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Realtime App Development with RethinkDB and React Native

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22 min read · View original · sitepoint.com

A realtime app keeps a user up-to-date with the information they want to know, when they need it. A user doesn’t need to constantly refresh a window to get the latest updates, a server behind the app will push updates to the app automatically. In this tutorial I’m going to cover realtime app development by creating a news sharing app with RethinkDB and React Native.

I’m going to assume that you already have experience creating React Native apps so I’m not going to go into detail on each line of code. If you’re a beginner, I recommend you read my previous tutorial, “Build an Android App with React Native“. If you want to follow along, you can find the code on Github.

Here’s how the final app will look:

news sharer app

I will start by looking at the code for the mobile app and then move on to the server component which uses Node, Express, Socket.io and RethinkDB.

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Install Dependencies

Inside your clone of the project, navigate the NewsSharer directory and execute npm install to install the following dependencies:

Linking Icons

After installing the dependencies, there’s one extra step to get the icons to work, linking them to the app. Do this by using rnpm, the React Native package manager.

Install rnpm with npm:

npm install rnpm -g

Then execute rnpm link in the root of the NewsSharer directory to link the icons.

The App

Below are the contents of the index.android.js file:

import React, { Component } from 'react';
import {
  AppRegistry,
  StyleSheet,
  View
} from 'react-native';

import Main from './components/Main';

class NewsSharer extends Component {

  render() {
    return (
      <View style={styles.container}>
        <Main />
      </View>
    );
  }
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: 'center',
    alignItems: 'center',
    backgroundColor: '#F5FCFF',
  }
});

AppRegistry.registerComponent('NewsSharer', () => NewsSharer);

This file is the entry-point file for the Android app. If you want to deploy to iOS, you can copy the code into a new index.ios.js file.

The main task of this file does is to import the Main component, where the core of the app is located. This reduces code repetition as you import a component instead of repeating code for each platform.

Main App Component

Inside components/Main.js:

import React, { Component } from 'react';
import {
  AppRegistry,
  StyleSheet,
  Text,
  View,
  TextInput,
  TouchableHighlight,
  Linking,
  ListView
} from 'react-native';

import Button from 'react-native-button';
import Modal from 'react-native-modalbox';
import Icon from 'react-native-vector-icons/Octicons';

import "../UserAgent";
import io from 'socket.io-client/socket.io';

import _ from 'lodash';

var base_url = 'http://YOUR_DOMAIN_NAME_OR_IP_ADDRESS:3000';

export default class Main extends Component {

  constructor(props){
    super(props);

    this.socket = io(base_url, {
      transports: ['websocket']
    });

    this.state = {
      is_modal_open: false,
      news_title: '',
      news_url: '',
      news_items_datasource: new ListView.DataSource({
        rowHasChanged: (row1, row2) => row1 !== row2,
      }),
      is_news_loaded: false,
      news: {},
      news_items: []
    };

  }

  getNewsItems(){

    fetch(base_url + '/news')
      .then((response) => {
        return response.json();
      })
      .then((news_items) => {

        this.setState({
          'news_items': news_items
        });

        var news_datasource = this.state.news_items_datasource.cloneWithRows(news_items);

        this.setState({
          'news': news_datasource,
          'is_news_loaded': true
        });

        return news_items;
      })
      .catch((error) => {
        alert('Error occured while fetching news items');
      });

  }

  componentWillMount(){

    this.socket.on('news_updated', (data) => {

      var news_items = this.state.news_items;
      if(data.old_val === null){

        news_items.push(data.new_val);

      }else{

        _.map(news_items, function(row, index){
          if(row.id == data.new_val.id){
            news_items[index].upvotes = data.new_val.upvotes;
          }

        });

      }

      this.updateUI(news_items);

    });

