Functionality to check handlers set on platform channels (#57696)

packages/flutter/lib/src/services/binary_messenger.dart

@@ -41,20 +41,35 @@ abstract class BinaryMessenger {
   /// argument.
   ///
   /// The handler's return value, if non-null, is sent as a response, unencoded.
-  void setMessageHandler(String channel, Future<ByteData> handler(ByteData message));
+  void setMessageHandler(String channel, MessageHandler handler);
+
+  /// Returns true if the `handler` argument matches the `handler` previously
+  /// passed to [setMessageHandler].
+  ///
+  /// This method is useful for tests or test harnesses that want to assert the
+  /// handler for the vspecified channel has not been altered by a previous test.
+  bool checkMessageHandler(String channel, MessageHandler handler);
 
   /// Set a mock callback for intercepting messages from the [send] method on
   /// this class, on the given channel, without decoding them.
   ///
   /// The given callback will replace the currently registered mock callback for
   /// that channel, if any. To remove the mock handler, pass null as the
-  /// [handler] argument.
+  /// `handler` argument.
   ///
   /// The handler's return value, if non-null, is used as a response, unencoded.
   ///
   /// This is intended for testing. Messages intercepted in this manner are not
   /// sent to platform plugins.
-  void setMockMessageHandler(String channel, Future<ByteData> handler(ByteData message));
+  void setMockMessageHandler(String channel, MessageHandler handler);
+
+  /// Returns true if the `handler` argument matches the `handler` previously
+  /// passed to [setMockMessageHandler].
+  ///
+  /// This method is useful for tests or test harnesses that want to assert the
+  /// mock handler for the specified channel has not been altered by a previous
+  /// test.
+  bool checkMockMessageHandler(String channel, MessageHandler handler);
 }
 
 /// The default instance of [BinaryMessenger].

packages/flutter/lib/src/services/binding.dart

@@ -293,6 +293,9 @@ class _DefaultBinaryMessenger extends BinaryMessenger {
     });
   }
 
+  @override
+  bool checkMessageHandler(String channel, MessageHandler handler) => _handlers[channel] == handler;
+
   @override
   void setMockMessageHandler(String channel, MessageHandler handler) {
     if (handler == null)
@@ -300,4 +303,7 @@ class _DefaultBinaryMessenger extends BinaryMessenger {
     else
       _mockHandlers[channel] = handler;
   }
+
+  @override
+  bool checkMockMessageHandler(String channel, MessageHandler handler) => _mockHandlers[channel] == handler;
 }

packages/flutter/lib/src/services/platform_channel.dart

@@ -98,6 +98,9 @@ class BasicMessageChannel<T> {
   }
 }
 
+Expando<Object> _methodChannelHandlers = Expando<Object>();
+Expando<Object> _methodChannelMockHandlers = Expando<Object>();
+
 /// A named channel for communicating with platform plugins using asynchronous
 /// method calls.
 ///
@@ -372,12 +375,22 @@ class MethodChannel {
   /// similarly to what happens if no method call handler has been set.
   /// Any other exception results in an error envelope being sent.
   void setMethodCallHandler(Future<dynamic> handler(MethodCall call)) {
+    _methodChannelHandlers[this] = handler;
     binaryMessenger.setMessageHandler(
       name,
-      handler == null ? null : (ByteData message) => _handleAsMethodCall(message, handler),
+      handler == null
+        ? null
+        : (ByteData message) => _handleAsMethodCall(message, handler),
     );
   }
 
+  /// Returns true if the `handler` argument matches the `handler` previously
+  /// passed to [setMethodCallHandler].
+  ///
+  /// This method is useful for tests or test harnesses that want to assert the
+  /// handler for the specified channel has not been altered by a previous test.
+  bool checkMethodCallHandler(Future<dynamic> handler(MethodCall call)) => _methodChannelHandlers[this] == handler;
+
   /// Sets a mock callback for intercepting method invocations on this channel.
   ///
   /// The given callback will replace the currently registered mock callback for
@@ -397,12 +410,20 @@ class MethodChannel {
   /// [MethodCodec.encodeSuccessEnvelope], to act as if platform plugin had
   /// returned that value.
   void setMockMethodCallHandler(Future<dynamic> handler(MethodCall call)) {
+    _methodChannelMockHandlers[this] = handler;
     binaryMessenger.setMockMessageHandler(
       name,
       handler == null ? null : (ByteData message) => _handleAsMethodCall(message, handler),
     );
   }
 
