mrverdant13/screen.dart

## screen.dart
import 'package:flutter/material.dart';

class SetStateScreen extends StatelessWidget {
  const SetStateScreen({
    Key? key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) => Scaffold(
        appBar: AppBar(
          title: const Text('`setState` test'),
        ),
        body: const Body(),
      );
}

class Body extends StatefulWidget {
  const Body({
    Key? key,
  }) : super(key: key);

  @override
  _BodyState createState() => _BodyState();
}

class _BodyState extends State<Body> {
  String? _myString;

  @override
  Widget build(BuildContext context) => Column(
        children: [
          Expanded(
            child: Center(
              child: SetterWidget(
                onCreate: (value) => setState(
                  () => _myString = value,
                ),
              ),
            ),
          ),
          Expanded(
            child: Center(
              child: Text('Actual value: <$_myString>'),
            ),
          ),
          Expanded(
            child: Center(
              child: _myString == null
                  ? const NullIndicatorWidget()
                  : const NonNullIndicatorWidget(),
            ),
          ),
        ],
      );
}

class SetterWidget extends StatefulWidget {
  const SetterWidget({
    Key? key,
    required this.onCreate,
  }) : super(key: key);

  final void Function(String? value) onCreate;

  @override
  _SetterWidgetState createState() => _SetterWidgetState();
}

class _SetterWidgetState extends State<SetterWidget> {
  @override
  void initState() {
    super.initState();
    Future.delayed(
      const Duration(seconds: 2),
    ).then(
      (_) => widget.onCreate('New value'),
    );
  }

  @override
  Widget build(BuildContext context) => Text('Setter widget');
}

class NullIndicatorWidget extends StatelessWidget {
  const NullIndicatorWidget({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) => const Text('The value is null');
}

class NonNullIndicatorWidget extends StatelessWidget {
  const NonNullIndicatorWidget({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) => const Text('The value is not null');
}

## screen_test.dart
import 'package:flutter/material.dart';
import 'package:flutter_playground/set_state_test/screen.dart';
import 'package:flutter_test/flutter_test.dart';

void main() {
  testWidgets(
    'Async `setState` test',
    (tester) async {
      await tester.pumpWidget(
        MaterialApp(
          home: const Body(),
        ),
      );

      final nullValueIndicatorFinder = find.byType(NullIndicatorWidget);
      expect(nullValueIndicatorFinder, findsOneWidget);

      await tester.pump(const Duration(seconds: 2));

      final nonNullValueIndicatorFinder = find.byType(NonNullIndicatorWidget);
      expect(nonNullValueIndicatorFinder, findsOneWidget);

      await tester.pump(const Duration(seconds: 1));
    },
  );
}
	import 'package:flutter/material.dart';

	class SetStateScreen extends StatelessWidget {
	const SetStateScreen({
	Key? key,
	}) : super(key: key);

	@override
	Widget build(BuildContext context) => Scaffold(
	appBar: AppBar(
	title: const Text('`setState` test'),
	),
	body: const Body(),
	);
	}

	class Body extends StatefulWidget {
	const Body({
	Key? key,
	}) : super(key: key);

	@override
	_BodyState createState() => _BodyState();
	}

	class _BodyState extends State<Body> {
	String? _myString;

	@override
	Widget build(BuildContext context) => Column(
	children: [
	Expanded(
	child: Center(
	child: SetterWidget(
	onCreate: (value) => setState(
	() => _myString = value,
	),
	),
	),
	),
	Expanded(
	child: Center(
	child: Text('Actual value: <$_myString>'),
	),
	),
	Expanded(
	child: Center(
	child: _myString == null
	? const NullIndicatorWidget()
	: const NonNullIndicatorWidget(),
	),
	),
	],
	);
	}

	class SetterWidget extends StatefulWidget {
	const SetterWidget({
	Key? key,
	required this.onCreate,
	}) : super(key: key);

	final void Function(String? value) onCreate;

	@override
	_SetterWidgetState createState() => _SetterWidgetState();
	}

	class _SetterWidgetState extends State<SetterWidget> {
	@override
	void initState() {
	super.initState();
	Future.delayed(
	const Duration(seconds: 2),
	).then(
	(_) => widget.onCreate('New value'),
	);
	}

	@override
	Widget build(BuildContext context) => Text('Setter widget');
	}

	class NullIndicatorWidget extends StatelessWidget {
	const NullIndicatorWidget({Key? key}) : super(key: key);

	@override
	Widget build(BuildContext context) => const Text('The value is null');
	}

	class NonNullIndicatorWidget extends StatelessWidget {
	const NonNullIndicatorWidget({Key? key}) : super(key: key);

	@override
	Widget build(BuildContext context) => const Text('The value is not null');
	}
	import 'package:flutter/material.dart';
	import 'package:flutter_playground/set_state_test/screen.dart';
	import 'package:flutter_test/flutter_test.dart';

	void main() {
	testWidgets(
	'Async `setState` test',
	(tester) async {
	await tester.pumpWidget(
	MaterialApp(
	home: const Body(),
	),
	);

	final nullValueIndicatorFinder = find.byType(NullIndicatorWidget);
	expect(nullValueIndicatorFinder, findsOneWidget);

	await tester.pump(const Duration(seconds: 2));

	final nonNullValueIndicatorFinder = find.byType(NonNullIndicatorWidget);
	expect(nonNullValueIndicatorFinder, findsOneWidget);

	await tester.pump(const Duration(seconds: 1));
	},
	);
	}