# Transition of platform channel test interfaces to flutter_test package > The setMockMessageHandler method related APIs have moved from package:flutter to package:flutter_test :::important These breaking change docs are accurate, as of the release under which they are published. Over time, the workarounds described here might become inaccurate. We don't, in general, keep these breaking change docs up to date as of each release. The [breaking change index file](/release/breaking-changes) lists the docs created for each release. ::: ## Summary The following methods have been replaced by APIs in the `flutter_test` package: * `BinaryMessenger.checkMessageHandler` * `BinaryMessenger.setMockMessageHandler` *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` * `MethodChannel.checkMockMethodCallHandler` The `onPlatformMessage` callback is no longer used by the Flutter framework. ## Context 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: ```plaintext 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` (for example, using `testWidgets`), it is recommended to use the following patterns to replace calls to those methods (where `tester` is the `WidgetTester` instance): ```dart // old code ServicesBinding.defaultBinaryMessenger.setMockMessageHandler(...); ServicesBinding.defaultBinaryMessenger.checkMockMessageHandler(...); // new code tester.binding.defaultBinaryMessenger.setMockMessageHandler(...); tester.binding.defaultBinaryMessenger.checkMockMessageHandler(...); ``` ```dart // old code myChannel.setMockMessageHandler(...); myChannel.checkMockMessageHandler(...); // new code tester.binding.defaultBinaryMessenger.setMockDecodedMessageHandler(myChannel, ...); tester.binding.defaultBinaryMessenger.checkMockMessageHandler(myChannel, ...); ``` ```dart // old code myMethodChannel.setMockMethodCallHandler(...); myMethodChannel.checkMockMethodCallHandler(...); // 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. ## Timeline Landed in version: 2.3.0-17.0.pre.1
In stable release: 2.5 ## References API documentation: * [`TestDefaultBinaryMessenger`][] * [`TestDefaultBinaryMessengerBinding`][] Relevant PR: * [PR #76288: Migrate to ChannelBuffers.push][] [`TestDefaultBinaryMessenger`]: https://api.flutter.dev/flutter/flutter_test/TestDefaultBinaryMessenger-class.html [`TestDefaultBinaryMessengerBinding`]: https://api.flutter.dev/flutter/flutter_test/TestDefaultBinaryMessengerBinding-mixin.html [PR #76288: Migrate to ChannelBuffers.push]: https://github.com/flutter/flutter/pull/76288