title: "realtime" ratingbar indicator in Flutter
body: <p>I’m new to Flutter and programming in general, so maybe my question is stupid, 
  but I need some help. On my app I have a screen with 4 ratingbars. (I use the ratingbar package from pub.dev)</p>
  
  <p>When you set a rating on these ratingbars the value of the rating is saved into a variable. 
  Now I have a ratingbar indicator on the same screen and i want that this indicator shows the average rating of the 4 ratingbars above. I tried that the indicator shows the 4 variables divided by 4 but in that case the indicator doesn’t update if i change the rating on one of the 4 ratingbars… </p>
  
  <p>so i want the indicator to listen in “realtime” if the variables change and then show a different rating… how is that possible?
  I hope someone understand my problem and is able to help :)
  Greetings, Yoqora</p>