packages/flutter/test/widgets/binding_attach_root_widget_test.dart

// Copyright 2014 The Flutter 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 'package:flutter/scheduler.dart';
import 'package:flutter/services.dart';
import 'package:flutter/widgets.dart';
import 'package:flutter_test/flutter_test.dart';

void main() {
  test('attachRootWidget will schedule a frame', () async {
    final WidgetsFlutterBindingWithTestBinaryMessenger binding = WidgetsFlutterBindingWithTestBinaryMessenger();
    expect(SchedulerBinding.instance.hasScheduledFrame, isFalse);
    // Framework starts with detached statue. Sends resumed signal to enable frame.
    final ByteData message = const StringCodec().encodeMessage('AppLifecycleState.resumed')!;
    await binding.defaultBinaryMessenger.handlePlatformMessage('flutter/lifecycle', message, (_) { });

    binding.attachRootWidget(const Placeholder());
    expect(SchedulerBinding.instance.hasScheduledFrame, isTrue);
  });
}

class WidgetsFlutterBindingWithTestBinaryMessenger extends WidgetsFlutterBinding with TestDefaultBinaryMessengerBinding { }