  }

  updateUI(news_items){
    var ordered_news_items = _.orderBy(news_items, 'upvotes', 'desc');
    var limited_news_items = _.slice(ordered_news_items, 0, 30);
    var news_datasource = this.state.news_items_datasource.cloneWithRows(limited_news_items);

    this.setState({
      'news': news_datasource,
      'is_news_loaded': true,
      'is_modal_open': false,
      'news_items': limited_news_items
    });
  }

  componentDidMount(){
    this.getNewsItems();
  }

  upvoteNewsItem(id, upvotes){

    fetch(base_url + '/upvote-newsitem', {
      method: 'POST',
      headers: {
        'Accept': 'application/json',
        'Content-Type': 'application/json',
      },
      body: JSON.stringify({
        news_id: id,
        upvotes: upvotes + 1
      })
    })
      .catch((err) => {
        alert('Error occured while trying to upvote');
      });    

  }

  openModal(){
    this.setState({
      is_modal_open: true
    });
  }

  closeModal(){
    this.setState({
      is_modal_open: false
    });
  }

  shareNews(){

    fetch(base_url + '/save-newsitem', {
      method: 'POST',
      headers: {
        'Accept': 'application/json',
        'Content-Type': 'application/json',
      },
      body: JSON.stringify({
        news_title: this.state.news_title,
        news_url: this.state.news_url,
      })
    })
      .then((response) => {
        alert('News was shared!');
        this.setState({
          news_title: '',
          news_url: ''
        });
      })
      .catch((err) => {
        alert('Error occured while sharing news');
      });

  }

  openPage(url){
    Linking.canOpenURL(url).then(supported => {
      if(supported){
        Linking.openURL(url);
      }
    });
  }

  renderNews(news){
    return (
      <View style={styles.news_item}>
        <TouchableHighlight onPress={this.upvoteNewsItem.bind(this, news.id, news.upvotes)} underlayColor={"#E8E8E8"}>
          <View style={styles.upvote}>
              <Icon name="triangle-up" size={30} color="#666" />
              <Text style={styles.upvote_text}>{news.upvotes}</Text>
          </View>
        </TouchableHighlight>
        <TouchableHighlight onPress={this.openPage.bind(this, news.url)} underlayColor={"#E8E8E8"}>
          <View style={styles.news_title}>
            <Text style={styles.news_item_text}>{news.title}</Text>
          </View>
        </TouchableHighlight>
      </View>
    );
  }

  render(){

    return (
      <View style={styles.container}>
        <View style={styles.header}>
          <View style={styles.app_title}>
            <Text style={styles.header_text}>News Sharer</Text>   
          </View>
          <View style={styles.header_button_container}>
            <Button onPress={this.openModal.bind(this)} style={styles.btn}>
              Share News
            </Button>
          </View>     
        </View>

        {
          this.state.is_news_loaded &&
          <View style={styles.body}>
            <ListView initialListSize={1} dataSource={this.state.news} style={styles.news} renderRow={this.renderNews.bind(this)}></ListView>
          </View>
        }

        <Modal
          isOpen={this.state.is_modal_open}
          style={styles.modal}
          position={"center"}
        >
          <View style={styles.modal_body}>
            <View style={styles.modal_header}>
              <Text style={styles.modal_header_text}>Share News</Text>
            </View>

            <View style={styles.input_row}>
              <TextInput
                style={{height: 40, borderColor: 'gray', borderWidth: 1}}
                onChangeText={(text) => this.setState({news_title: text})}
                value={this.state.news_title}
                placeholder="Title"
              />
            </View>

            <View style={styles.input_row}>
              <TextInput
                style={{height: 40, borderColor: 'gray', borderWidth: 1}}
                onChangeText={(text) => this.setState({news_url: text})}
                value={this.state.news_url}
                placeholder="URL"
                keyboardType="url"
              />
            </View>

            <View style={styles.input_row}>
              <Button onPress={this.shareNews.bind(this)} style={[styles.btn, styles.share_btn]}>
                Share
              </Button>
            </View>
          </View>

