Amplifyを使ってReact Native + Cognitoでログイン機能を爆速で作成する

# プロジェクトのテンプレを作成
最近`react-native-cli`が更新されて、`npx`で利用するのが推奨されているようです。

```bash
# 既にreact-native-cliをグローバルインストールしてる場合は削除
npm uninstall -g react-native-cli
npx react-native init MyApp --template react-native-template-typescript
cd MyApp
```

# npmの依存モジュールを追加
```bash
npm i -S aws-amplify aws-amplify-react-native amazon-cognito-identity-js react-native-vector-icons
npx react-native link amazon-cognito-identity-js
npx react-native link react-native-vector-icons
(cd ios && pod update && pod install)
```

# amplifyの設定
```bash
npx amplify init
npx amplify add api
# 認証はCognito、種類はGraphQL、それ以外はデフォルトで
npx amplify push
```

# ログイン画面の作成
```App.tsx
/**
 * Sample React Native App
 * https://github.com/facebook/react-native
 *
 * Generated with the TypeScript template
 * https://github.com/react-native-community/react-native-template-typescript
 *
 * @format
 */

import React, {Fragment} from 'react';
import {
  SafeAreaView,
  StyleSheet,
  ScrollView,
  View,
  Text,
  StatusBar,
} from 'react-native';

import {
  Header,
  LearnMoreLinks,
  Colors,
  DebugInstructions,
  ReloadInstructions,
} from 'react-native/Libraries/NewAppScreen';

import Amplify from 'aws-amplify';
import awsconfig from './aws-exports';
import { withAuthenticator } from 'aws-amplify-react-native';

Amplify.configure(awsconfig);

const App = () => {
  return (
    <Fragment>
      <StatusBar barStyle="dark-content" />
      <SafeAreaView>
        <ScrollView
          contentInsetAdjustmentBehavior="automatic"
          style={styles.scrollView}>
          <Header />
          {global.HermesInternal == null ? null : (
            <View style={styles.engine}>
              <Text style={styles.footer}>Engine: Hermes</Text>
            </View>
          )}
          <View style={styles.body}>
            <View style={styles.sectionContainer}>
              <Text style={styles.sectionTitle}>Step One</Text>
              <Text style={styles.sectionDescription}>
                Edit <Text style={styles.highlight}>App.tsx</Text> to change this
                screen and then come back to see your edits.
              </Text>
            </View>
            <View style={styles.sectionContainer}>
              <Text style={styles.sectionTitle}>See Your Changes</Text>
              <Text style={styles.sectionDescription}>
                <ReloadInstructions />
              </Text>
            </View>
            <View style={styles.sectionContainer}>
              <Text style={styles.sectionTitle}>Debug</Text>
              <Text style={styles.sectionDescription}>
                <DebugInstructions />
              </Text>
            </View>
            <View style={styles.sectionContainer}>
              <Text style={styles.sectionTitle}>Learn More</Text>
              <Text style={styles.sectionDescription}>
                Read the docs to discover what to do next:
              </Text>
            </View>
            <LearnMoreLinks />
          </View>
        </ScrollView>
      </SafeAreaView>
    </Fragment>
  );
};

const styles = StyleSheet.create({
  scrollView: {
    backgroundColor: Colors.lighter,
  },
  engine: {
    position: 'absolute',
    right: 0,
  },
  body: {
    backgroundColor: Colors.white,
  },
  sectionContainer: {
    marginTop: 32,
    paddingHorizontal: 24,
  },
  sectionTitle: {
    fontSize: 24,
    fontWeight: '600',
    color: Colors.black,
  },
  sectionDescription: {
    marginTop: 8,
    fontSize: 18,
    fontWeight: '400',
    color: Colors.dark,
  },
  highlight: {
    fontWeight: '700',
  },
  footer: {
    color: Colors.dark,
    fontSize: 12,
    fontWeight: '600',
    padding: 4,
    paddingRight: 12,
    textAlign: 'right',
  },
});

export default withAuthenticator(App, true)

```
```bash
npx react-native run-ios
```

`error: Cycle inside`のエラーが出た場合は、
`ios/MyApp.xcworkspace`を開いて、`File>Workspace Settings...`のBuild System:を`Legacy Build System`に変更する。