How can I mock/stub out a Flutter platform channel/plugin?

Question

I read the introduction to platform-specific plugins/channels on the Flutter website and I browsed some simple examples of a plugin, like url_launcher:

// Copyright 2017 The Chromium 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';

const _channel = const MethodChannel('plugins.flutter.io/url_launcher');

/// Parses the specified URL string and delegates handling of it to the
/// underlying platform.
///
/// The returned future completes with a [PlatformException] on invalid URLs and
/// schemes which cannot be handled, that is when [canLaunch] would complete
/// with false.
Future<Null> launch(String urlString) {
  return _channel.invokeMethod(
    'launch',
    urlString,
  );
}

In widgets tests or integration tests, how can I mock out or stub channels so I don't have to rely on the real device (running Android or iOS) say, actually launching a URL?

Answer 1

MethodChannel#setMockMethodCallHandler is deprecated and removed as of now.

Looks like this is the way to go now:

import 'package:flutter/services.dart'; 
import 'package:flutter_test/flutter_test.dart';

void mockUrlLauncher() {
  const channel = MethodChannel('plugins.flutter.io/url_launcher');

  handler(MethodCall methodCall) async {
    if (methodCall.method == 'yourMethod') {
      return 42;
    }
    return null;
  }

  TestWidgetsFlutterBinding.ensureInitialized();

  TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
      .setMockMethodCallHandler(channel, handler);
}

The details are on GitHub.

And here is a tested example for package_info plugin for future references:

import 'package:flutter/services.dart'; 
import 'package:flutter_test/flutter_test.dart';

void mockPackageInfo() {
  const channel = MethodChannel('plugins.flutter.io/package_info');

  handler(MethodCall methodCall) async {
    if (methodCall.method == 'getAll') {
      return <String, dynamic>{
        'appName': 'myapp',
        'packageName': 'com.mycompany.myapp',
        'version': '0.0.1',
        'buildNumber': '1'
      };
    }
    return null;
  }

  TestWidgetsFlutterBinding.ensureInitialized();

  TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
      .setMockMethodCallHandler(channel, handler);
}