        </Modal>
      </View>
    );
  }
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    alignSelf: 'stretch',
    backgroundColor: '#F5FCFF',
  },
  header: {
    flex: 1,
    backgroundColor: '#3B3738',
    flexDirection: 'row'
  },
  app_title: {
    flex: 7,
    padding: 10
  },
  header_text: {
    fontSize: 20,
    color: '#FFF',
    fontWeight: 'bold'
  },
  header_button_container: {
    flex: 3
  },
  body: {
    flex: 19
  },
  btn: {
    backgroundColor: "#05A5D1",
    color: "white",
    margin: 10
  },
  modal: {
    height: 300
  },
  modal_header: {
    margin: 20,
  },
  modal_body: {
    alignItems: 'center'
  },
  input_row: {
    padding: 20
  },
  modal_header_text: {
    fontSize: 18,
    fontWeight: 'bold'
  },
  share_btn: {
    width: 100
  },
  news_item: {
    paddingLeft: 10,
    paddingRight: 10,
    paddingTop: 15,
    paddingBottom: 15,
    marginBottom: 5,
    borderBottomWidth: 1,
    borderBottomColor: '#ccc',
    flex: 1,
    flexDirection: 'row'
  },
  news_item_text: {
    color: '#575757',
    fontSize: 18
  },
  upvote: {
    flex: 2,
    paddingRight: 15,
    paddingLeft: 5,
    alignItems: 'center'
  },
  news_title: {
    flex: 18,
    justifyContent: 'center'
  },
  upvote_text: {
    fontSize: 18,
    fontWeight: 'bold'
  }
});

AppRegistry.registerComponent('Main', () => Main);

Breaking down the code above. First import the built-in React Native and third-party components needed.

import React, { Component } from 'react';
import {
  AppRegistry,
  StyleSheet,
  Text,
  View,
  TextInput,
  TouchableHighlight,
  Linking,
  ListView
} from 'react-native';

import Button from 'react-native-button';
import Modal from 'react-native-modalbox';
import Icon from 'react-native-vector-icons/Octicons';

import "../UserAgent";
import io from 'socket.io-client/socket.io';

import _ from 'lodash';

Note that you’re importing custom code here:

import "../UserAgent";

This is the UserAgent.js file that you see at the root of the NewsSharer directory. It contains code to set the user agent to react-native, needed by Socket.io to work, or it will assume that it’s in a browser environment.

window.navigator.userAgent = 'react-native';

Next, the base URL to which the app is going to make requests. If you’re testing locally this can be the internal IP address of your computer. For this to work, you have to make sure that your phone or tablet is connected to the same network as your computer.

var base_url = 'http://YOUR_DOMAIN_NAME_OR_IP_ADDRESS:3000';

Inside the constructor, initialize the socket connection:

this.socket = io(base_url, {
  transports: ['websocket']
});

Set the default app state:

this.state = {
  is_modal_open: false, //for showing/hiding the modal
  news_title: '', //default value for news title text field
  news_url: '', //default value for news url text field
  //initialize a datasource for the news items
  news_items_datasource: new ListView.DataSource({
    rowHasChanged: (row1, row2) => row1 !== row2,
  }),
  //for showing/hiding the news items
  is_news_loaded: false,
  news: {}, //the news items datasource
  news_items: [] //the news items
};

This function fetches the news items from the server, using the built-in fetch method. It performs a GET request to the news route and then extracts the news_items object from the response. This is then used to create the news datasource which required by the ListView component. Once created, it updates the state with the news datasource so that the UI gets updated with the news items in it.

getNewsItems(){

  fetch(base_url + '/news')
    .then((response) => {
      return response.json();
    })
    .then((news_items) => {

      this.setState({
        'news_items': news_items
      });

      var news_datasource = this.state.news_items_datasource.cloneWithRows(news_items);

      this.setState({
        'news': news_datasource,
        'is_news_loaded': true
      });

      return news_items;
    })
    .catch((error) => {
      alert('Error occured while fetching news items');
    });

}

The componentWillMount method is one of React’s lifecycle methods. This allows you to execute code before the initial rendering occurs. This is where you listen for the news_updated event emitted by Socket.io’s server component, and when this event happens, it can be one of two things. When a users shares a news item or when they upvote an existing news item.

