title: Differences between getRootNav() and navCtrl() methods
body: <p>Can you tell me what are the differences of below 2 methods? Which moment should I use it?</p>
  
  <p><em>book.ts</em></p>
  
  <pre><code>this.app.getRootNav().push('FromBook', { bookId: this.data.id })
  
  this.navCtrl.push('FromBook', { bookId: this.data.id });
  </code></pre>
  
  <p>When we use inner components like below sometimes it works.Sometimes it is not.Why this kind of different behavior with above 2 navigation methods?</p>
  
  <p><em>author-page.html</em></p>
  
  <pre><code>&lt;div&gt;
          &lt;book *ngFor="let book of authorData?.books" [data]="book"&gt;&lt;/book&gt;
  &lt;/div&gt;
  </code></pre>