import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  Future process;
  bool showFutureBuilder = false;

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        FlatButton(
          child: const Text('WITH FUTURE EXCEPTION'),
          onPressed: () {
            processButtonWithFuture(futureWithException);
          },
        ),
        if (showFutureBuilder)
          FutureBuilder(
            future: process,
            builder: (context, snapshot) {
              if (snapshot.connectionState == ConnectionState.waiting)
                return const Text('WAITING');
              if (snapshot.hasError) return const Text('ERROR HAPPENED');
              if (snapshot.hasData) return const Text('ALL IS FINE');
            },
          )
      ],
    );
  }

  void processButtonWithFuture(Function future) {
    setState(() {
      showFutureBuilder = true;
      process = future();
    });
  }

  Future<String> futureWithException() async {
    throw const FormatException();
  }
}

void main() {
  testWidgets('with Future finds exception', (WidgetTester tester) async {
    await tester.pumpWidget(MaterialApp(home: MyWidget()));

    var handler = FlutterError.onError;

    FlutterError.onError = (FlutterErrorDetails errorDetails) {
      if (errorDetails.exception is FormatException)
        print('YES');
      FlutterError.onError = handler;
    };

    await tester.tap(find.text('WITH FUTURE EXCEPTION'));

    await tester.pump();
    await tester.pump();

  });
}