RethinkDB’s changefeed returns a null value for the old_val if it’s a new item. This is how you distinguish between the two possibilities. If a user has shared a new news item, push it to the news_items array. Otherwise, look for the upvoted news item and update its upvote count. Now you can update the UI to reflect the changes.

componentWillMount(){

  this.socket.on('news_updated', (data) => {

    var news_items = this.state.news_items;
    if(data.old_val === null){ //a new news item is shared
      //push the new item to the news_items array
      news_items.push(data.new_val);

    }else{ //an existing news item is upvoted
      //find the news item that was upvoted and update its upvote count
      _.map(news_items, function(row, index){
        if(row.id == data.new_val.id){
          news_items[index].upvotes = data.new_val.upvotes;
        }

      });

    }

    //update the UI to reflect the changes
    this.updateUI(news_items);

  });

}

The updateUI function orders the news items by their upvote count, highest to lowest. Once sorted, extract the first 30 news items and update the state.

updateUI(news_items){
  var ordered_news_items = _.orderBy(news_items, 'upvotes', 'desc');
  var limited_news_items = _.slice(ordered_news_items, 0, 30);
  var news_datasource = this.state.news_items_datasource.cloneWithRows(limited_news_items);

  this.setState({
    'news': news_datasource,
    'is_news_loaded': true,
    'is_modal_open': false,
    'news_items': limited_news_items
  });
}

The componentDidMount method is another React lifecycle method invoked after the initial rendering occurs. This is where you fetch the news items from the server.

Note: You can also do this inside the componentWillMount method if you want to make the request before the component is mounted.

componentDidMount(){
  this.getNewsItems();
}

The upvoteNewsItem method makes an upvote news item request to the server.

upvoteNewsItem(id, upvotes){

  fetch(base_url + '/upvote-newsitem', {
    method: 'POST',
    headers: {
      'Accept': 'application/json',
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({
      news_id: id,
      upvotes: upvotes + 1
    })
  })
    .catch((err) => {
      alert('Error occured while trying to upvote');
    });    

}

The openModal and closeModal methods show and hide the modal for sharing a news item.

openModal(){
  this.setState({
    is_modal_open: true
  });
}

closeModal(){
  this.setState({
    is_modal_open: false
  });
}

The shareNews function sends a request for creating a news item.

shareNews(){

  fetch(base_url + '/save-newsitem', {
    method: 'POST',
    headers: {
      'Accept': 'application/json',
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({
      news_title: this.state.news_title,
      news_url: this.state.news_url,
    })
  })
    .then((response) => {
      alert('News was shared!');
      this.setState({
        news_title: '',
        news_url: ''
      });
    })
    .catch((err) => {
      alert('Error occured while sharing news');
    });

}

The openPage function opens the URL of the news item in the browser.

openPage(url){
  Linking.canOpenURL(url).then(supported => {
    if(supported){
      Linking.openURL(url);
    }
  });
}

The renderNews function returns the UI for each of the news items. This displays the upvote button, the number of upvotes and the news title. The news title is wrapped inside a TouchableHighlight component. This allows you to execute the openPage function to open the URL. You do the same thing for the upvote count.

Note: The code uses the TouchableHighlight component instead of the Button component because the Button component cannot have View or Text components inside it.

renderNews(news){
  return (
    <View style={styles.news_item}>
      <TouchableHighlight onPress={this.upvoteNewsItem.bind(this, news.id, news.upvotes)} underlayColor={"#E8E8E8"}>
        <View style={styles.upvote}>
            <Icon name="triangle-up" size={30} color="#666" />
            <Text style={styles.upvote_text}>{news.upvotes}</Text>
        </View>
      </TouchableHighlight>
      <TouchableHighlight onPress={this.openPage.bind(this, news.url)} underlayColor={"#E8E8E8"}>
        <View style={styles.news_title}>
          <Text style={styles.news_item_text}>{news.title}</Text>
        </View>
      </TouchableHighlight>
    </View>
  );
}

The render function returns the UI of the whole app.

render(){
    ...
}

Inside the render function you have the header which contains the title of the app and a button for opening the modal for sharing a news item.

