index.js

import { AppRegistry } from 'react-native';
import { name as appName } from './app.json';
import BackgroundFetch from 'react-native-background-fetch';
import { loadEvents, persistEvents } from './src/headless';
import { getTimestamp } from './src/utils';

/// Android-only:  BackgroundFetch event-handler when app is terminated.
/// NOTE:  This handler must be placed and registered here in index.js -- DO NOT place this in your App components.
///
const headlessTask = async ({ taskId }) => {
    // Get task id from event {}:
    console.log('[BackgroundFetch] 💀 HeadlessTask start: ', taskId);

    // Persist the event in AsyncStorage to render in list when app is relaunched.
    const events = await loadEvents() || [];

    events.unshift({
        isHeadless: true,
        taskId,
        timestamp: getTimestamp(),
    });

    await persistEvents(events);

    // Required:  Signal to native code that your task is complete.
    // If you don't do this, your app could be terminated and/or assigned
    // battery-blame for consuming too much time in background.
    BackgroundFetch.finish(taskId);
};

// Register your BackgroundFetch HeadlessTask
BackgroundFetch.registerHeadlessTask(headlessTask);

//sync app state with device internal storage
const setup = require('./src/SyncWithInternalStorageSetup');

AppRegistry.registerComponent(appName, setup);