title: how to get AttributeSet properties
body: <p>Suppose i have a class which extends ViewGroup</p>
  
  <pre><code>public class MapView extends ViewGroup
  </code></pre>
  
  <p>It is included in the layout <code>map_controls.xml</code> like this</p>
  
  <pre><code>&lt;com.xxx.map.MapView
      android:id="@+id/map"
      android:background="@drawable/address"
      android:layout_width="fill_parent"
      android:layout_height="fill_parent"&gt;
  &lt;/com.xxx.map.MapView&gt;
  </code></pre>
  
  <p>How do i retrieve properties in the constructor from AttributeSet ? Let's say the drawable in the background field.</p>
  
  <pre><code>public MapView(Context context, AttributeSet attrs) {
  }
  </code></pre>