<View style={styles.header}>
  <View style={styles.app_title}>
    <Text style={styles.header_text}>News Sharer</Text>   
  </View>
  <View style={styles.header_button_container}>
    <Button onPress={this.openModal.bind(this)} style={styles.btn}>
      Share News
    </Button>
  </View>     
</View>

For the body, you have the ListView component for rendering the news items. It has three required parameters, initialListSize, dataSource, and renderRow. The initialListSize is set to 1 so that the ListView renders each row one by one over the course of multiple frames. You can also update this to a higher value if you want the rows to appear all at once. The dataSource are the news items and the renderRow is the function for rendering each individual news item row.

{
  this.state.is_news_loaded &&
  <View style={styles.body}>
    <ListView initialListSize={1} dataSource={this.state.news} style={styles.news} renderRow={this.renderNews.bind(this)}></ListView>
  </View>
}

Next is the modal for sharing news. This has two text fields for entering the title and the URL of the news, and a button for submitting it to the server. The text fields uses the TextInput component. There are no labels so add placeholder text to guide the user to what they need to input.

Both text fields have the onChangeText method which updates the value for each one. The keyboardType of url is used for the news URL text field so that it opens the keyboard optimized for entering URL’s in the device. The user is not expected to enter it manually, they could use copy and paste, but this is a ‘nice to have’ in case they decide to enter it manually. Below the text fields is the button for sharing the news. This calls the shareNews function defined earlier.

<Modal
  isOpen={this.state.is_modal_open}
  style={styles.modal}
  position={"center"}
>
  <View style={styles.modal_body}>
    <View style={styles.modal_header}>
      <Text style={styles.modal_header_text}>Share News</Text>
    </View>

    <View style={styles.input_row}>
      <TextInput
        style={{height: 40, borderColor: 'gray', borderWidth: 1}}
        onChangeText={(text) => this.setState({news_title: text})}
        value={this.state.news_title}
        placeholder="Title"
      />
    </View>

    <View style={styles.input_row}>
      <TextInput
        style={{height: 40, borderColor: 'gray', borderWidth: 1}}
        onChangeText={(text) => this.setState({news_url: text})}
        value={this.state.news_url}
        placeholder="URL"
        keyboardType="url"
      />
    </View>

    <View style={styles.input_row}>
      <Button onPress={this.shareNews.bind(this)} style={[styles.btn, styles.share_btn]}>
        Share
      </Button>
    </View>
  </View>

</Modal>

Set the styles for the component:

const styles = StyleSheet.create({
  container: {
    flex: 1,
    alignSelf: 'stretch',
    backgroundColor: '#F5FCFF',
  },
  header: {
    flex: 1,
    backgroundColor: '#3B3738',
    flexDirection: 'row'
  },
  app_title: {
    flex: 7,
    padding: 10
  },
  header_text: {
    fontSize: 20,
    color: '#FFF',
    fontWeight: 'bold'
  },
  header_button_container: {
    flex: 3
  },
  body: {
    flex: 19
  },
  btn: {
    backgroundColor: "#05A5D1",
    color: "white",
    margin: 10
  },
  modal: {
    height: 300
  },
  modal_header: {
    margin: 20,
  },
  modal_body: {
    alignItems: 'center'
  },
  input_row: {
    padding: 20
  },
  modal_header_text: {
    fontSize: 18,
    fontWeight: 'bold'
  },
  share_btn: {
    width: 100
  },
  news_item: {
    paddingLeft: 10,
    paddingRight: 10,
    paddingTop: 15,
    paddingBottom: 15,
    marginBottom: 5,
    borderBottomWidth: 1,
    borderBottomColor: '#ccc',
    flex: 1,
    flexDirection: 'row'
  },
  news_item_text: {
    color: '#575757',
    fontSize: 18
  },
  upvote: {
    flex: 2,
    paddingRight: 15,
    paddingLeft: 5,
    alignItems: 'center'
  },
  news_title: {
    flex: 18,
    justifyContent: 'center'
  },
  upvote_text: {
    fontSize: 18,
    fontWeight: 'bold'
  }
});

The Server Component

Now it’s time to move to the server component of the app, where you’ll learn how to save and upvote news items in RethinkDB, and how to inform the app that a change has happened in the database.

Creating the Database

I’m going to assume that you have already installed RethinkDB on your computer. If not, follow the installation and getting started docs on the RethinkDB website.

Once that’s done, you can now access http://localhost:8080 in your browser to view the RethinkDB admin console. Click on the Tables tab then click on the Add Database button. This will open a modal box that lets you enter the name of the database, call it ‘newssharer’ and click Add.

create new database

Now create the table where you’re going to save the news items. Click the Add Table button, name it ‘news_items’, then click Create Table.

create news_items table

Install Dependencies

You can install the server dependencies by navigating to the root of the project directory (with the newssharer-server.js and package.json files), and execute npm install to install the following dependencies:

The Server-Side Code

Inside newssharer-server.js:

var r = require('rethinkdb');

var express = require('express');
var app = express();

var server = require('http').createServer(app);
var io = require('socket.io')(server);

var bodyParser = require('body-parser');
app.use(bodyParser.json());

var connection;

r.connect({host: 'localhost', port: 28015}, function(err, conn) {
    if(err) throw err;
    connection = conn;

    r.db('newssharer').table('news_items')
        .orderBy({index: r.desc('upvotes')})
        .changes()
        .run(connection, function(err, cursor){

            if (err) throw err;
            io.sockets.on('connection', function(socket){
                cursor.each(function(err, row){
                    if(err) throw err;
                    io.sockets.emit('news_updated', row);   
                });
            });
    });

});

app.get('/create-table', function(req, res){
    r.db('newssharer').table('news_items').indexCreate('upvotes').run(connection, function(err, result){
        console.log('boom');
        res.send('ok')
    });
});

app.get('/fill', function(req, res){
    r.db('newssharer').table('news_items').insert([
        {
            title: 'A Conversation About Fantasy User Interfaces',
            url: 'https://www.subtraction.com/2016/06/02/a-conversation-about-fantasy-user-interfaces/',
            upvotes: 30
        },
        {
            title: 'Apple Cloud Services Outage',
            url: 'https://www.apple.com/support/systemstatus/',
            upvotes: 20
        }
    ]).run(connection, function(err, result){
        if (err) throw err;
        res.send('news_items table was filled!');
    });
});

app.get('/news', function(req, res){
    res.header("Content-Type", "application/json");
    r.db('newssharer').table('news_items')
        .orderBy({index: r.desc('upvotes')})
        .limit(30)
        .run(connection, function(err, cursor) {
            if (err) throw err;
            cursor.toArray(function(err, result) {
                if (err) throw err;
                res.send(result);
            });
    });
});

app.post('/save-newsitem', function(req, res){

    var news_title = req.body.news_title;
    var news_url = req.body.news_url;

    r.db('newssharer').table('news_items').insert([
       {
        'title': news_title,
        'url': news_url,
        'upvotes': 100
       },
    ]).run(connection, function(err, result){
        if (err) throw err;
        res.send('ok');
    });

});

app.post('/upvote-newsitem', function(req, res){

    var id = req.body.news_id;
    var upvote_count = req.body.upvotes;

    r.db('newssharer').table('news_items')
        .filter(r.row('id').eq(id))
        .update({upvotes: upvote_count})
        .run(connection, function(err, result) {
            if (err) throw err;
            res.send('ok');
        });
});

app.get('/test/upvote', function(req, res){
    var id = '144f7d7d-d580-42b3-8704-8372e9b2a17c';
    var upvote_count = 350;

    r.db('newssharer').table('news_items')
        .filter(r.row('id').eq(id))
        .update({upvotes: upvote_count})
        .run(connection, function(err, result) {
            if (err) throw err;
            res.send('ok');
        });
});

app.get('/test/save-newsitem', function(req, res){

    r.db('newssharer').table('news_items').insert([
       {
        'title': 'banana',
        'url': 'http://banana.com',
        'upvotes': 190,
        'downvotes': 0
       },
    ]).run(connection, function(err, result){
        if(err) throw err;
        res.send('ok');
    });

