必要に迫られたのでFlutterを学びます③～外部package編～

ニュイ・ソシエール氏が歌ってみたを投稿してました。
テンション上がりますね。

[環境構築編](https://qiita.com/Takahiro-Hirai/items/4fee6ad3e807254cae3c)　
[Hello World編](https://qiita.com/Takahiro-Hirai/items/67f501ee8ed51982f5dc)
[外部パッケージ利用編](https://qiita.com/Takahiro-Hirai/items/9c5ede0d83951cd6c1b8)　⇐　今ここ
[StatefulWidget利用編](https://qiita.com/Takahiro-Hirai/items/f9b0a4d4a9011eab92dc)

## 第四章 外部packageの利用
Flutterだとpubspec.yamlで外部からimportするpackageを決めているようです。
Javaでいうpom.xmlやbuild.gradleみたいなものでしょうか。
ちなみにデフォルトで生成されるpubspec.yamlは下記のとおりです。

```pubspec.yaml
name: プロジェクト名
description: A new Flutter project.

# The following line prevents the package from being accidentally published to
# pub.dev using `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: ">=2.7.0 <3.0.0"

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: ^0.1.3

dev_dependencies:
  flutter_test:
    sdk: flutter

# 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
```
デフォルトだとプラスボタン用のiconを導入する外部packageが記載されてますね。
今回は[Codelabs](https://codelabs.developers.google.com/codelabs/first-flutter-app-pt1/index.html?index=..%2F..index#3)に書いてある[english_words](https://pub.dev/packages/english_words)を導入してみます。
ちなみにMavenRepository的なのは[こちら](https://pub.dev/)になるみたいです。
メソッドとかのドキュメントは[こちら](https://pub.dev/documentation/english_words/latest/english_words/english_words-library.html)。

それでは導入しましょう。
導入方法は先のpub.devのInstallingのところに載っているので簡単ですね。
今回は画面左上部に3つの文字列を表示させます。
なおここではライセンスの記載を省略してますが、MITライセンスなので、
本来はどこかしらにライセンスを記載する必要があるはずです。

```pubspec.yaml
name: hello_world
description: A new Flutter project.

publish_to: 'none' 

version: 1.0.0+1

environment:
  sdk: ">=2.7.0 <3.0.0"

dependencies:
  flutter:
    sdk: flutter

  cupertino_icons: ^0.1.3

  english_words: ^3.1.5


dev_dependencies:
  flutter_test:
    sdk: flutter

flutter:

  uses-material-design: true

```

```main.dart
import 'package:flutter/material.dart';
import 'package:english_words/english_words.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    var num = 3;
    return MaterialApp(
      title: 'Welcome to Flutter',
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Welcome to Flutter'),
        ),
        body: 
          ListView.builder(
            itemCount: num,
            itemBuilder:(context, int index){
              return Padding(
                padding: EdgeInsets.all(5.0),
                child: Text(
                  nouns[index],
                ));
            }
          )
      ),
    );
  }
}
```
``noun``を使うとList型で返却してくれるそうなので、[ListView](https://api.flutter.dev/flutter/widgets/ListView-class.html)を用いてそれを表示させることにしました。
結果はこんな感じ。
![2020-05-24_12h16_14.png](https://qiita-image-store.s3.ap-northeast-1.amazonaws.com/0/318745/843157a0-4815-3628-9068-796fa4ef9dda.png)

Gradleみたいに使えるのは感覚的に分かりやすくて助かります。