title: Callback to activity from other class
body: <p>I have  <code>Activity</code> class, <code>Controller</code> class (normal java class use to control number of activity) and <code>BusinessEngine</code> class (normal java class use to process data).</p>
  
  <p>When I need to do some calculation from activity, <code>Activity</code> will call <code>Controller</code> and <code>Controller</code> will call <code>BusinessEngine</code> to do the calculation. When <code>BusinessEngine</code> done with the calculation, it will pass the value back to <code>Controller</code> and finally let the activity know the calculation is complete.</p>
  
  <p>The problem is how I callback <code>Activity</code> from <code>Controller</code> class? Or pass any data to <code>Activity</code> and notify it the data has been change?</p>