みんな大好きFlutterの人気講座「The Complete 2021 Flutter Development Bootcamp with Dart」、前から知ってはいたのですが英語への苦手意識のため放置、どうやらかなり良い講座のようでこのたび通しでやってみることにしました。全部で30時間近くあるらしいので気長に進めていきます。学習の要点メモをまとめていきます。

Section1-9の記事はこちら：
https://qiita.com/igakeso/items/b6e602449cfb2d586f36

Section10-11の記事はこちら：
https://qiita.com/igakeso/items/00a5dcca6b40c0a3e644

・セクション12：theme、final/const、GestureDetector、enum、三項演算子、Slider、custom widget、Map、pow、ページ遷移時の変数の受け渡し、等
標準のWidgetのデザインをカスタムするところはデザイン上の必要性を少しも感じられず苦痛だったw
ページ遷移のやり方、ページ遷移時の変数の受け渡しのやり方はもっと早く学んでおいても良いかも、
後、main.dart、screen、constants等、フォルダ分けをして分業しやすくするようにするチーム開発の方法も紹介
参考リンク：
https://flutter.dev/docs/cookbook
https://api.flutter.dev/flutter/widgets/GestureDetector-class.html
https://api.flutter.dev/flutter/material/Slider-class.html

import 'package:flutter/material.dart';
import 'dart:math';
import 'package:font_awesome_flutter/font_awesome_flutter.dart';

void main() => runApp(BMICalculator());

enum Gender {
  male,
  female,
}

class BMICalculator extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(
        primaryColor: const Color(0xFF010E21),
        scaffoldBackgroundColor: const Color(0xFF010E21),
      ),
      home: InputPage(),
    );
  }
}

class InputPage extends StatefulWidget {
  @override
  _InputPageState createState() => _InputPageState();
}