+  /// Returns true if the `handler` argument matches the `handler` previously
+  /// passed to [setMockMethodCallHandler].
+  ///
+  /// This method is useful for tests or test harnesses that want to assert the
+  /// handler for the specified channel has not been altered by a previous test.
+  bool checkMockMethodCallHandler(Future<dynamic> handler(MethodCall call)) => _methodChannelMockHandlers[this] == handler;
+
   Future<ByteData> _handleAsMethodCall(ByteData message, Future<dynamic> handler(MethodCall call)) async {
     final MethodCall call = codec.decodeMethodCall(message);
     try {

packages/flutter/test/services/autofill_test.dart

@@ -211,9 +211,15 @@ class FakeTextChannel implements MethodChannel {
     incoming = handler;
   }
 
+  @override
+  bool checkMethodCallHandler(Future<void> Function(MethodCall call) handler) => throw UnimplementedError();
+
   @override
   void setMockMethodCallHandler(Future<void> Function(MethodCall call) handler)  => throw UnimplementedError();
 
+  @override
+  bool checkMockMethodCallHandler(Future<void> Function(MethodCall call) handler) => throw UnimplementedError();
+
   void validateOutgoingMethodCalls(List<MethodCall> calls) {
     expect(outgoingCalls.length, calls.length);
     bool hasError = false;

packages/flutter/test/services/default_binary_messenger_test.dart

@@ -34,4 +34,26 @@ void main() {
     });
     expect(count, equals(1));
   });
+
+  test('can check the handler', () {
+    Future<ByteData> handler(ByteData call) => Future<ByteData>.value(null);
+    final BinaryMessenger messenger = ServicesBinding.instance.defaultBinaryMessenger;
+
+    expect(messenger.checkMessageHandler('test_channel', null), true);
+    expect(messenger.checkMessageHandler('test_channel', handler), false);
+    messenger.setMessageHandler('test_channel', handler);
+    expect(messenger.checkMessageHandler('test_channel', handler), true);
+    messenger.setMessageHandler('test_channel', null);
+  });
+
+  test('can check the mock handler', () {
+    Future<ByteData> handler(ByteData call) => Future<ByteData>.value(null);
+    final BinaryMessenger messenger = ServicesBinding.instance.defaultBinaryMessenger;
+
+    expect(messenger.checkMockMessageHandler('test_channel', null), true);
+    expect(messenger.checkMockMessageHandler('test_channel', handler), false);
+    messenger.setMockMessageHandler('test_channel', handler);
+    expect(messenger.checkMockMessageHandler('test_channel', handler), true);
+    messenger.setMockMessageHandler('test_channel', null);
+  });
 }

packages/flutter/test/services/platform_channel_test.dart

@@ -58,6 +58,7 @@ void main() {
       final String result = await channel.invokeMethod('sayHello', 'hello');
       expect(result, equals('hello world'));
     });
+
     test('can invoke list method and get result', () async {
       ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler(
         'ch7',
@@ -89,7 +90,6 @@ void main() {
       expect(await channel.invokeListMethod<String>('sayHello', 'hello'), null);
     });
 
