Create initial version of WebSocket service

src/app/websocket.service.spec.ts

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+import { TestBed, inject } from '@angular/core/testing';
+
+import { WebSocketService } from './websocket.service';
+
+describe('WebSocketService', () => {
+  beforeEach(() => {
+    TestBed.configureTestingModule({
+      providers: [WebSocketService]
+    });
+  });
+
+  it('should be created', inject([WebSocketService], (service: WebSocketService) => {
+    expect(service).toBeTruthy();
+  }));
+});

src/app/websocket.service.ts

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+import { Injectable } from '@angular/core';
+import { QueueingSubject } from 'queueing-subject';
+import { Observable } from 'rxjs/Observable';
+import websocketConnect from 'rxjs-websockets';
+import { WSMessage } from './wsmessage';
+import 'rxjs/add/operator/map';
+import 'rxjs/add/operator/filter';
+import 'rxjs/add/operator/share';
+
+
+function jsonWebsocketConnect(url: string, input: Observable<object>, protocols?: string | string[]) {
+  const jsonInput = input.map(message => JSON.stringify(message));
+  const { connectionStatus, messages } = websocketConnect(url, jsonInput, protocols);
+  const jsonMessages = messages.map(message => JSON.parse(message));
+  return { connectionStatus, messages: jsonMessages };
+}
+
+@Injectable()
+export class WebSocketService {
+  private uplink: QueueingSubject<object>;
+  public downlink: Observable<WSMessage>;
+
+  constructor() {
+
+  }
+
+  public connect() {
+    if (this.downlink) {
+      return;
+    }
+
+    // Using share() causes a single WebSocket to be created when the first
+    // observer subscribes. This socket is shared with subsequent observers
+    // and closed when the observer count falls to zero.
+    this.downlink = jsonWebsocketConnect(
+      'ws://localhost:4242/ws',
+      this.uplink = new QueueingSubject<object>()
+    ).messages.map(message => <WSMessage> message).share();
+    console.log('ws connected');
+  }
+
+  public observeAnchor(anchor: string): Observable<WSMessage> {
+    return this.downlink.filter(message => message.anchor === anchor);
+  }
+
+  public send(anchor: string, data: any): void {
+    // If the WebSocket is not connected then the QueueingSubject will ensure
+    // that messages are queued and delivered when the WebSocket reconnects.
+    // A regular Subject can be used to discard messages sent when the WebSocket
+    // is disconnected.
+    this.uplink.next({
+      'anchor': anchor,
+      'data': data
+    });
+  }
+}

src/app/wsmessage.ts

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+export class WSMessage {
+  anchor: string;
+  data: any;  // TODO: sane annotation
+}