class _InputPageState extends State<InputPage> {
  Gender? selectedGender;
  int height = 170;
  int weight = 60;
  int age = 20;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('BMI CALCULATOR'),
      ),
      body: Column(
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: [
          Expanded(
            child: Row(
              children: [
                Expanded(
                  child: ReuseableCard(
                    onPress: () {
                      selectedGender = Gender.male;
                      setState(() {});
                    },
                    color: selectedGender == Gender.male ? kActiveCardColor : kInactiveCardColor,
                    cardChild: IconContent(
                      icon: FontAwesomeIcons.mars,
                      label: 'MELE',
                    ),
                  ),
                ),
                Expanded(
                  child: ReuseableCard(
                    onPress: () {
                      selectedGender = Gender.female;
                      setState(() {});
                    },
                    color: selectedGender == Gender.female ? kActiveCardColor : kInactiveCardColor,
                    cardChild: IconContent(
                      icon: FontAwesomeIcons.venus,
                      label: 'FEMELE',
                    ),
                  ),
                ),
              ],
            ),
          ),
          Expanded(
            child: ReuseableCard(
              color: kActiveCardColor,
              cardChild: Column(
                mainAxisAlignment: MainAxisAlignment.center,
                children: [
                  const Text(
                    'HEIGHT',
                    style: kLabelTextStyle,
                  ),
                  Row(
                    mainAxisAlignment: MainAxisAlignment.center,
                    crossAxisAlignment: CrossAxisAlignment.baseline,
                    textBaseline: TextBaseline.alphabetic,
                    children: [
                      Text(
                        height.toString(),
                        style: kNumberTextStyle,
                      ),
                      const Text(
                        'cm',
                        style: kLabelTextStyle,
                      ),
                    ],
                  ),
                  SliderTheme(
                    data: SliderTheme.of(context).copyWith(
                      thumbColor: const Color(0xFFEB1555),
                      thumbShape: const RoundSliderThumbShape(enabledThumbRadius: 15.0),
                      overlayColor: const Color(0x29EB1555),
                      overlayShape: const RoundSliderOverlayShape(overlayRadius: 30.0),
                      activeTrackColor: Colors.white,
                      inactiveTrackColor: const Color(0xFF8D8E98),
                    ),
                    child: Slider(
                      value: height.toDouble(),
                      min: 120.0,
                      max: 220.0,
                      onChanged: (double newValue) {
                        setState(() {
                          height = newValue.round();
                        });
                      },
                    ),
                  ),
                ],
              ),
            ),
          ),
          Expanded(
            child: Row(
              children: [
                Expanded(
                  child: ReuseableCard(
                    color: kActiveCardColor,
                    cardChild: Column(
                      mainAxisAlignment: MainAxisAlignment.center,
                      children: [
                        const Text(
                          'WEIGHT',
                          style: kLabelTextStyle,
                        ),
                        Text(
                          weight.toString(),
                          style: kNumberTextStyle,
                        ),
                        Row(
                          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                          children: [
                            RoundIconButton(
                              icon: FontAwesomeIcons.minus,
                              onPressed: () {
                                weight--;
                                setState(() {});
                              },
                            ),
                            RoundIconButton(
                              icon: FontAwesomeIcons.plus,
                              onPressed: () {
                                weight++;
                                setState(() {});
                              },
                            ),
                          ],
                        ),
                      ],
                    ),
                  ),
                ),
                Expanded(
                  child: ReuseableCard(
                    color: kActiveCardColor,
                    cardChild: Column(
                      mainAxisAlignment: MainAxisAlignment.center,
                      children: [
                        const Text(
                          'AGE',
                          style: kLabelTextStyle,
                        ),
                        Text(
                          age.toString(),
                          style: kNumberTextStyle,
                        ),
                        Row(
                          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                          children: [
                            RoundIconButton(
                              icon: FontAwesomeIcons.minus,
                              onPressed: () {
                                age--;
                                setState(() {});
                              },
                            ),
                            RoundIconButton(
                              icon: FontAwesomeIcons.plus,
                              onPressed: () {
                                age++;
                                setState(() {});
                              },
                            ),
                          ],
                        ),
                      ],
                    ),
                  ),
                ),
              ],
            ),
          ),
          BottomButton(
            buttonTitle: 'CALCULATE',
            onTap: () {
              Calculator calc = Calculator(height: height, weight: weight);
              Navigator.push(context, MaterialPageRoute(builder: (context) => ResultPage(
                resultText: calc.getResult(),
                resultBMI: calc.getBMI(),
                resultInterpretation: calc.getInterpretation(),
              )),);
            },
          ),
        ],
      ),
    );
  }
}

class Calculator {
  Calculator({required this.height, required this.weight});
  final int height;
  final int weight;
  late double _bmi = 22.0;

  String getBMI() {
    _bmi = weight / pow(height / 100, 2);
    return _bmi.toStringAsFixed(1);
  }

  String getResult() {
    if(_bmi > 25) {
      return 'Overweight';
    } else if(_bmi <18) {
      return 'Underweight';
    } else {
      return 'Normal';
    }
  }

  String getInterpretation() {
    if(_bmi > 25) {
      return '標準体重を超えています。運動しましょう。';
    } else if(_bmi <18) {
      return '標準体重を下回っています。もっと食べましょう。';
    } else {
      return '標準体重です。';
    }
  }
}

class ResultPage extends StatelessWidget {
  ResultPage({required this.resultText, required this.resultBMI, required this.resultInterpretation});
  final String resultText;
  final String resultBMI;
  final String resultInterpretation;
  
