WebSocket in Angular 4

Question

I am creating a chat application with Angular 4 and websocket. For this, I've followed this Angular websocket tutorial

Here is the WebsocketService source code:

import { Injectable } from '@angular/core';
import * as Rx from 'rxjs/Rx';

@Injectable()
export class WebsocketService {
  constructor() { }

  private subject: Rx.Subject<MessageEvent>;

  public connect(url): Rx.Subject<MessageEvent> {
    if (!this.subject) {
      this.subject = this.create(url);
      console.log("Successfully connected: " + url);
    } 
    return this.subject;
  }

  private create(url): Rx.Subject<MessageEvent> {
    let ws = new WebSocket(url);

    let observable = Rx.Observable.create(
    (obs: Rx.Observer<MessageEvent>) => {
        ws.onmessage = obs.next.bind(obs);
        ws.onerror = obs.error.bind(obs);
        ws.onclose = obs.complete.bind(obs);
        return ws.close.bind(ws);
    })
let observer = {
        next: (data: Object) => {
            if (ws.readyState === WebSocket.OPEN) {
                ws.send(JSON.stringify(data));
            }
        }
    }
    return Rx.Subject.create(observer, observable);
  }

}

And it is my ChatService:

import { Injectable } from '@angular/core';
import { Observable, Subject } from 'rxjs/Rx';
import { WebsocketService } from './websocket.service';

const CHAT_URL = 'ws://echo.websocket.org/';

export interface Message {
    author: string,
    message: string
}

@Injectable()
export class ChatService {
    public messages: Subject<Message>;

    constructor(wsService: WebsocketService) {
        this.messages = <Subject<Message>>wsService
            .connect(CHAT_URL)
            .map((response: MessageEvent): Message => {
                let data = JSON.parse(response.data);
                return {
                    author: data.author,
                    message: data.message
                }
            });
    }
}

It works fine, but I want to detect the connection status. I want to know if the connection has been interrupted or the server is down.

For that, I tried to implement an isServerOn() function inside the WebsocketService class like this:

isServerOn(): Observable<boolean> {
    return Observable.of(!!this.subject);
}

But it has not resolve the problem. Is there anyone who has encourtered the same problem?

Thank you in advance.

what is "this.server" ? i doubt that it has a type of which it is possible to create an observable which does what you want by just calling Observable.of on it. — Patrick Kelleter
– Patrick Kelleter, Commented Dec 18, 2017 at 13:12

luixaviles · Accepted Answer · 2018-01-05 19:11:48Z

I suggest you use type definitions for socket.io-client in your Angular application. Then define a service as follows:

import { Injectable } from '@angular/core';
import { Observable } from 'rxjs/Observable';
import { Observer } from 'rxjs/Observer';
import { Message } from '../model/message';
import { Event } from '../model/event';

import * as socketIo from 'socket.io-client';

const SERVER_URL = 'https://yourserverhost.com';

@Injectable()
export class SocketService {
    private socket;

    public initSocket(): void {
        this.socket = socketIo(SERVER_URL);
    }

    public send(message: Message): void {
        this.socket.emit('message', message);
    }

    public onEvent(event: Event): Observable<any> {
        return new Observable<Event>(observer => {
            this.socket.on(event, () => observer.next());
        });
    }
}

Define an Event enum:

export enum Event {
    CONNECT = 'connect',
    DISCONNECT = 'disconnect'
}

Then subscribe to your service functions from your Angular component:

export class ChatComponent implements OnInit {
  constructor(private socketService: SocketService) { }

   ngOnInit(): void {
    this.initIoConnection();
  }

  private initIoConnection(): void {
    this.socketService.initSocket();

    this.ioConnection = this.socketService.onMessage()
      .subscribe((message: Message) => {
        this.messages.push(message);
      });


    this.socketService.onEvent(Event.CONNECT)
      .subscribe(() => {
        console.log('Connected to the server');
      });

    this.socketService.onEvent(Event.DISCONNECT)
      .subscribe(() => {
        console.log('Disconnected');
      });
  }
}

Find the complete chat project, which is using Node.js, WebSockets and Angular here: https://github.com/luixaviles/socket-io-typescript-chat

Patrick Kelleter · Accepted Answer · 2017-12-19 08:37:09Z

1

I am not sure what you want to achieve with

 return Observable.of(!!this.subject);

I do not think that it does what you think it does. Instead you should rather create your own BehaviorSubject and return the corresponding Observable like

isServerOn(): Observable<boolean> {
    return this.myServerOnSubject.asObservable();
}

At the corresponding online/offline code positions you then can emit the next value with

this.myServerOnSubject.next(true/false);

answered Dec 19, 2017 at 8:37

Patrick Kelleter

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