Calculator from Real (BRL) to Euro (EUR) using API

conversorProcessor.dart

class Conversor {
  double value;
  double price;

  Conversor(double value, double price) {
    this.value = value;
    this.price = price;
  }

  double convert() {
    return value / price;
  }

  @override
  String toString() {
    return '€ ${convert().toStringAsFixed(2)}';
  }
}

conversorWidget.dart

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

import 'webRequest.dart';
import 'conversorProcessor.dart';

class Homepage extends StatefulWidget {
  Homepage({Key key, title: 'Homepage'}) : super(key: key);

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

class _HomepageState extends State<Homepage> {
  GlobalKey<FormState> _formKey = GlobalKey<FormState>();
  TextEditingController _requiredValue = TextEditingController();
  TextEditingController _euroValue = TextEditingController();
  String _result;
  Future<Map> _futureData;

  void clearData() {
    _requiredValue.text = '';
    _euroValue.text = 'Loading data...';

    setState(() {
      var request = new WebRequest();
      _result = 'Insert value to convert';

      _futureData = request.collectData();
    });
  }

  @override
  void initState() {
    super.initState();
    clearData();
  }

  void convertCoin() {
    double requiredValue = double.parse(_requiredValue.text);
    double coinValue = double.parse(_euroValue.text);

    setState(
        () => _result = new Conversor(requiredValue, coinValue).toString());
  }

  FloatingActionButton calcButton() {
    return FloatingActionButton(
        onPressed: () {
          if (_formKey.currentState.validate()) {
            convertCoin();
          }
        },
        tooltip: 'Calcular',
        child: Icon(Icons.calculate_outlined));
  }

  IconButton clearButton() {
    return IconButton(
      icon: Icon(Icons.refresh),
      onPressed: () {
        clearData();
      },
    );
  }

  AppBar appBarBuilder() {
    return AppBar(
      title: Text('Home'),
      actions: <Widget>[
        clearButton(),
      ],
    );
  }

  TextFormField textField(
      {TextEditingController controller,
      String errorMessage,
      String requisiteMessage}) {
    return TextFormField(
      keyboardType: TextInputType.numberWithOptions(decimal: true),
      inputFormatters: <TextInputFormatter>[
        FilteringTextInputFormatter.allow(RegExp('[0-9.,]'))
      ],
      decoration: InputDecoration(labelText: requisiteMessage),
      controller: controller,
      validator: (text) {
        return text.isEmpty ? errorMessage : null;
      },
    );
  }

  Padding resultText() {
    return Padding(
        padding: EdgeInsets.symmetric(vertical: 36),
        child: Text(
          _result,
          textAlign: TextAlign.center,
        ));
  }

  Form formBuilder() {
    return Form(
        key: _formKey,
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            textField(
                requisiteMessage: 'R\$ to convert',
                errorMessage: 'Insert the value to convert',
                controller: _requiredValue),
            textField(
                requisiteMessage: 'Value of 1€ in R\$',
                errorMessage: 'Insert euro value',
                controller: _euroValue),
            resultText(),
          ],
        ));
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: appBarBuilder(),
      body: FutureBuilder<Map>(
        future: _futureData,
        builder: (context, snapshot) {
          if (snapshot.hasData) {
            var priceData = snapshot.data["EURBRL"]["bid"];
            _euroValue.text = priceData.toString();

            return SingleChildScrollView(
                padding: EdgeInsets.all(25), child: formBuilder());
          } else if (snapshot.hasError) {
            return Text(snapshot.error);
          } else {
            return Center(
                child: LinearProgressIndicator(backgroundColor: Colors.purple));
          }
        },
      ),
      floatingActionButton: calcButton(),
    );
  }
}

main.dart

import 'package:flutter/material.dart';
import 'conversorWidget.dart';

const appName = "Flutter Class";
void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: appName,
      darkTheme: ThemeData.dark().copyWith(primaryColor: Colors.green),
      theme: ThemeData(
        primarySwatch: Colors.red,
      ),
      home: Homepage(),
      debugShowCheckedModeBanner: false,
    );
  }
}

webRequest.dart

import 'dart:convert';
import 'package:http/http.dart' as http;

class WebRequest {
  static const endpoint = 'https://economia.awesomeapi.com.br/json/last/EUR';

  Future<Map> collectData() async {
    http.Response response = await http.get(endpoint);
    return json.decode(response.body);
  }
}