title: angular2 style guide - property with dollar sign?
body: <p><a href="https://angular.io/guide/component-interaction#parent-and-children-communicate-via-a-service" rel="noreferrer">Parent and children communicate via a service example</a> from the official guide on <a href="https://angular.io" rel="noreferrer">Angular.io</a> makes use of dollar signs in Observable stream names.</p>
  <p>Notice <code>missionAnnounced$</code> and <code>missionConfirmed$</code> in the following example:</p>
  <pre class="lang-javascript prettyprint-override"><code>import { Injectable } from '@angular/core';
  import { Subject } from 'rxjs';
  
  @Injectable()
  export class MissionService {
  
    // Observable string sources
    private missionAnnouncedSource = new Subject&lt;string&gt;();
    private missionConfirmedSource = new Subject&lt;string&gt;();
  
    // Observable string streams
    missionAnnounced$ = this.missionAnnouncedSource.asObservable();
    missionConfirmed$ = this.missionConfirmedSource.asObservable();
  
    // Service message commands
    announceMission(mission: string) {
      this.missionAnnouncedSource.next(mission);
    }
  
    confirmMission(astronaut: string) {
      this.missionConfirmedSource.next(astronaut);
    }
  }
  </code></pre>
  <p>Can anyone explain:</p>
  <ul>
  <li>why <code>$</code> is used? What's the reason behind this notation? Do I always need to use this for public properties?</li>
  <li>public properties are used but not methods (e.g. missionAnnouncements(), missionConfirmations()) - again, is this a convention for Angular2 apps?</li>
  </ul>