Nrthugu

So I'm trying to create an animation in Flutter that requires a different outcome every time the user presses a button.

I've implemented the following code according to the Flutter Animations tutorial and created a function to update it.

class _RoulettePageWidgetState extends State<RoulettePageWidget>

with SingleTickerProviderStateMixin {

   Animation<double> _animation;

   Tween<double> _tween;

   AnimationController _animationController;



   int position = 0;



   @override

   void initState() {

      super.initState();

      _animationController =

          AnimationController(duration: Duration(seconds: 2), vsync: this);

      _tween = Tween(begin: 0.0, end: 100.0);

      _animation = _tween.animate(_animationController)

          ..addListener(() {

              setState(() {});

          });

   }



   void setNewPosition(int newPosition) {

      _tween = Tween(

        begin: 0.0,

        end: math.pi*2/25*newPosition);

      _animationController.reset();

      _tween.animate(_animationController);

      _animationController.forward();

   }



   @override

   Widget build(BuildContext context) {

      return Container(

         child: Column(

            children: <Widget>[

               Center(

                  child: Transform.rotate(

                     angle: _animationController.value,

                     child: Icon(

                        Icons.arrow_upward,

                     size: 250.0,

                  ),

               )),

               Expanded(

                  child: Container(),

               ),

               RaisedButton(

                  child: Text('SPIN'),

                  onPressed: () {

                     setState(() {

                        setNewPosition(math.Random().nextInt(25));

                     });

                  },

               )

            ],

         )

      );

   }

}

As you can see I'm updating the _tween's begin: and end: but this doesn't seem to change the animation.

So what should I be doing to create a 'different' animation every time the users presses the button?

The general idea is to make the animations build upon each other with a random new value so for example:

• first spin: 0 -> 10


• second spin: 10 -> 13


• third spin: 13 -> 18


• ... etc

So I wondered if I could update the animation, or should I create a new animation every time?
Another thing I could think of was tracking the positions and use the same animation every time (0.0 -> 100.0) to act as a percentage of the transfer.

So instead of creating a new animation from 10 -> 15 I would be doing something like:
currentValue = 10 + (15-10)/100*_animationController.value

I'm going to skip your code a bit, and focus on what you're really asking:

The general idea is to make the animations build upon each other with a random new value so for example:

• first spin: 0 -> 10




• second spin: 10 -> 13




• third spin: 13 -> 18




• ... etc

With an explicit animation like this, there are three objects you are interested in:

• a controller, which is a special kind of Animation that simply generates values linearly from its lower to its upper bound (both doubles, typically 0.0 and 1.0). You can control the flow of the animation - send it running forward, reverse it, stop it, or reset it.




• a tween, which isn't an Animation but rather an Animatable. A tween defines the interpolation between two values, which don't even have to be numbers. It implements a transform method under the hood that takes in the current value of an animation and spits out the actual value you want to work with: another number, a color, a linear gradient, even a whole widget. This is what you should use to generate your angles of rotation.




• an animation, which is the animation whose value you're actually going to work with (so this is where you'd grab values to build with). You get this by giving your tween a parent Animation to transform - this might be your controller directly but can also be some other sort of animation you've built on it (like a CurvedAnimation, which would give you easing or bouncy/elastic curves and so on). Flutter's animations are highly composable that way.

Your code is failing largely because you're not actually using the top-level animation you created in your build method and you're creating a new tween and animation every time you call setNewPosition. You can use the same tween and animation for multiple animation "cycles" - simply change the begin and end properties of the existing tween and it bubbles up to the animation. That ends up something like this:

class _RoulettePageWidgetState extends State<RoulettePageWidget>

with SingleTickerProviderStateMixin {

   Animation<double> _animation;

   Tween<double> _tween;

   AnimationController _animationController;

   math.Random _random = math.Random();



   int position = 0;



   double getRandomAngle() {

      return math.pi * 2 / 25 * _random.nextInt(25);