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('BMI RESULT'),
      ),
      body: Column(
        mainAxisAlignment: MainAxisAlignment.spaceEvenly,
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: [
          Expanded(
              child: Container(
                padding: const EdgeInsets.all(15.0),
                alignment: Alignment.bottomLeft,
                child: const Text(
                  'Your Result',
                  style: kTitleTextStyle,
                ),
              )
          ),
          Expanded(
            flex: 5,
            child: ReuseableCard(
              color: kActiveCardColor,
              cardChild: Column(
                mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                crossAxisAlignment: CrossAxisAlignment.center,
                children: [
                  Text(
                    resultText.toUpperCase(),
                    style: kResultTextStyle,
                  ),
                  Text(
                    resultBMI,
                    style: kBMITextStyle,
                  ),
                  Padding(
                    padding: const EdgeInsets.all(20.0),
                    child: Text(
                      resultInterpretation,
                      style: kBodyTextStyle,
                      textAlign: TextAlign.center,
                    ),
                  ),
                ],
              ),
            ),
          ),
          BottomButton(
              buttonTitle: 'RETURN',
              onTap: () {
                Navigator.pop(context);
              },
          )
        ],
      ),
    );
  }
}

class BottomButton extends StatelessWidget {
  BottomButton({required this.buttonTitle, required this.onTap});
  final String buttonTitle;
  final dynamic onTap;

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: onTap,
      child: Container(
        child: Center(
          child: Text(
            buttonTitle,
            style: kLargeButtonTextStyle,
          ),
        ),
        color: kBottomContainerColor,
        margin: const EdgeInsets.only(top: 10.0),
        padding: const EdgeInsets.only(bottom: 20.0),
        width: double.infinity,
        height: kBottomContainerHeight,
      ),
    );
  }
}



class IconContent extends StatelessWidget {
  IconContent({required this.icon, required this.label});
  final IconData icon;
  final String label;

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: [
        Icon(
          icon,
          size: 80.0,
        ),
        const SizedBox(
          height: 15.0,
        ),
        Text(
          label,
          style: kLabelTextStyle,
        ),
      ],
    );
  }
}

class ReuseableCard extends StatelessWidget {
  ReuseableCard({required this.color, this.cardChild, this.onPress});
  final Color color;
  final Widget? cardChild;
  final dynamic onPress;

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: onPress,
      child: Container(
        child: cardChild,
        margin: const EdgeInsets.all(15.0),
        decoration: BoxDecoration(
          color: color,
          borderRadius: BorderRadius.circular(10.0),
        ),
      ),
    );
  }
}

class RoundIconButton extends StatelessWidget {
  RoundIconButton({required this.icon, required this.onPressed});
  final IconData icon;
  final dynamic onPressed;

  @override
  Widget build(BuildContext context) {
    return RawMaterialButton(
      child: Icon(icon),
      onPressed: onPressed,
      shape: RoundedRectangleBorder(
        borderRadius: BorderRadius.circular(24.0),
      ),
      fillColor: const Color(0xFF4C4F5E),
      constraints: const BoxConstraints.tightFor(
        width: 56.0,
        height: 56.0,
      ),
      elevation: 0.0,
    );
  }
}

double kBottomContainerHeight = 80;
Color kActiveCardColor = const Color(0xFF1D1E33);
Color kInactiveCardColor = const Color(0xFF111328);
Color kBottomContainerColor = const Color(0xFFEB1555);

const kLabelTextStyle = TextStyle(
  fontSize: 18.0,
  color: Color(0xFF8D8E98),
);

const kNumberTextStyle = TextStyle(
  fontSize: 50.0,
  fontWeight: FontWeight.w900,
);

const kLargeButtonTextStyle = TextStyle(
  fontSize: 25.0,
  fontWeight: FontWeight.bold,
);

const kTitleTextStyle = TextStyle(
  fontSize: 50.0,
  fontWeight: FontWeight.bold,
);

const kResultTextStyle = TextStyle(
  fontSize: 22.0,
  fontWeight: FontWeight.bold,
  color: Color(0xFF24D876),
);

const kBMITextStyle = TextStyle(
  fontSize: 100.0,
  fontWeight: FontWeight.bold,
);

const kBodyTextStyle = TextStyle(
  fontSize: 22.0,
);

・セクション12-133：Route/Navigation
セクション12の途中で登場したページ遷移の解説、パンケーキの例えがわかりやすいw
参考リンク：
https://flutter.dev/docs/cookbook/navigation/navigation-basics
https://flutter.dev/docs/cookbook/navigation/named-routes

