Transition of platform channel test interfaces to flutter_test package


The following methods have been replaced by APIs in the flutter_test package: BinaryMessenger.checkMessageHandler, BinaryMessenger.setMockMessageHandler, BinaryMessenger.checkMockMessageHandler, BasicMessageChannel.setMockMessageHandler, MethodChannel.checkMethodCallHandler, MethodChannel.setMockMethodCallHandler, and MethodChannel.checkMockMethodCallHandler. The onPlatformMessage callback is no longer used by the Flutter framework.


As part of a refactoring of the low-level plugin communications architecture, we have moved from the previous onPlatformMessage/handlePlatformMessage logic to a per-channel buffering system implemented in the engine in the ChannelBuffers class. To maintain compatibility with existing code, the existing BinaryMessenger.setMessageHandler API has been refactored to use the new ChannelBuffers API.

One difference between the ChannelBuffers API and the previous API is that the new API is more consistent in its approach to asynchrony. As a side-effect, the APIs around message passing are now entirely asynchronous.

This posed a problem for the implementation of the legacy testing APIs which, for historical reasons, were previously in the flutter package. Since they relied on the underlying logic being partly synchronous, they required refactoring. To avoid adding even more test logic into the flutter package, a decision was made to move this logic to the flutter_test package.

Description of change

Specifically, the following APIs were affected:

  • BinaryMessenger.checkMessageHandler: Obsolete.
  • BinaryMessenger.setMockMessageHandler: Replaced by TestDefaultBinaryMessenger.setMockMessageHandler.
  • BinaryMessenger.checkMockMessageHandler: Replaced by TestDefaultBinaryMessenger.checkMockMessageHandler.
  • BasicMessageChannel.setMockMessageHandler: Replaced by TestDefaultBinaryMessenger.setMockDecodedMessageHandler.
  • MethodChannel.checkMethodCallHandler: Obsolete.
  • MethodChannel.setMockMethodCallHandler: Replaced by TestDefaultBinaryMessenger.setMockMethodCallHandler.
  • MethodChannel.checkMockMethodCallHandler: Replaced by TestDefaultBinaryMessenger.checkMockMessageHandler.

These replacements are only available to code using the new TestDefaultBinaryMessengerBinding (such as any code using testWidgets in a flutter_test test). There is no replacement for production code that was using these APIs, as they were not intended for production code use.

Tests using checkMessageHandler have no equivalent in the new API, since message handler registration is handled directly by the ChannelBuffers object, which does not expose the currently registered listener for a channel. (Tests verifying handler registration appear to be rare.)

Code that needs migrating may see errors such as the following:

  error - The method 'setMockMessageHandler' isn't defined for the type 'BinaryMessenger' at test/sensors_test.dart:64:8 - (undefined_method)

  error • The method 'setMockMethodCallHandler' isn't defined for the type 'MethodChannel' • test/widgets/editable_text_test.dart:5623:30 • undefined_method

[error] The method 'setMockMessageHandler' isn't defined for the type 'BasicMessageChannel' (test/material/feedback_test.dart:37:36)

In addition, the onPlatformMessage callback, which previously was hooked by the framework to receive messages from plugins, is no longer used (and will be removed in due course). As a result, calling this callback to inject messages into the framework no longer has an effect.

Migration guide

The flutter_test package provides some shims so that uses of the obsolete setMock... and checkMock... methods will continue to work. Tests that previously did not import package:flutter_test/flutter_test.dart can do so to enable these shims; this should be sufficient to migrate most code.

These shim APIs are deprecated, however. Instead, in code using WidgetTester (e.g. using testWidgets), it is recommended to use the following patterns to replace calls to those methods (where tester is the WidgetTester instance):

// old code
// new code
// old code
// new code
tester.binding.defaultBinaryMessenger.setMockDecodedMessageHandler(myChannel, ...);
tester.binding.defaultBinaryMessenger.checkMockMessageHandler(myChannel, ...);
// old code
// new code
tester.binding.defaultBinaryMessenger.setMockMethodCallHandler(myMethodChannel, ...);
tester.binding.defaultBinaryMessenger.checkMockMessageHandler(myMethodChannel, ...);

Tests that use package:test and test() can be changed to use package:flutter_test and testWidgets() to get access to a WidgetTester.

Code that does not have access to a WidgetTester can refer to TestDefaultBinaryMessengerBinding.instance!.defaultBinaryMessenger instead of tester.binding.defaultBinaryMessenger.

Tests that do not use the default test widgets binding (AutomatedTestWidgetsFlutterBinding, which is initialized by testWidgets) can mix the TestDefaultBinaryMessengerBinding mixin into their binding to get the same results.

Tests that manipulate onPlatformMessage will no longer function as designed. To send mock messages to the framework, consider using ChannelBuffers.push. There is no mechanism to intercept messages from the plugins and forward them to the framework in the new API. If your use case requires such a mechanism, please file a bug.


Landed in version: 2.3.0-17.0.pre.1
In stable release: not yet


API documentation:

Relevant PRs: