title: What's tools:layout in fragment xml file?
body: <p>I started a new Android Application based on master/detail flow template using ADT Eclipse. This template creates two activities, a master fragment and a detail fragment in order to fit small and larger screens.</p>
  
  <p>I noticed the <code>activity_item_list.xml</code> file has the <code>tools:layout</code> attribute:</p>
  
  <pre><code>&lt;fragment xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:tools="http://schemas.android.com/tools"
      android:id="@+id/item_list"
      android:name="com.example.fragmenttwopanel.ItemListFragment"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:layout_marginLeft="16dp"
      android:layout_marginRight="16dp"
      tools:context=".ItemListActivity"
      tools:layout="@android:layout/list_content" /&gt;
  </code></pre>
  
  <p>I tried to delete the attribute and the app runs in the same way but in the graphical layout tab of ADT, a message ask me for: </p>
  
  <blockquote>
    <p>pick preview layout from the "Fragment Layout" context menu</p>
  </blockquote>
  
  <p>What is its purpose? Is it just for preview in graphical layout?</p>