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      initialRoute: '/',
      routes: {
        '/': (context) => Screen0(),
        '/first': (context) => Screen1(),
        '/second': (context) => Screen2(),
      },
    );
  }
}

class Screen0 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Colors.purple,
        title: Text('Screen 0'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            ElevatedButton(
              child: Text('Go To Screen 1'),
              onPressed: () {
                Navigator.pushNamed(context, '/first');
              },
                style: ElevatedButton.styleFrom(
                  primary: Colors.purple,
                )
            ),
            ElevatedButton(
              child: Text('Go To Screen 2'),
              onPressed: () {
                Navigator.pushNamed(context, '/second');
              },
            ),
          ],
        ),
      ),
    );
  }
}

class Screen1 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Colors.red,
        title: Text('Screen 1'),
      ),
      body: Center(
        child: ElevatedButton(
          child: Text('Go Forwards To Screen 2'),
          onPressed: () {
            Navigator.push(context, MaterialPageRoute(builder: (context) => Screen2()), );
          },
            style: ElevatedButton.styleFrom(
              primary: Colors.red,
            ),
        ),
      ),
    );
  }
}

class Screen2 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Colors.blue,
        title: Text('Screen 2'),
      ),
      body: Center(
        child: ElevatedButton(
          child: Text('Go Back To Screen 1'),
          onPressed: () {
            Navigator.pop(context);
          },
          style: ElevatedButton.styleFrom(
            primary: Colors.blue,
          ),
        ),
      ),
    );
  }
}

・セクション13：Geolocation、非同期処理（Future、async/await）、例外処理（try/catch）、null認識演算子、API、JSON、http、get、jsonDecode、TextField等
参考リンク：
https://pub.dev/packages/geolocator
https://openweathermap.org/api
https://flutter.dev/docs/cookbook/networking/fetch-data
https://www.restapitutorial.com/httpstatuscodes.html

import 'package:flutter/material.dart';
import 'package:geolocator/geolocator.dart';
import 'package:http/http.dart' as http;
import 'dart:convert';
import 'package:flutter_spinkit/flutter_spinkit.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark(),
      home: LoadingScreen(),
    );
  }
}

class LoadingScreen extends StatefulWidget {
  @override
  _LoadingScreenState createState() => _LoadingScreenState();
}

class _LoadingScreenState extends State<LoadingScreen> {
  @override
  void initState() {
    getLocationData();
    super.initState();
  }

  void getLocationData() async{
    var weatherData = await WeatherModel().getLocationWeather();

    Navigator.push(context, MaterialPageRoute(builder: (context) => LocationScreen(locationWeather: weatherData)));
  }

  @override
  Widget build(BuildContext context) {
    return const Scaffold(
      body: Center(
        child: SpinKitFadingCircle(
          color: Colors.white,
          size: 100.0,
        ),
      ),
    );
  }
}

class LocationScreen extends StatefulWidget {
  LocationScreen({required this.locationWeather});
  final Map<String, dynamic> locationWeather;
  @override
  _LocationScreenState createState() => _LocationScreenState(locationWeather: locationWeather);
}

class _LocationScreenState extends State<LocationScreen> {
  _LocationScreenState({required this.locationWeather});
  Map<String, dynamic> locationWeather;
  WeatherModel wetherModel = WeatherModel();
  String? temperature;
  String? weatherIcon;
  String? message;
  String? sityName;
  
