title: AnimationController smooth stop in flutter
body: <p>I have a spinning object in my flutter-app.
  When the user clickes on it, it is supose to slow down until it's stopped 100%.
  I can't make this happen, how do I do somthing like that?</p>
  <pre><code> AnimationController angleController;
  
    slowDown () {
      setState(() {
        rotationSpeed += 180;
        Future.delayed(Duration(milliseconds: 100), () {
          slowDown ();
        });
      });
    }
  
    @override
    void initState() {
      super.initState();
      angleController = AnimationController(vsync: this, duration: Duration(milliseconds: rotationSpeed));
      angleController .addListener(() {
        setState(() {
          angle = angleController.value * rotationAngle / 360 * pi * 2;
        });
      });
    }
  </code></pre>