encoding
+  /// them using the channel's codec.
+  ///
+  /// To send an error, pass the error information to the handler's event sink.
+  ///
+  /// {@macro flutter.flutter_test.TestDefaultBinaryMessenger.handlePlatformMessage.asyncHandlers}
+  ///
+  /// Registered handlers are cleared after each test.
+  ///
+  /// See also:
+  ///
+  ///  * [setMockMethodCallHandler], which is the similar method for
+  ///    [MethodChannel].
+  ///
+  ///  * [setMockMessageHandler], which is similar but provides raw
+  ///    access to the underlying bytes.
+  ///
+  ///  * [setMockDecodedMessageHandler], which is similar but decodes
+  ///    the messages using a [MessageCodec].
+  void setMockStreamHandler(EventChannel channel, MockStreamHandler? handler) {
+    if (handler == null) {
+      setMockMessageHandler(channel.name, null);
+      return;
+    }
+
+    final StreamController<Object?> controller = StreamController<Object?>();
+    addTearDown(controller.close);
+
+    setMockMethodCallHandler(MethodChannel(channel.name, channel.codec), (MethodCall call) async {
+      switch (call.method) {
+        case 'listen':
+          return handler.onListen(call.arguments, MockStreamHandlerEventSink(controller.sink));
+        case 'cancel':
+          return handler.onCancel(call.arguments);
+        default:
+          throw UnimplementedError('Method ${call.method} not implemented');
+      }
+    });
+
+    final StreamSubscription<Object?> sub = controller.stream.listen(
+      (Object? e) => channel.binaryMessenger.handlePlatformMessage(
+        channel.name,
+        channel.codec.encodeSuccessEnvelope(e),
+        null,
+      ),
+    );
+    addTearDown(sub.cancel);
+    sub.onError((Object? e) {
+      if (e is! PlatformException) {
+        throw ArgumentError('Stream error must be a PlatformException');
+      }
+      channel.binaryMessenger.handlePlatformMessage(
+        channel.name,
+        channel.codec.encodeErrorEnvelope(
+          code: e.code,
+          message: e.message,
+          details: e.details,
+        ),
+        null,
+      );
+    });
+    sub.onDone(() => channel.binaryMessenger.handlePlatformMessage(channel.name, null, null));
+  }
+
   /// Returns true if the `handler` argument matches the `handler`
   /// previously passed to [setMockMessageHandler],
   /// [setMockDecodedMessageHandler], or [setMockMethodCallHandler].

packages/flutter_test/test/test_default_binary_messenger_test.dart

@@ -69,6 +69,32 @@ void main() {
     expect(result?.buffer.asUint8List(), Uint8List.fromList(<int>[2, 3, 4]));
   });
 
+  test('Mock StreamHandler is set correctly', () async {
+    const EventChannel channel = EventChannel('');
+    TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger.setMockStreamHandler(
+      channel,
+      MockStreamHandler.inline(onListen: (Object? arguments, MockStreamHandlerEventSink events) {
+        events.success(arguments);
+        events.error(code: 'code', message: 'message', details: 'details');
+        events.endOfStream();
+      })
+    );
+
+    expect(
+      channel.receiveBroadcastStream('argument'),
+      emitsInOrder(<Object?>[
+        'argument',
+        emitsError(
+          isA<PlatformException>()
+            .having((PlatformException e) => e.code, 'code', 'code')
+            .having((PlatformException e) => e.message, 'message', 'message')
+            .having((PlatformException e) => e.details, 'details', 'details'),
+        ),
+        emitsDone,
+      ]),
+    );
+  });
+
   testWidgets('Mock AllMessagesHandler is set correctly',
       (WidgetTester tester) async {
     final TestDefaultBinaryMessenger binaryMessenger =