title: In FireStore how to know if client is listening to particular document
body: <p>I have the following scenario.</p>
  <ol>
  <li>Sensor generates data every 30 seconds.</li>
  <li>Update sensor data on FireStore document.</li>
  <li>Mobile app shows the sensor data in realtime.</li>
  </ol>
  <p>One problem with above steps is that lot of data writes takes place, which increases billing. I want to optimize it by changing step 2.</p>
  <ol start="2">
  <li>Update sensor data on FireStore document <strong>only if mobile app is listening to realtime updates</strong></li>
  </ol>
  <p>To do this optimisation I need a way to know if app is listening to realtime update. So is there a FireStore event/callback fired when app starts/stops to listen to realtime update.</p>