I am using expo FileSystem to download the pdf file. The API response lands into success function. However, I am not able to show the downloaded file to the user.

The expected behaviour should be like we usually see notification icon on the status bar and on click on icon its opens your file.

FileSystem.downloadAsync(

  'https://bitcoin.org/bitcoin.pdf',

  FileSystem.documentDirectory + 'Stay_Overview.xlsx'

).then(({ uri }) => {

   console.log('Finished downloading to ', uri);

})

 .catch(error => {

    console.error(error);

 });

DownloadManager

I believe that you are looking to use the DownloadManager for handling your downloads on Android (be aware there is no DownloadManager for iOS so you would have to handle this differently) The DownloadManager either savse the file to a shared system cache or it would save it to external storage.

However, at this time I do not believe that Expo allows you to use the DownloadManager, instead handling all the downloads itself. The reason could be that Expo doesn't allow you access to external storage, as it states in the documentation:

Each app only has read and write access to locations under the following directories:

• Expo.FileSystem.documentDirectory


• Expo.FileSystem.cacheDirectory

https://docs.expo.io/versions/latest/sdk/filesystem

So there would be no way to access the file in Expo once it was downloaded.

Possible Solution

A possible solution would be to use React-Native Fetch Blob. It does allow you to use the DownloadManager.
https://github.com/joltup/rn-fetch-blob#android-media-scanner-and-download-manager-support

Using the DownloadManager can be achieved in rn-fetch-blob by setting the addAndroidDownloads with the useDownloadManager key set to true.

However, that would mean ejecting your application from Expo.

This one had one or two tricks, but here is a solution to this using Expo that works on both iOS and Android.

In a new Expo project, amend the following two files:

• App.js

import React, { Component } from 'react';

import { View, ScrollView, StyleSheet, Button, Alert, Platform, Text, TouchableWithoutFeedback } from 'react-native';

import { FileSystem, Constants, Notifications, Permissions } from 'expo';

import Toast, {DURATION} from 'react-native-easy-toast';



async function getiOSNotificationPermission() {

  const { status } = await Permissions.getAsync(

    Permissions.NOTIFICATIONS

  );

  if (status !== 'granted') {

    await Permissions.askAsync(Permissions.NOTIFICATIONS);

  }

}



export default class App extends Component {

  constructor(props) {

    super(props);

    // this.toast = null;

    this.listenForNotifications = this.listenForNotifications.bind(this);

    // this.openFile = this.openFile.bind(this);

    this.state = {

      filePreviewText: ''

    }

  }



  _handleButtonPress = () => {

    let fileName = 'document.txt';

    let fileUri = FileSystem.documentDirectory + fileName;

    FileSystem.downloadAsync(

      "https://raw.githubusercontent.com/expo/expo/master/README.md",

      fileUri

    ).then(({ uri }) => {

      console.log('Finished downloading to ', uri);



      const localnotification = {

        title: 'Download has finished',

        body: fileName + " has been downloaded. Tap to open file.",

        android: {

          sound: true,

        },

        ios: {

          sound: true,

        },



        data: {

          fileUri: uri

        },

      };

      localnotification.data.title = localnotification.title;

      localnotification.data.body = localnotification.body;

      let sendAfterFiveSeconds = Date.now();

      sendAfterFiveSeconds += 3000;



      const schedulingOptions = { time: sendAfterFiveSeconds };

      Notifications.scheduleLocalNotificationAsync(

        localnotification,

        schedulingOptions

      );

    })

    .catch(error => {

        console.error(error);

        Alert.alert(error);

    });

  };

  listenForNotifications = () => {

    const _this = this;



    Notifications.addListener(notification => {

      if (notification.origin === 'received') {

        // We could also make our own design for the toast

        // _this.refs.toast.show(<View><Text>hello world!</Text></View>);



        const toastDOM = 

          <TouchableWithoutFeedback 

            onPress={() => {this.openFile(notification.data.fileUri)}}

            style={{padding: '10', backgroundColor: 'green'}}>

            <Text style={styles.toastText}>{notification.data.body}</Text>

          </TouchableWithoutFeedback>;