-
     test('can invoke map method and get result', () async {
       ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler(
         'ch7',
@@ -143,6 +143,7 @@ void main() {
         fail('PlatformException expected');
       }
     });
+
     test('can invoke unimplemented method', () async {
       ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler(
         'ch7',
@@ -158,6 +159,7 @@ void main() {
         fail('MissingPluginException expected');
       }
     });
+
     test('can invoke unimplemented method (optional)', () async {
       ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler(
         'ch8',
@@ -166,6 +168,7 @@ void main() {
       final String result = await optionalMethodChannel.invokeMethod<String>('sayHello', 'hello');
       expect(result, isNull);
     });
+
     test('can handle method call with no registered plugin', () async {
       channel.setMethodCallHandler(null);
       final ByteData call = jsonMethod.encodeMethodCall(const MethodCall('sayHello', 'hello'));
@@ -175,6 +178,7 @@ void main() {
       });
       expect(envelope, isNull);
     });
+
     test('can handle method call of unimplemented method', () async {
       channel.setMethodCallHandler((MethodCall call) async {
         throw MissingPluginException();
@@ -186,6 +190,7 @@ void main() {
       });
       expect(envelope, isNull);
     });
+
     test('can handle method call with successful result', () async {
       channel.setMethodCallHandler((MethodCall call) async => '${call.arguments}, world');
       final ByteData call = jsonMethod.encodeMethodCall(const MethodCall('sayHello', 'hello'));
@@ -195,6 +200,7 @@ void main() {
       });
       expect(jsonMethod.decodeEnvelope(envelope), equals('hello, world'));
     });
+
     test('can handle method call with expressive error result', () async {
       channel.setMethodCallHandler((MethodCall call) async {
         throw PlatformException(code: 'bad', message: 'sayHello failed', details: null);
@@ -214,6 +220,7 @@ void main() {
         fail('PlatformException expected');
       }
     });
+
     test('can handle method call with other error result', () async {
       channel.setMethodCallHandler((MethodCall call) async {
         throw ArgumentError('bad');
@@ -233,6 +240,26 @@ void main() {
         fail('PlatformException expected');
       }
     });
+
+    test('can check the handler', () {
+      Future<dynamic> handler(MethodCall call) => Future<dynamic>.value(null);
+
+      const MethodChannel channel = MethodChannel('test_handler');
+      expect(channel.checkMethodCallHandler(null), true);
+      expect(channel.checkMethodCallHandler(handler), false);
+      channel.setMethodCallHandler(handler);
+      expect(channel.checkMethodCallHandler(handler), true);
+    });
+
+    test('can check the mock handler', () {
+      Future<dynamic> handler(MethodCall call) => Future<dynamic>.value(null);
+
+      const MethodChannel channel = MethodChannel('test_handler');
+      expect(channel.checkMockMethodCallHandler(null), true);
+      expect(channel.checkMockMethodCallHandler(handler), false);
+      channel.setMockMethodCallHandler(handler);
+      expect(channel.checkMockMethodCallHandler(handler), true);
+    });
   });
   group('EventChannel', () {
     const MessageCodec<dynamic> jsonMessage = JSONMessageCodec();

packages/flutter/test/services/text_input_test.dart

@@ -266,9 +266,16 @@ class FakeTextChannel implements MethodChannel {
     incoming = handler;
   }
 
+  @override
+  bool checkMethodCallHandler(Future<void> Function(MethodCall call) handler) => throw UnimplementedError();
+
+
   @override
   void setMockMethodCallHandler(Future<void> Function(MethodCall call) handler)  => throw UnimplementedError();
 
+  @override
+  bool checkMockMethodCallHandler(Future<void> Function(MethodCall call) handler) => throw UnimplementedError();
+
   void validateOutgoingMethodCalls(List<MethodCall> calls) {
     expect(outgoingCalls.length, calls.length);
     bool hasError = false;

packages/flutter_test/lib/src/binding.dart

@@ -134,10 +134,20 @@ class TestDefaultBinaryMessenger extends BinaryMessenger {
     delegate.setMessageHandler(channel, handler);
   }
 
+  @override
+  bool checkMessageHandler(String channel, MessageHandler handler) {
+    return delegate.checkMessageHandler(channel, handler);
+  }
+
   @override
   void setMockMessageHandler(String channel, MessageHandler handler) {
     delegate.setMockMessageHandler(channel, handler);
   }
+
+  @override
+  bool checkMockMessageHandler(String channel, MessageHandler handler) {
+    return delegate.checkMockMessageHandler(channel, handler);
+  }
 }
 
 /// Base class for bindings used by widgets library tests.

packages/flutter_test/lib/src/test_text_input.dart

@@ -52,20 +52,14 @@ class TestTextInput {
     register();
   }
   /// Installs this object as a mock handler for [SystemChannels.textInput].
-  void register() {
-    SystemChannels.textInput.setMockMethodCallHandler(_handleTextInputCall);
-    _isRegistered = true;
-  }
+  void register() => SystemChannels.textInput.setMockMethodCallHandler(_handleTextInputCall);
 