});


server.listen(3000);

In the code above, first you import dependencies:

var r = require('rethinkdb');

var express = require('express');

var app = express();

var server = require('http').createServer(app);
var io = require('socket.io')(server);

var bodyParser = require('body-parser');
app.use(bodyParser.json());

Create the variable for storing the current RethinkDB connection.

var connection;

Listening for Changes

Connect to the RethinkDB database, by default RethinkDB runs on port 28015 so that is where you connect. If you’ve used a different port, replace 28015 with the port that you used.

r.connect({host: 'localhost', port: 28015}, function(err, conn) {
    if(err) throw err;
    connection = conn;

    ...

});

Still inside the database connection code, query the news_items table in the newssharer database, ordering the items by their upvotes count. Then use RethinkDB’s Changefeeds feature to listen for changes in the table (a database log of sorts). Every time a change happens in the table (CRUD operations), it’s notified of the change.

r.db('newssharer').table('news_items')
    .orderBy({index: r.desc('upvotes')})
    .changes()
    .run(connection, function(err, cursor){

        ...
});

Inside the callback function for the run method, initialize the socket connection and loop through the contents of the cursor. The cursor represents the changes made in the table. Every time a change occurs, it triggers the cursor.each function.

Note: The function doesn’t contain all the data changes. The previous changes gets replaced whenever a new change is made. This means that it only loops through a single row at any given time. This allows you to send the changes to the client using socket.io.

if (err) throw err; //check if there are errors and return it if any
io.sockets.on('connection', function(socket){
    cursor.each(function(err, row){
        if(err) throw err;
        io.sockets.emit('news_updated', row);   
    });
});

Each row has the following structure if a news item is shared:

{
  "old_val": null,
  "new_val": {
    "id": 1,
    "news_title": "Google",
    "news_url": "http://google.com",
    "upvotes": 0
  }
}

This is why you checked for null earlier, because a newly shared news item won’t have an old_val.

If a user upvotes a news item:

{
  "old_val": {
    "id": 1,
    "news_title": "Google",
    "news_url": "http://google.com",
    "upvotes": 0
  }
  "new_val": {
    "id": 1,
    "news_title": "Google",
    "news_url": "http://google.com",
    "upvotes": 1
  }
}

It returns both the whole structure for the old value and the new value of the row. This means you can update more than one field in one client and send those changes to all the other connected clients. RethinkDB makes implementing realtime apps simple with it’s changfeeds feature.

Adding an Index to the Upvotes Field

This is the route that adds an index to the upvotes field:

app.get('/add-index', function(req, res){
    r.db('newssharer').table('news_items').indexCreate('upvotes').run(connection, function(err, result){
        res.send('ok')
    });
});

You need this for the orderBy function to work, because it needs the field you are sorting with to have an index.

.orderBy({index: r.desc('upvotes')})

When the server is running, make sure to open http://localhost:3000/add-index in you browser before you test out the app. This route only needs to be called once.