  @override
  void initState() {
    updateScreen(locationWeather);
    super.initState();
  }

  void updateScreen (Map<String, dynamic> weatherData) {
    if(weatherData == null) {
      temperature = '0';
      weatherIcon = 'Error';
      message = 'Unable to get location data.';
      sityName = '';
    } else {
      temperature = weatherData['main']['temp'].toStringAsFixed(1);
      weatherIcon = wetherModel.getWeatherIcon(weatherData['weather'][0]['id']);
      message = wetherModel.getMessage(weatherData['main']['temp'].toInt());
      sityName = weatherData['name'];
      setState(() {});
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Container(
        decoration: BoxDecoration(
          image: DecorationImage(
            image: const AssetImage('images/location_background.jpeg'),
            fit: BoxFit.cover,
            colorFilter: ColorFilter.mode(
                Colors.white.withOpacity(0.8),
                BlendMode.dstATop
            ),
          ),
        ),
        constraints: const BoxConstraints.expand(),
        child: SafeArea(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.spaceBetween,
            crossAxisAlignment: CrossAxisAlignment.stretch,
            children: [
              Row(
                mainAxisAlignment: MainAxisAlignment.spaceBetween,
                children: [
                  TextButton(
                    onPressed: () async {
                      var weatherData = await wetherModel.getLocationWeather();
                      updateScreen(weatherData);
                    },
                    child: const Icon(
                      Icons.near_me,
                      size: 50.0,
                    ),
                  ),
                  TextButton(
                    onPressed: () async{
                      var cityName = await Navigator.push(context, MaterialPageRoute(builder: (context) => CityScreen(locationWeather: locationWeather)));
                      if(cityName != null ) {
                        var weatherData = await wetherModel.getCityWeather(cityName);
                        updateScreen(weatherData);
                      }
                    },
                    child: const Icon(
                      Icons.location_city,
                      size: 50.0,
                    ),
                  ),
                ],
              ),
              Padding(
                padding: const EdgeInsets.only(left: 15.0),
                child: Row(
                  children: [
                    Text(
                      '$temperature°',
                      style: kTempTextStyle,
                    ),
                    Text(
                      weatherIcon ?? '🤷',
                      style: kConditionTextStyle,
                    ),
                  ],
                ),
              ),
              Padding(
                padding: const EdgeInsets.only(right: 15.0),
                child: Text(
                  '$message in $sityName',
                  textAlign: TextAlign.right,
                  style: kMessageTextStyle,
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

class CityScreen extends StatefulWidget {
  CityScreen({required this.locationWeather});
  Map<String, dynamic> locationWeather;
  @override
  _CityScreenState createState() => _CityScreenState();
}

class _CityScreenState extends State<CityScreen> {
  String cityName = '';
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Container(
        decoration: const BoxDecoration(
          image: DecorationImage(
            image: AssetImage('images/city_background.jpeg'),
            fit: BoxFit.cover,
          ),
        ),
        constraints: const BoxConstraints.expand(),
        child: SafeArea(
          child: Column(
            children: [
              Align(
                alignment: Alignment.topLeft,
                child: TextButton(
                  onPressed: () {
                    Navigator.pop(context);
                  },
                  child: const Icon(
                    Icons.arrow_back_ios,
                    size: 50.0,
                  ),
                ),
              ),
              Container(
                padding: const EdgeInsets.all(20.0),
                child: TextField(
                  style: const TextStyle(
                    color: Colors.black,
                  ),
                  decoration: kTextFieldInputDecoration,
                  onChanged: (value) {
                    cityName = value;
                  },
                ),
              ),
              TextButton(
                onPressed: () {
                  Navigator.pop(context, cityName);
                },
                child: const Text(
                  'GET WEATHER',
                  style: kButtonTextStyle,
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

class Location {
  late double latitude;
  late double longitude;

  Future<void> getCurrentLocation() async {
    try {
      Position position = await Geolocator.getCurrentPosition(desiredAccuracy: LocationAccuracy.low);
      latitude = position.latitude;
      longitude = position.longitude;
    } catch(e) {
      print(e);
    }
  }
}

class NetworkHelper {
  NetworkHelper(this.url);
  final String url;

