Is there a way to know if rxjs websocket is opened

I am using RxJS in an angular 4 project.

I am trying to initiate a Websocket, and especially to know when this one is opened.

I am currently using WebSocket from RxJS (v5). https://github.com/ReactiveX/rxjs/blob/master/src/observable/dom/WebSocketSubject.ts

I noticed there is an openObserver in WebSocketSubjectConfig but I cannot find how to create the Observer. I have been locked on it from several hours.

Here is an excerpt of my code so far:

import { Injectable } from '@angular/core';
import { webSocket} from 'rxjs/observable/dom/webSocket';
import { WebSocketSubject, WebSocketSubjectConfig} from 'rxjs/observable/dom/WebSocketSubject';

@Injectable()
export class MzkWebsocketJsonRpcService {
    subject: WebSocketSubject<any>;
    jsonRpcId: number;

    constructor() {

        this.subject = webSocket('ws://localhost:12345');
        this.subject.openObserver =
            /// Find a way to create the openObserver


        this.subject.subscribe(
            this.onMessage,
            this.onError,
            this.onClose,
        );
        console.log('Socket connected');
        this.jsonRpcId = 1;
    }

    public send(method: string, params: any[]) {

        let jsonFrame: any = {id: this.jsonRpcId, 'json-rpc': '2.0', method: method};

        if (params) {
            jsonFrame['params'] = params;
        }
        this.subject.next(JSON.stringify(jsonFrame));
        this.jsonRpcId ++;
    }

    onMessage(data: string) {
        console.log('Websocket message: ', data);
    }

    onError(data: string) {
        console.log('Websocket error:', data);
    }

    onClose() {
        console.log('Websocket closing');
    }
}