Proper way of using WebSockets with React Native

Question

I'm new to React Native, but very familiar with React. As a beginner I'm looking to setup a connection between a cloud server and react-native with websockets as I've seen in the documentation. Unfortunately, there's no decent example out there that could help me out. This is all that I've got so far:

import React, { Component } from 'react';

import {
  AppRegistry,
  StyleSheet,
  Text,
  View,
  Button
} from 'react-native';

export default class raspberry extends Component {
  constructor(props) {
    super(props);

    this.state = { open: false };
    this.socket = new WebSocket('ws://127.0.0.1:3000');
    this.emit = this.emit.bind(this);
  }

  emit() {
    this.setState(prevState => ({ open: !prevState.open }))
    this.socket.send("It worked!")
  }

  render() {

    const LED = {
      backgroundColor: this.state.open ? 'lightgreen' : 'red',
      height: 30,
      position: 'absolute',
      flexDirection: 'row',
      bottom: 0,
      width: 100,
      height: 100,
      top: 120,
      borderRadius: 40,
      justifyContent: 'space-between'

    }

    return (
      <View style={styles.container}>
        <Button
          onPress={this.emit}
          title={this.state.open ? "Turn off" : "Turn on"}
          color="#21ba45"
          accessibilityLabel="Learn more about this purple button"
        />
        <View style={LED}></View>
      </View>
    );
  }

  componentDidMount() {
    this.socket.onopen = () => socket.send(JSON.stringify({ type: 'greet', payload: 'Hello Mr. Server!' }))
    this.socket.onmessage = ({ data }) => console.log(JSON.parse(data).payload)
  }

}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: 'center',
    alignItems: 'center',
    backgroundColor: '#F5FCFF',
  },
  welcome: {
    fontSize: 20,
    textAlign: 'center',
    margin: 10,
  },
  instructions: {
    textAlign: 'center',
    color: '#333333',
    marginBottom: 5,
  },
});

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

Everything works fine, but when I press the button to send a message, this is the error I get:

Cannot send a message. Unknown WebSocket id 1

I also made a test with a js client and everything worked smooth..looking to see how I could get this fixed or some example sources where I can figure it out.

Answer 1

change the code

socket.send(JSON.stringify({ type: 'greet', payload: 'Hello Mr. Server!' }))

to

this.socket.send(JSON.stringify({ type: 'greet', payload: 'Hello Mr. Server!' }))

it should work.

here is my code to test, based on your code and RN 0.45 (and project generated by create-react-native-app), connects to a public websocket server wss://echo.websocket.org/, on my android it works fine and I can see the websocket server's echo message after I push the button.

import React, { Component } from 'react';

import {
    StyleSheet,
    Text,
    View,
    Button
} from 'react-native';

export default class App extends React.Component {

    constructor() {
        super();

        this.state = {
            open: false
        };
        this.socket = new WebSocket('wss://echo.websocket.org/');
        this.emit = this.emit.bind(this);
    }

    emit() {
        this.setState(prevState => ({
            open: !prevState.open
        }))
        this.socket.send("It worked!")
    }

    componentDidMount() {
        this.socket.onopen = () => this.socket.send(JSON.stringify({type: 'greet', payload: 'Hello Mr. Server!'}));
        this.socket.onmessage = ({data}) => console.log(data);
    }

    render() {

        const LED = {
            backgroundColor: this.state.open
            ? 'lightgreen'
            : 'red',
            height: 30,
            position: 'absolute',
            flexDirection: 'row',
            bottom: 0,
            width: 100,
            height: 100,
            top: 120,
            borderRadius: 40,
            justifyContent: 'space-between'
        }

        return (
            <View style={styles.container}>
                <Button onPress={this.emit} title={this.state.open
        ? "Turn off"
        : "Turn on"} color="#21ba45" accessibilityLabel="Learn more about this purple button"/>
                <View style={LED}></View>
            </View>
        );
    }
}


const styles = StyleSheet.create({
    container: {
        flex: 1,
        justifyContent: 'center',
        alignItems: 'center',
        backgroundColor: '#F5FCFF'
    },
    welcome: {
        fontSize: 20,
        textAlign: 'center',
        margin: 10
    },
    instructions: {
        textAlign: 'center',
        color: '#333333',
        marginBottom: 5
    }
});

Answer 2

After I've done some researches I found that the WebSocket should be

new WebSocket("ws://10.0.2.2:PORT/")

where 10.0.2.2 means the localhost

Answer 3

According to documentation you need to add state connected to your component. And send anything only if connected state is true.

export default class raspberry extends Component {
  constructor(props) {
    super(props);
    this.state = {
      open: false,
      connected: false
    };
    this.socket = new WebSocket('ws://127.0.0.1:3000');
    this.socket.onopen = () => {
      this.setState({connected:true})
    }; 
    this.emit = this.emit.bind(this);
  }

  emit() {
    if( this.state.connected ) {
      this.socket.send("It worked!")
      this.setState(prevState => ({ open: !prevState.open }))
    }
  }
}