Play & pause a Flutter animation

Short answer:

AnimationController _controller = ...;

// To stop animation
_controller.stop();

// To start from beginning
_controller.forward();

// To start from a point other than the very beginning.
_controller.forward(from: 0.8);

---

Long answer:

I wasn't aware of that code, here is how I did. All you need is Controller.reset() to stop the animation and Controller.repeat() to start it.

However if you need to start the animation just once, use Controller.forward() and Controller.reverse().

void main() => runApp(MaterialApp(home: Scaffold(body: HomePage())));

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> with SingleTickerProviderStateMixin {
  AnimationController _controller;
  bool _isPlaying = true;

  @override
  void initState() {
    super.initState();
    _controller = AnimationController(
      vsync: this,
      lowerBound: 0.3,
      duration: Duration(seconds: 3),
    )..repeat();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Animation")),
      body: Stack(
        alignment: Alignment.center,
        children: <Widget>[
          _buildCircularContainer(200),
          _buildCircularContainer(250),
          _buildCircularContainer(300),
          Align(child: CircleAvatar(backgroundImage: AssetImage("assets/images/profile.png"), radius: 72)),
          Align(
            alignment: Alignment(0, 0.5),
            child: RaisedButton(
              child: Text(_isPlaying ? "STOP" : "START"),
              onPressed: () {
                if (_isPlaying) _controller.reset();
                else _controller.repeat();
                setState(() => _isPlaying = !_isPlaying);
              },
            ),
          ),
        ],
      ),
    );
  }

  Widget _buildCircularContainer(double radius) {
    return AnimatedBuilder(
      animation: CurvedAnimation(parent: _controller, curve: Curves.fastLinearToSlowEaseIn),
      builder: (context, child) {
        return Container(
          width: _controller.value * radius,
          height: _controller.value * radius,
          decoration: BoxDecoration(color: Colors.black54.withOpacity(1 - _controller.value), shape: BoxShape.circle),
        );
      },
    );
  }
}

When you directly have access to the AnimationController this snippet will start/stop the animation, wherever it left off.

animationController.isAnimating
    ? animationController.stop()
    : animationController.forward();

Here the .isAnimating property is of type bool and is true when the animationController is animating at the moment. Depending on the result .stop()/.forward() will stop/start the animation respectively.