integrate expo to the existing project

gistfile1.md

1. add app.json, set the newest sdkVersion

{
  "name": "<yourname>",
  "displayName": "<yourname>",
  "expo": {
    "sdkVersion": "25.0.0"
  }
}

2. add to package.json, something:

"react-native": "https://github.com/expo/react-native/archive/sdk-25.0.0.tar.gz" // according to package.json's sdkVersion

3. add to index.ts:

import { KeepAwake, registerRootComponent } from 'expo';
import { App } from './app';

if (__DEV__) {
  KeepAwake.activate();
}

registerRootComponent(App);

4. add to app.tsx

import { Font } from 'expo';

interface AppProps {
  dispatch: Dispatch<any>;
  nav: NavigationState;
}

export class App extends Component<AppProps> {
  constructor(props: any) {
    super(props)
    this.state = {
      isFontLoaded: false
    };
  }
  public componentDidMount() {
    Font.loadAsync({
      '<customFontName>': require('../assets/fonts/<customFontName>.ttf')
    }).then(() => this.setState({ isFontLoaded: true }));
  }

  public render() {
    return (this.state as any).isFontLoaded ? <App /> : null;
  }
}

5. npm i expo
6. npm i exponent
7. exp start
8. Enjoy

You may put it the playground and see the output:
https://www.typescriptlang.org/play?#code/C4TwDgpgBAIglgZzAQ2AYwBYB4AqA+KAXigG8BfAKFEigDlkA3OAc1TgHsA7AZWFWmLkKaADbIECKAGF2AWzBcInYLgJCKcZRABOAM2RpoAQTBgACtvZhJJClCgATRCnQYAXLGepMWZJxB4ANx2UJyMHvRMrMAcPHzAEMGUFBAAHgrawFCi4pImYFBpCZwOkjLyispY+RZWCGohaFwIwNoArmjA7NoAFGCW1h5+IACUpCH2CG2Qvf11IxNQwBiIAHQt-ETj9jtQiABiXMAAMuzIDhAOHvoiCBCLZMH2lPZgbQBGInBo2XIKnEpgPAHABZdhtZQ9Ma2XaHZSrERnBxGBAgThoHow3ZQADkWDQbRacjhwHosggeBxHm0EAAjm04DSejjVqsAPS5CDABBs3RHHn4wldWQkskU1bAYC6HELXZkEYSjBKHpQogEZZrO7AXj8TF7BAk07nS4eVptaDykZPKDJV4fL4-GklHSqrH2GnANraThQHoahDreLQcRQYYKg5HI0XBxQAD8UGqpigbIIHk4bREImtlDIQA