How to test Flutter widgets on different screen sizes?

You can specify custom surface size by using WidgetTester

The following code will run a test with a screen size of 42x42

import 'package:flutter/widgets.dart';import 'package:flutter_test/flutter_test.dart';void main() {  testWidgets("foo", (tester) async {    tester.binding.window.physicalSizeTestValue = Size(42, 42);    // resets the screen to its orinal size after the test end    addTearDown(tester.binding.window.clearPhysicalSizeTestValue);    // TODO: do something  });}

Not sure why but solution of @rémi-rousselet didn't work for me. I've had to specify screen size using binding.window.physicalSizeTestValue and binding.window.devicePixelRatioTestValue so that output is fully deterministic

I've added a little bit more code for flutter beginners like me. Check this:

void main() {  final TestWidgetsFlutterBinding binding =    TestWidgetsFlutterBinding.ensureInitialized();  testWidgets("Basic layout test (mobile device)", (tester) async {    binding.window.physicalSizeTestValue = Size(400, 200);    binding.window.devicePixelRatioTestValue = 1.0;    await tester.pumpWidget(new MyApp());    expect(find.byType(MyHomePage), findsOneWidget);    // etc.  });}

There is a package called device_preview that can simulate your flutter app running on different devices.