   /// Removes this object as a mock handler for [SystemChannels.textInput].
   ///
   /// After calling this method, the channel will exchange messages with the
   /// Flutter engine. Use this with [FlutterDriver] tests that need to display
   /// on-screen keyboard provided by the operating system.
-  void unregister() {
-    SystemChannels.textInput.setMockMethodCallHandler(null);
-    _isRegistered = false;
-  }
+  void unregister() => SystemChannels.textInput.setMockMethodCallHandler(null);
 
   /// Log for method calls.
   ///
@@ -76,12 +70,13 @@ class TestTextInput {
   /// Whether this [TestTextInput] is registered with [SystemChannels.textInput].
   ///
   /// Use [register] and [unregister] methods to control this value.
-  // TODO(dnfield): This is unreliable. https://github.com/flutter/flutter/issues/47180
-  bool get isRegistered => _isRegistered;
-  bool _isRegistered = false;
+  bool get isRegistered => SystemChannels.textInput.checkMockMethodCallHandler(_handleTextInputCall);
 
   /// Whether there are any active clients listening to text input.
-  bool get hasAnyClients => _client > 0;
+  bool get hasAnyClients {
+    assert(isRegistered);
+    return _client > 0;
+  }
 
   int _client = 0;
 
@@ -122,11 +117,15 @@ class TestTextInput {
   }
 
   /// Whether the onscreen keyboard is visible to the user.
-  bool get isVisible => _isVisible;
+  bool get isVisible {
+    assert(isRegistered);
+    return _isVisible;
+  }
   bool _isVisible = false;
 
   /// Simulates the user changing the [TextEditingValue] to the given value.
   void updateEditingValue(TextEditingValue value) {
+    assert(isRegistered);
     // Not using the `expect` function because in the case of a FlutterDriver
     // test this code does not run in a package:test test zone.
     if (_client == 0)
@@ -149,6 +148,7 @@ class TestTextInput {
   /// - User pressed the home button and sent the application to background.
   /// - User closed the virtual keyboard.
   void closeConnection() {
+    assert(isRegistered);
     // Not using the `expect` function because in the case of a FlutterDriver
     // test this code does not run in a package:test test zone.
     if (_client == 0)
@@ -167,6 +167,7 @@ class TestTextInput {
 
   /// Simulates the user typing the given text.
   void enterText(String text) {
+    assert(isRegistered);
     updateEditingValue(TextEditingValue(
       text: text,
     ));
@@ -176,6 +177,7 @@ class TestTextInput {
   /// Does not check that the [TextInputAction] performed is an acceptable one
   /// based on the `inputAction` [setClientArgs].
   Future<void> receiveAction(TextInputAction action) async {
+    assert(isRegistered);
     return TestAsyncUtils.guard(() {
       // Not using the `expect` function because in the case of a FlutterDriver
       // test this code does not run in a package:test test zone.
@@ -215,6 +217,7 @@ class TestTextInput {
 
   /// Simulates the user hiding the onscreen keyboard.
   void hide() {
+    assert(isRegistered);
     _isVisible = false;
   }
 }

packages/flutter_web_plugins/lib/src/plugin_registry.dart

@@ -118,8 +118,7 @@ class _PlatformBinaryMessenger extends BinaryMessenger {
   }
 
   @override
-  void setMessageHandler(
-      String channel, Future<ByteData> Function(ByteData message) handler) {
+  void setMessageHandler(String channel, MessageHandler handler) {
     if (handler == null)
       _handlers.remove(channel);
     else
@@ -129,12 +128,21 @@ class _PlatformBinaryMessenger extends BinaryMessenger {
     });
   }
 
+  @override
+  bool checkMessageHandler(String channel, MessageHandler handler) => _handlers[channel] == handler;
+
   @override
   void setMockMessageHandler(
       String channel, Future<ByteData> Function(ByteData message) handler) {
     throw FlutterError(
         'Setting mock handlers is not supported on the platform side.');
   }
+
+  @override
+  bool checkMockMessageHandler(String channel, MessageHandler handler) {
+    throw FlutterError(
+        'Setting mock handlers is not supported on the platform side.');
+  }
 }
 
 /// The default [BinaryMessenger] for Flutter Web plugins.