  Future<Map<String, dynamic>> getData() async {
    http.Response response = await http.get(Uri.parse(url));
    if(response.statusCode == 200) {
      Map<String, dynamic> decodedData = jsonDecode(response.body);
      return decodedData;
    } else {
      print(response.statusCode);
      return {};
    }
  }
}

const apiKey = '********************************';
const apiUrl = 'https://api.openweathermap.org/data/2.5/weather';

class WeatherModel {
  Future<dynamic> getLocationWeather() async {
    Location location = Location();
    await location.getCurrentLocation();
    String url = '$apiUrl?lat=${location.latitude}&lon=${location.longitude}&units=metric&appid=$apiKey';
    NetworkHelper networkHelper = NetworkHelper(url);
    var weatherData = await networkHelper.getData();
    return weatherData;
  }

  Future<dynamic> getCityWeather(String cityName) async {
    String url = '$apiUrl?q=$cityName&units=metric&appid=$apiKey';
    NetworkHelper networkHelper = NetworkHelper(url);
    var weatherData = await networkHelper.getData();
    return weatherData;
  }
  
  String getWeatherIcon(int condition) {
    if (condition < 300) {
      return '🌩';
    } else if (condition < 400) {
      return '🌧';
    } else if (condition < 600) {
      return '☔️';
    } else if (condition < 700) {
      return '☃️';
    } else if (condition < 800) {
      return '🌫';
    } else if (condition == 800) {
      return '☀️';
    } else if (condition <= 804) {
      return '☁️';
    } else {
      return '🤷‍';
    }
  }

  String getMessage(int temp) {
    if (temp > 25) {
      return 'It\'s 🍦 time';
    } else if (temp > 20) {
      return 'Time for shorts and 👕';
    } else if (temp < 10) {
      return 'You\'ll need 🧣 and 🧤';
    } else {
      return 'Bring a 🧥 just in case';
    }
  }
}

const kTempTextStyle = TextStyle(
  fontFamily: 'Spartan MB',
  fontSize: 80.0,
);

const kMessageTextStyle = TextStyle(
  fontFamily: 'Spartan MB',
  fontSize: 50.0,
);

const kButtonTextStyle = TextStyle(
  fontSize: 30.0,
  fontFamily: 'Spartan MB',
);

const kConditionTextStyle = TextStyle(
  fontSize: 100.0,
);

const kTextFieldInputDecoration = InputDecoration(
    filled: true,
    fillColor: Colors.white,
    icon: Icon(
      Icons.location_city,
      color: Colors.white,
    ),
    hintText: 'City Name',
    hintStyle: TextStyle(
      color: Colors.grey,
    ),
    border: OutlineInputBorder(
      borderRadius: BorderRadius.all(
          Radius.circular(10.0)
      ),
      borderSide: BorderSide.none,
    ),
);

・セクション14：セクション13の復習チャレンジ、DropdownButton、反復処理（for）、cupertinoPicker、Platform識別等
実際に使えそうなサービスで楽しく学べた、Widgetを外に切り出すところがまだ慣れない
参考リンク：
https://api.flutter.dev/flutter/material/DropdownButton-class.html
https://flutter.dev/docs/development/ui/widgets/cupertino
https://www.coinapi.io/Pricing

import 'package:flutter/material.dart';
import 'package:flutter/cupertino.dart';
import 'dart:io' show Platform;
import 'dart:convert';
import 'package:http/http.dart' as http;

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(
          primaryColor: Colors.lightBlue,
          scaffoldBackgroundColor: Colors.white),
      home: PriceScreen(),
    );
  }
}

class PriceScreen extends StatefulWidget {
  @override
  _PriceScreenState createState() => _PriceScreenState();
}

class _PriceScreenState extends State<PriceScreen> {
  String selectedCurrency = 'USD';
  String $selectedCrypto = 'BTC';
  Map<String, String> currencyValues = {};

  DropdownButton dropdownButton() {
    List<DropdownMenuItem<String>> dropdownItems =[];
    for (String currency in currenciesList) {
      DropdownMenuItem<String> newItem = DropdownMenuItem(
        child: Text(currency),
        value: currency,
      );
      dropdownItems.add(newItem);
    }
    return DropdownButton<String>(
      value: selectedCurrency,
      items: dropdownItems,
      onChanged: (String? value) {
        selectedCurrency = value!;
        getData();
        setState(() {});
      },
    );
  }

