FutureProviderとは?

## FutureProviderについて学んでみた
どこかの国の人のサンプルが分かりやすかったのだが、なんか適当な書き方だったので、リファクタリングしたみた。

Flutter大学の勉強会でriverpodを使って非同期処理を行うFutureProviderというものを知ってよくあるjsonplaceholderを使ったサンプルないか調べていたら、Githubにあった!

**GithubのURL**
https://github.com/claytonfaria/flutter-riverpod-fetch-data

**riverpod packageのURL**
https://pub.dev/documentation/riverpod/latest/riverpod/FutureProvider-class.html

**Flutter公式ドキュメントのHTTP通信を行う方法**
https://docs.flutter.dev/cookbook/networking/send-data

**Flutter公式ドキュメントのJSONについて書かれたページ**
https://docs.flutter.dev/development/data-and-backend/json

## ソースコード
main.dart
```dart
import 'package:flutter/material.dart';
import 'package:flutter_riverpod/flutter_riverpod.dart';
import 'package:riverpod_jsonplaceholder/posts.dart';

void main() {
  runApp(const ProviderScope(child: MyApp()));
}

class MyApp extends ConsumerWidget {
  const MyApp({Key? key}) : super(key: key);

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context, WidgetRef ref) {
    var value = ref.watch(postsProvider);

    return MaterialApp(
      title: 'Flutter Demo',
      home: Scaffold(
        appBar: AppBar(
          title: Text('flutter_riverpod http'),
          backgroundColor: Colors.lightBlue,
        ),
        body: value.when(
            data: (posts) {
              return ListView.builder(
                itemCount: posts.length,
                itemBuilder: (context, index) {
                  return ListTile(
                    title: Text(posts[index].title),
                  );
                },
              );
            },
            error: (err, stack) => Center(child: Text(err.toString())),
            loading: () => const Center(child: CircularProgressIndicator())),
      ),
    );
  }
}
```

posts.dart
```dart
import 'dart:convert' as convert;

import 'package:flutter/foundation.dart';
import 'package:flutter_riverpod/flutter_riverpod.dart';
import 'package:http/http.dart' as http;

final postsProvider = FutureProvider<List<Post>>((ref) async {
  return fetchPosts();
});

class Post {
  final int userId;
  final int id;
  final String title;
  final String body;

  Post(
      {required this.userId,
      required this.body,
      required this.id,
      required this.title});

  Post.fromJson(Map<String, dynamic> json)
      : userId = json['userId'],
        id = json['id'],
        title = json['title'],
        body = json['body'];

  Map<String, dynamic> toJson() => {
        'userId': userId,
        'id': id,
        'title': title,
        'body': body,
      };
}

List<Post> parsePosts(String responseBody) {
  final parsed = convert.jsonDecode(responseBody).cast<Map<String, dynamic>>();

  return parsed.map<Post>((json) => Post.fromJson(json)).toList();
}

Future<List<Post>> fetchPosts() async {
  var url = Uri.parse('https://jsonplaceholder.typicode.com/posts');

  var response = await http.get(url);
  if (response.statusCode == 200) {
    // Use the compute function to run parsePhotos in a separate isolate.
    return compute(parsePosts, response.body);
  } else {
    throw Exception('Failed to load post');
  }
}
```

pabspec.yaml
```yaml
name: riverpod_jsonplaceholder
description: A new Flutter project.

# The following line prevents the package from being accidentally published to
# pub.dev using `flutter pub publish`. This is preferred for private packages.
publish_to: 'none' # Remove this line if you wish to publish to pub.dev

# The following defines the version and build number for your application.
# A version number is three numbers separated by dots, like 1.2.43
# followed by an optional build number separated by a +.
# Both the version and the builder number may be overridden in flutter
# build by specifying --build-name and --build-number, respectively.
# In Android, build-name is used as versionName while build-number used as versionCode.
# Read more about Android versioning at https://developer.android.com/studio/publish/versioning
# In iOS, build-name is used as CFBundleShortVersionString while build-number used as CFBundleVersion.
# Read more about iOS versioning at
# https://developer.apple.com/library/archive/documentation/General/Reference/InfoPlistKeyReference/Articles/CoreFoundationKeys.html
version: 1.0.0+1

environment:
  sdk: ">=2.16.1 <3.0.0"

# Dependencies specify other packages that your package needs in order to work.
# To automatically upgrade your package dependencies to the latest versions
# consider running `flutter pub upgrade --major-versions`. Alternatively,
# dependencies can be manually updated by changing the version numbers below to
# the latest version available on pub.dev. To see which dependencies have newer
# versions available, run `flutter pub outdated`.
dependencies:
  flutter:
    sdk: flutter


  # The following adds the Cupertino Icons font to your application.
  # Use with the CupertinoIcons class for iOS style icons.
  cupertino_icons: ^1.0.2

dev_dependencies:
  flutter_test:
    sdk: flutter
  flutter_riverpod: ^1.0.3
  http: ^0.13.4

  # The "flutter_lints" package below contains a set of recommended lints to
  # encourage good coding practices. The lint set provided by the package is
  # activated in the `analysis_options.yaml` file located at the root of your
  # package. See that file for information about deactivating specific lint
  # rules and activating additional ones.
  flutter_lints: ^1.0.0

# For information on the generic Dart part of this file, see the
# following page: https://dart.dev/tools/pub/pubspec

# The following section is specific to Flutter.
flutter:

  # The following line ensures that the Material Icons font is
  # included with your application, so that you can use the icons in
  # the material Icons class.
  uses-material-design: true

  # To add assets to your application, add an assets section, like this:
  # assets:
  #   - images/a_dot_burr.jpeg
  #   - images/a_dot_ham.jpeg

  # An image asset can refer to one or more resolution-specific "variants", see
  # https://flutter.dev/assets-and-images/#resolution-aware.

  # For details regarding adding assets from package dependencies, see
  # https://flutter.dev/assets-and-images/#from-packages

  # To add custom fonts to your application, add a fonts section here,
  # in this "flutter" section. Each entry in this list should have a
  # "family" key with the font family name, and a "fonts" key with a
  # list giving the asset and other descriptors for the font. For
  # example:
  # fonts:
  #   - family: Schyler
  #     fonts:
  #       - asset: fonts/Schyler-Regular.ttf
  #       - asset: fonts/Schyler-Italic.ttf
  #         style: italic
  #   - family: Trajan Pro
  #     fonts:
  #       - asset: fonts/TrajanPro.ttf
  #       - asset: fonts/TrajanPro_Bold.ttf
  #         weight: 700
  #
  # For details regarding fonts from package dependencies,
  # see https://flutter.dev/custom-fonts/#from-packages

```

## スクリーンショット

![スクリーンショット 2022-03-31 22.21.23.png](https://qiita-image-store.s3.ap-northeast-1.amazonaws.com/0/2303722/dc668513-bf4c-f222-8a62-167811f0fce7.png)

## 最後に
実際の開発現場では、バックエンドはFirebaseは認証機能ぐらいでしか使っていないと聞いたことがあるのですが、その場合は、Web通信を行なって、get、post、put、deleteを行なって普段使っているアプリのデータベースを操作しているのでしょうね。
CRUDってやつですね。

例を出すとSlack、KINGOFTIMEが当てはまる。データベースはクラウドを使っているんでしょうね。

ベンチャーでは、バックエンドはFirebaseのところもあるので、個人開発では無理してバックエンドのAPIまで作らなくてもいいんだなと最近転職活動して思った。