Enable TestDefaultBinaryMessenger to intercept all platform channels. (#100049)

packages/flutter_test/lib/src/test_default_binary_messenger.dart

@@ -8,6 +8,11 @@ import 'dart:ui' as ui;
 import 'package:fake_async/fake_async.dart';
 import 'package:flutter/services.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(
+    String channel, MessageHandler? handler, ByteData? message);
+
 /// A [BinaryMessenger] subclass that is used as the default binary messenger
 /// under testing environment.
 ///
@@ -116,11 +121,17 @@ class TestDefaultBinaryMessenger extends BinaryMessenger {
   // can implement the [checkMockMessageHandler] method.
   final Map<String, Object> _outboundHandlerIdentities = <String, Object>{};
 
+  /// Handler that intercepts and responds to outgoing messages, pretending
+  /// to be the platform, for all channels.
+  AllMessagesHandler? allMessagesHandler;
+
   @override
   Future<ByteData?>? send(String channel, ByteData? message) {
     final Future<ByteData?>? resultFuture;
     final MessageHandler? handler = _outboundHandlers[channel];
-    if (handler != null) {
+    if (allMessagesHandler != null) {
+      resultFuture = allMessagesHandler!(channel, handler, message);
+    } else if (handler != null) {
       resultFuture = handler(message);
     } else {
       resultFuture = delegate.send(channel, message);

packages/flutter_test/test/test_default_binary_messenger_test.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:typed_data';
 import 'dart:ui' as ui;
 
 import 'package:flutter/services.dart';
@@ -26,6 +27,19 @@ class TestDelegate extends BinaryMessenger {
   void setMessageHandler(String channel, MessageHandler? handler) => throw UnimplementedError();
 }
 
+class WorkingTestDelegate extends BinaryMessenger {
+  @override
+  Future<ByteData?>? send(String channel, ByteData? message) async {
+    return ByteData.sublistView(Uint8List.fromList(<int>[1, 2, 3]));
+  }
+
+  // Rest of the API isn't needed for this test.
+  @override
+  Future<void> handlePlatformMessage(String channel, ByteData? data, ui.PlatformMessageResponseCallback? callback) => throw UnimplementedError();
+  @override
+  void setMessageHandler(String channel, MessageHandler? handler) => throw UnimplementedError();
+}
+
 void main() {
   testWidgets('Caught exceptions are caught by the test framework', (WidgetTester tester) async {
     final BinaryMessenger delegate = TestDelegate();
@@ -39,4 +53,29 @@ void main() {
       expect(error, const RecognizableTestException());
     }
   });
+
+  testWidgets('Mock MessageHandler is set correctly',
+      (WidgetTester tester) async {
+    final TestDefaultBinaryMessenger binaryMessenger =
+        TestDefaultBinaryMessenger(WorkingTestDelegate());
+    binaryMessenger.setMockMessageHandler(
+        '',
+        (ByteData? message) async =>
+            ByteData.sublistView(Uint8List.fromList(<int>[2, 3, 4])));
+
+    final ByteData? result = await binaryMessenger.send('', null);
+    expect(result?.buffer.asUint8List(), Uint8List.fromList(<int>[2, 3, 4]));
+  });
+
+  testWidgets('Mock AllMessagesHandler is set correctly',
+      (WidgetTester tester) async {
+    final TestDefaultBinaryMessenger binaryMessenger =
+        TestDefaultBinaryMessenger(WorkingTestDelegate());
+    binaryMessenger.allMessagesHandler =
+        (String channel, MessageHandler? handler, ByteData? message) async =>
+            ByteData.sublistView(Uint8List.fromList(<int>[2, 3, 4]));
+
+    final ByteData? result = await binaryMessenger.send('', null);
+    expect(result?.buffer.asUint8List(), Uint8List.fromList(<int>[2, 3, 4]));
+  });
 }