title: refresh activity from service after notification
body: <p>I have a service</p>
  
  <pre><code>public class GcmIntentService extends IntentService{...}
  </code></pre>
  
  <p>that manages  the notifications.
  When a notification arrives if the user is out of the application, with a tap on the notification the main activity is resumed and updated</p>
  
  <pre><code>public class MainActivity extends Activity {
  
  ...
   lv = (ListView) findViewById(R.id.lv);
  adapter = new Adapter(this, item);
  lv .setAdapter(adapter);
  
  ...
  }
  </code></pre>
  
  <p>but how can I update the activity if the user is already on it?</p>