Basic WebSocket implementaion

Downgrade to Socket.io 2.3.0

src/app/backend.service.ts

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+import { HttpClient } from '@angular/common/http';
+import { Injectable } from '@angular/core';
+import { Socket } from 'ngx-socket-io';
+
+/*
+ * The following interfaces are copied from the backend.
+ * 
+ * Checkout src/helper/types.ts and src/models/invoice/invoice.interface.ts
+ * (in the backend repository) for more information.
+ */
+export enum CryptoUnits {
+  BITCOIN = 'BTC',
+  BITCOINCASH = 'BCH',
+  ETHEREUM = 'ETH',
+  LITECOIN = 'LTC',
+  DOGECOIN = 'DOGE',
+  MONERO = 'XMR'
+}
+
+export interface ICart {
+  price: number;
+  name: string;
+  image: string;
+  quantity: number;
+}
+
+export interface IPaymentMethod {
+  method: any;
+  amount: number
+}
+
+export enum PaymentStatus {
+  PENDING = 0,
+  PARTIALLY = 1,
+  UNCONFIRMED = 2,
+  DONE = 3,
+  CANCELLED = 4
+}
+
+export interface IInvoice {
+  selector: string;
+  paymentMethods: IPaymentMethod[];
+  receiveAddress: string;
+  paidWith?: CryptoUnits;
+  paid?: number;
+  transcationHashes?: string[];
+  cart?: ICart[];
+  totalPrice?: number;
+  currency: string;
+  dueBy: number;
+  status?: PaymentStatus;
+  email?: string;
+  successUrl: string;
+  cancelUrl: string;
+  createdAt?: number;
+}
+
+@Injectable({
+  providedIn: 'root'
+})
+export class BackendService {
+
+  SERVER_URL = 'http://localhost:2009';
+
+  constructor(
+    private socket: Socket,
+    private http: HttpClient
+  ) {
+    this.socket.on('status', (data: any) => {
+      console.log('Status has been updated to: ', data);
+    });
+    this.socket.on('subscribe', (success: boolean) => {
+      if (success) { console.log('We\'re getting the progress of this invoice!'); }
+      else { console.log('Subscription failed'); }
+    });
+  }
+
+  subscribeTo(selector: string) {
+    this.socket.emit('subscribe', { selector });
+  }
+
+  getInvoice(selector: string): Promise<IInvoice> {
+    return new Promise(async (resolve, reject) => {
+      this.http.get(this.SERVER_URL + '/invoice/' + selector, {
+        observe: 'body',
+        responseType: 'json'
+      }).toPromise().then((invoice) => {
+        resolve(invoice as IInvoice);
+      }).catch(err => {
+        reject(err);
+      });
+    });
+  }
+}