Adding Dummy News Items

This route inserts dummy entries to the news_items table. This is optional for testing purposes so that you can see news items immediately without having to add them through the app.

app.get('/fill', function(req, res){
    r.db('newssharer').table('news_items').insert([
        {
            title: 'A Conversation About Fantasy User Interfaces',
            url: 'https://www.subtraction.com/2016/06/02/a-conversation-about-fantasy-user-interfaces/',
            upvotes: 30
        },
        {
            title: 'Apple Cloud Services Outage',
            url: 'https://www.apple.com/support/systemstatus/',
            upvotes: 20
        }
    ]).run(connection, function(err, result){
        if (err) throw err;
        res.send('news_items table was filled!');
    });
});

Returning News Items

This route returns the news items:

app.get('/news', function(req, res){
    res.header("Content-Type", "application/json");
    r.db('newssharer').table('news_items')
        .orderBy({index: r.desc('upvotes')})
        .limit(30)
        .run(connection, function(err, cursor) {
            if (err) throw err;
            cursor.toArray(function(err, result) {
                if (err) throw err;
                res.send(result);
            });
    });
});

The news items are ordered from the highest upvote count to the lowest and limited to 30. Instead of using cursor.each to loop through the news items, use cursor.toArray to convert it to an array with the following structure:

[
    {
        "title": "A Conversation About Fantasy User Interfaces",
        "url": "https://www.subtraction.com/2016/06/02/a-conversation-about-fantasy-user-interfaces/",
        "upvotes": 30
    },
    {
        "title": "Apple Cloud Services Outage",
        "url": "https://www.apple.com/support/systemstatus/",
        "upvotes": 20
    }    
]

Creating a News Item

This route saves a news item:

app.post('/save-newsitem', function(req, res){

    var news_title = req.body.news_title;
    var news_url = req.body.news_url;

    r.db('newssharer').table('news_items').insert([
       {
        'title': news_title,
        'url': news_url,
        'upvotes': 100
       },
    ]).run(connection, function(err, result){
        if (err) throw err;
        res.send('ok');
    });

});

This is called when a user shares a news item in the app. It accepts the news title and URL from the request body. The initial upvote count is set to 100 but you can pick another number.

Upvoting a News Item

This is the route for upvoting a news item:

app.post('/upvote-newsitem', function(req, res){

    var id = req.body.news_id;
    var upvote_count = req.body.upvotes;

    r.db('newssharer').table('news_items')
        .filter(r.row('id').eq(id))
        .update({upvotes: upvote_count})
        .run(connection, function(err, result) {
            if (err) throw err;
            res.send('ok');
        });
});

This is called when a user upvotes a news item in the app. It uses the ID of the news item to fetch and then update it.

Note: You’ve already incremented the upvotes inside the app so are supplying the value that’s in the request body.

Test Saving and Upvoting News Items

I’ve also included a couple of routes for testing saving and upvoting news items. The best time to access these is when the app is already running on your device. That way, you’ll see that the UI is updated. How to run the app will be covered in the next section.

This is the route for testing saving a news item:

app.get('/test/save-newsitem', function(req, res){

    r.db('newssharer').table('news_items').insert([
       {
        'title': 'banana',
        'url': 'http://banana.com',
        'upvotes': 190,
        'downvotes': 0
       },
    ]).run(connection, function(err, result){
        if(err) throw err;
        res.send('ok');
    });

});

And this is the route for testing the upvoting of a news item. Be sure to replace the ID with the ID of an existing news item to make it work.

app.get('/test/upvote', function(req, res){
    var id = '144f7d7d-d580-42b3-8704-8372e9b2a17c';
    var upvote_count = 350;

    r.db('newssharer').table('news_items')
        .filter(r.row('id').eq(id))
        .update({upvotes: upvote_count})
        .run(connection, function(err, result) {
            if (err) throw err;
            res.send('ok');
        });
});

Running the Server

At this point I assume that RethinkDB is still running in the background. Run it if it’s not already running. Once it’s running, execute node newssharer-server.js at the root of the project directory to run the server component of the app.

Running the App

You can run the app the same way you run any React Native app. Below are the links for running the app in the platform of your choice:

If you’re having a trouble running the app, you can check the Common Problems section on my previous article on Building an Android App with React Native.

Once the app is running, try it out or access any of the test routes in your browser.

What’s Next

Here are some suggestions for further improving the app:

And that’s it! In this tutorial you created a realtime news sharing app and learned how to use Socket.io and RethinkDB’s changefeeds to create a realtime app.

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