title: How to animate a widget when it becomes visible in flutter?
body: <p>I want to add some animations to some widgets in my app, but these widget are not visible until the user scroll down.</p>
  
  <p>I'd like for these animations to happen when the widget becomes visible, Generally I'd start the animation in <code>initState</code> but this only make the animation when the widget is first drawn, not when it becomes visible to the user .</p>
  
  <p>Is there any such event to listen to ? If not I imagine I can play with Viewport or aspect ratio or MediaQuery?</p>