title: call a method on a fragment in a ViewPager
body: <p>All I want to do is calling a function of my Fragment's class. But I can't seem to find a way to access the instance of my fragments which are in a ViewPager.</p>
  
  <p>my activity's xml:</p>
  
  <pre><code>&lt;android.support.v4.view.ViewPager xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:tools="http://schemas.android.com/tools"
      android:id="@+id/pager"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      tools:context=".MainActivity" /&gt;
  </code></pre>
  
  <p>There is no  so I can't call findFragmentById() (can I?)
  also I don't know what to give to findFragmentByTag(), it always return null :/</p>