  CupertinoPicker cupertinoPicker() {
    List<Widget> pickerItems = [];
    for(String currency in currenciesList) {
      pickerItems.add(Text(currency));
    }
    return CupertinoPicker(
      itemExtent: 32.0,
      backgroundColor: Colors.lightBlue,
      onSelectedItemChanged: (pickedIndex) {
        selectedCurrency = currenciesList[pickedIndex];
        getData();
        setState(() {});
      },
      children: pickerItems,
    );
  }

  void getData() async{
    try {
      currencyValues = await CoinData().getCoinData(selectedCurrency);
      print(currencyValues);
    } catch(e) {
      print(e);
    }
  }

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('仮想通貨価格取得API'),
      ),
      body: Column(
        mainAxisAlignment: MainAxisAlignment.spaceBetween,
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: [
          Column(
            crossAxisAlignment: CrossAxisAlignment.stretch,
            children: [
              CryptoCard(
                cryptoCurrency: 'BTC',
                selectedCurrency: selectedCurrency,
                value: currencyValues['BTC'] ?? '',
              ),
              CryptoCard(
                cryptoCurrency: 'ETH',
                selectedCurrency: selectedCurrency,
                value: currencyValues['ETH'] ?? '',
              ),
              CryptoCard(
                cryptoCurrency: 'LTC',
                selectedCurrency: selectedCurrency,
                value: currencyValues['LTC'] ?? '',
              ),
            ],
          ),
          Container(
            height: 150.0,
            alignment: Alignment.center,
            padding: const EdgeInsets.only(bottom: 30.0),
            color: Colors.lightBlue,
            child: Container(
              child: Platform.isAndroid ? dropdownButton() : cupertinoPicker(),
            ),
          ),
        ],
      ),
    );
  }
}

class CryptoCard extends StatelessWidget {
  CryptoCard({required this.selectedCurrency, required this.cryptoCurrency, required this.value});
  final String selectedCurrency;
  final String cryptoCurrency;
  final String value;

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.fromLTRB(18.0, 18.0, 18.0, 0),
      child: Card(
        color: Colors.blue,
        elevation: 5.0,
        shape: RoundedRectangleBorder(
          borderRadius: BorderRadius.circular(10.0),
        ),
        child: Padding(
          padding: const EdgeInsets.symmetric(vertical: 15.0, horizontal: 28.0),
          child: Text(
            '1 $cryptoCurrency = $value $selectedCurrency',
            textAlign: TextAlign.center,
            style: const TextStyle(
              fontSize: 20.0,
              color: Colors.white,
            ),
          ),
        ),
      ),
    );
  }
}


const List<String> currenciesList = [
  'AUD', 'BRL', 'CAD', 'CNY', 'EUR', 'GBP', 'HKD', 'IDR', 'ILS', 'INR', 'JPY', 'MXN', 'NOK', 'NZD', 'PLN', 'RON', 'RUB', 'SEK', 'SGD', 'USD', 'ZAR'
];

const List<String> cryptoList = [
  'BTC', 'ETH', 'LTC',
];

const coinApi = 'https://rest.coinapi.io/v1/exchangerate';
const apiKey = '**********';

class CoinData {
  Future getCoinData(String selectedCurrency) async {
    Map<String, String> cryptoPrices = {};
    for (String crypto in cryptoList) {
      String url = '$coinApi/$crypto/$selectedCurrency?apikey=$apiKey';
      http.Response response = await http.get(Uri.parse(url));
      if (response.statusCode == 200) {
        var decodedData = jsonDecode(response.body);
        double lastPrice = decodedData['rate'];
        cryptoPrices[crypto] = lastPrice.toStringAsFixed(1);
      } else {
        print(response.statusCode);
        throw 'Problem with the get request';
      }
    }
    return cryptoPrices;
  }
}

残り1/4くらい！次はみんな大好きFirebase！
https://qiita.com/igakeso/items/29ae39cb9cafffe45492