App.js

import React from 'react';
import { Root } from 'native-base';
import * as Expo from 'expo';

import { AppNavigator } from './routes';

export default class App extends React.Component {

  constructor() {
    super();
    this.state = {
      readyFlag: false,
    };
  }

  async componentWillMount() {
    await Expo.Font.loadAsync({
      'Ionicons': require('@expo/vector-icons/fonts/Ionicons.ttf'),
    });
    this.setState({ readyFlag: true });
  }

  render() {
    if (!this.state.readyFlag) {
      return <Expo.AppLoading />;
    }
    return (
      <Root>
        <AppNavigator />
      </Root>
    );
  }
}