title: How subscribe to change variable?
body: <p>Please help my subscribe to changed variable. I make simply spinner. Spinner state(true|false) storage in service:</p>
  
  <pre><code>import { Injectable } from '@angular/core';
  import { Response, Headers, URLSearchParams } from '@angular/http';
  import { Observable } from 'rxjs/Observable';
  
  @Injectable()
  export class GlobalVarsService {
      private isVisibleSpinner: boolean = false;
  
    getSpinnerState(): Observable&lt;boolean&gt; {
      return this.isVisibleSpinner;
    }; 
  
    setSpinnerState(state): void {
      console.log('setSpinnerState', state);
      this.isVisibleSpinner = state;
    };    
  }
  </code></pre>
  
  <p>In component-template i display spinner via condition:</p>
  
  <pre><code>&lt;div class="nav"&gt;
    &lt;a [routerLink]="['/select']"&gt;select&lt;/a&gt;
    &lt;a [routerLink]="['/output']"&gt;output&lt;/a&gt;
  &lt;/div&gt;
  
  &lt;router-outlet&gt;&lt;/router-outlet&gt;
  
  &lt;div class="spinner-backdrop" *ngIf="isVisibleSpinner"&gt;&lt;/div&gt;
  &lt;div class="spinner-area" *ngIf="isVisibleSpinner"&gt;
      &lt;span class="spinner"&gt;loading...&lt;/span&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>In component i try subscribe to change in service isVisibleSpinner variable:</p>
  
  <pre><code>import { Component } from '@angular/core';
  import { Response } from '@angular/http';
  import 'rxjs/add/operator/map'
  import { GlobalVarsService } from './services/global-vars.service';
  
  @Component({
    selector: 'app-root',
    templateUrl: './app.component.html',
    styleUrls: ['./app.component.css']
  })
  export class AppComponent {
      private isVisibleSpinner: boolean;
  
    constructor(private globalVarsService: GlobalVarsService) {  
          this.globalVarsService.getSpinnerState().subscribe(data =&gt; {
            console.log(data);
            this.isVisibleSpinner = data;
          });     
    }
  }
  </code></pre>
  
  <p>But console output follow eror message:</p>
  
  <blockquote>
    <p>Type 'boolean' is not assignable to type 'Observable'.</p>
  </blockquote>