MVVM学んでみる

## 配列を操作する簡単なサンプルを作った

flutterバージョン: 2.5.2
状態管理: Provider6.0.2

フォルダ構成
```
lib
├── main.dart
├── models
│   └── todo.dart
└── views
    └── home_page.dart
```

main.dart
```dart
import 'package:flutter/material.dart';
import 'package:todo_sample/models/todo.dart';
import 'package:todo_sample/views/home_page.dart';
import 'package:provider/provider.dart';

// dartプログラムのエントリーポイントとなる関数
void main() {
  // Flutterのパッケージで、Widgetを定義する関数。引数にWidgetを渡すことでウィジェットツリーのルートとなる
  runApp(MultiProvider(
    providers: [
      ChangeNotifierProvider(create: (_) => ToDo(),)
    ],
    child: const MyApp(),
  ));
}

// Widgetとは、UI構築するために必要なクラス
// MyAppクラスは、StatelessWidgetを継承している
class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      home: HomePage(),
    );
  }
}
```

views/home_page.dart
```dart
import 'package:flutter/material.dart';
import 'package:provider/src/provider.dart';
import 'package:todo_sample/models/todo.dart';

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      // 画面にヘッダーを追加する
      appBar: AppBar(
        title: const Text('ToDo'),
      ),
      body: ListView.builder(
        itemCount: context.watch<ToDo>().todoItems.length,
        itemBuilder: (context, index) => ListTile(
          title: Text('配列の中身:${context.watch<ToDo>().todoItems[index]}'),
        ),
      ),
      // 追加ボタン
      floatingActionButton: FloatingActionButton(
        child: const Icon(Icons.add),
        onPressed: () {
          context.read<ToDo>().listAdd();
        },
      ),
    );
  }
}
```

models/todo.dart
```dart
import 'package:flutter/material.dart';

class ToDo with ChangeNotifier {
  // 仮のデータ
  final todoItems = ['one', 'tow', 'three'];

  // 仮のデータに文字を追加するメソッド
  void listAdd() {
    todoItems.add('four');
    notifyListeners();
  }
}

```

pabspec.yaml
```yaml
name: todo_sample
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: ">=2.12.0 <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
  provider: ^6.0.2

  # 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

```

![list.gif](https://qiita-image-store.s3.ap-northeast-1.amazonaws.com/0/2303722/fb8573f1-dbfb-bf36-3f77-0869c56441c3.gif)

## 最後に
ChangeNotifireとCousmaerしか知らなかった自分が、Dartで、ビジネスロジックを考えることが出来たのは、Swiftの学習が役に立っているのかもしれない...