        _this.toast.show(toastDOM, DURATION.FOREVER);

      } else if (notification.origin === 'selected') {

        this.openFile(notification.data.fileUri);

      }

        // Expo.Notifications.setBadgeNumberAsync(number);

        // Notifications.setBadgeNumberAsync(10);

        // Notifications.presentLocalNotificationAsync(notification);

        // Alert.alert(notification.title, notification.body);

    });

  };

  componentWillMount() {

    getiOSNotificationPermission();

    this.listenForNotifications();

  }

  componentDidMount() {

    // let asset = Asset.fromModule(md);

    // Toast.show('Hello World');

  }

  openFile = (fileUri) => {

    this.toast.close(40);

    console.log('Opening file ' + fileUri);

    FileSystem.readAsStringAsync(fileUri)

    .then((fileContents) => {

      // Get file contents in binary and convert to text

      // let fileTextContent = parseInt(fileContents, 2);

      this.setState({filePreviewText: fileContents});

    });

  }

  render() {

    return (

      <View style={styles.container}>

        <View style={styles.buttonsContainer}>

          <Button style={styles.button}

            title={"Download text file"}

            onPress={this._handleButtonPress}

          />

          <Button style={styles.button}

            title={"Clear File Preview"}

            onPress={() => {this.setState({filePreviewText: ""})}}

          />

        </View>

        <ScrollView style={styles.filePreview}>

          <Text>{this.state.filePreviewText}</Text>

        </ScrollView>

        <Toast ref={ (ref) => this.toast=ref }/>

      </View>

    );

            // <Toast

        //   ref={ (ref) => this.toast=ref }

        //   style={{backgroundColor:'green'}}

        //   textStyle={{color:'white'}}

        //   position={'bottom'}

        //   positionValue={100}

        //   opacity={0.8}

        // />

  }

}



const styles = StyleSheet.create({

  container: {

    flex: 1,

    alignItems: 'center',

    justifyContent: 'center',

    paddingTop: Constants.statusBarHeight,

    backgroundColor: '#ecf0f1',

  },

  buttonsContainer: {

    flexDirection: 'row',

  },

  button: {

    flex: 1

  },

  filePreview: {

    flex: 1,

    padding: 10,

  },

  toastText: {

    color: 'white',

    padding: 5,

    justifyContent: 'flex-start',

  },

});

• package.json: Add the following dependency (fork of react-native-easy-toast)

"react-native-easy-toast": "git+https://github.com/SiavasFiroozbakht/react-native-easy-toast.git"

There are a couple of important notes about this solution:

• Uses Expo API the most, for external local notifications and writing to / reading from files, which limits the current solution to being unable to write to other locations than Expo's own directory.




• Once the file is downloaded, either a customisable toast is shown to the user if the app is active (Expo currently does not support foreground notifications), or sends a local Push Notification to let the user know the download has finished. Clicking on any of these two will show the contents of the file in a View, using the <Text> component.




• The crazycodeboy/react-native-easy-toast repo has not been used directly due to a limitation of the toast, which is that the touch events are currently disregarded. The forked repo makes this functionality available before the merge request is implemented in the original. I recommend switching back to the original one once it gets patched as I will likely not maintain mine.




• Although this project is also available in Snack, it will not run due to the need of using a git repository in package.json as mentioned above, and other apparent inconsistencies in variable scoping. This would be fixed by either the merge request or the new feature in Snack.




• Other file types may be supported, either by Expo itself or via external packages, such as this PDF viewer. However, the code will have to be further adapted.




• The toast (internal notification) is created with a TouchableWithoutFeedback component, although there are other similar ones in React Native with various differences. This component can be customised in the code (search for toastDOM), but might even be replaceable in the future by internal notifications available in Expo.




• Lastly, an intentional three-second delay is applied to the notification once the file is downloaded – this allows us to test the notification when the app is in background. Feel free to remove the delay and trigger the notification immediately.

And that's it! I think this gives a good starting point for file downloading and previewing with Expo.

Codebase also available on GitHub.