   }



   @override

   void initState() {

      super.initState();

      _animationController =

          AnimationController(duration: Duration(seconds: 2), vsync: this);

      _tween = Tween(begin: 0.0, end: getRandomAngle());

      _animation = _tween.animate(_animationController)

          ..addListener(() {

              setState(() {});

          });

   }



   void setNewPosition() {

      _tween.begin = _tween.end;

      _animationController.reset();

      _tween.end = getRandomAngle();

      _animationController.forward();

   }



   @override

   Widget build(BuildContext context) {

      return Container(

         child: Column(

            children: <Widget>[

               Center(

                  child: Transform.rotate(

                     angle: _animation.value,

                     child: Icon(

                        Icons.arrow_upward,

                     size: 250.0,

                  ),

               )),

               Expanded(

                  child: Container(),

               ),

               RaisedButton(

                  child: Text('SPIN'),

                  onPressed: setNewPosition,

               )

            ],

         )

      );

   }

}

Hope that helps!

While working, in no situation will you actually want to make these animations within your layout as explained by @filleduchaos.

This is under optimized, as you're rebuilding far more than you should for the animation. And it's a pain to write yourself.

You'll want to use the AnimatedWidget family for this. They are divided into two
kinds:

• XXTransition


• AnimatedXX

The first is a low layer that consumes an Animation and listens to it so that you don't need to do that ugly :

..addListener(() {

  setState(() {});

});

The second handles the remaining pieces: AnimationController, TickerProvider and Tween.

This makes using animations much easier as it's almost entirely automatical.

In your case a rotation example would be as followed:

class RotationExample extends StatefulWidget {

  final Widget child;



  const RotationExample({

    Key key,

    this.child,

  }) : super(key: key);



  @override

  RotationExampleState createState() {

    return new RotationExampleState();

  }

}



class RotationExampleState extends State<RotationExample> {

  final _random = math.Random();

  double rad = 0.0;



  @override

  Widget build(BuildContext context) {

    return GestureDetector(

      onTap: _rotate,

      child: AnimatedTransform(

        duration: const Duration(seconds: 1),

        alignment: Alignment.center,

        transform: Matrix4.rotationZ(rad),

        child: Container(

          color: Colors.red,

          height: 42.0,

          width: 42.0,

        ),

      ),

    );

  }



  void _rotate() {

    setState(() {

      rad = math.pi * 2 / 25 * _random.nextInt(25);

    });

  }

}

Easier right?

The irony is that Flutter forgot to provide an AnimatedTransform (even although we have many others !). But no worries, I made it for you!

The AnimatedTransform implementation is as followed :

class AnimatedTransform extends ImplicitlyAnimatedWidget {

  final Matrix4 transform;

  final AlignmentGeometry alignment;

  final bool transformHitTests;

  final Offset origin;

  final Widget child;



  const AnimatedTransform({

    Key key,

    @required this.transform,

    @required Duration duration,

    this.alignment,

    this.transformHitTests = true,

    this.origin,

    this.child,

    Curve curve = Curves.linear,

  })  : assert(transform != null),

        assert(duration != null),

        super(

          key: key,

          duration: duration,

          curve: curve,

        );



  @override

  _AnimatedTransformState createState() => _AnimatedTransformState();

}



class _AnimatedTransformState

    extends AnimatedWidgetBaseState<AnimatedTransform> {

  Matrix4Tween _transform;



  @override

  void forEachTween(TweenVisitor<dynamic> visitor) {

    _transform = visitor(_transform, widget.transform,

        (dynamic value) => Matrix4Tween(begin: value));

  }



  @override

  Widget build(BuildContext context) {

    return Transform(

      alignment: widget.alignment,

      transform: _transform.evaluate(animation),

      transformHitTests: widget.transformHitTests,

      origin: widget.origin,

      child: widget.child,

    );

  }

}

I will submit a pull request so that in the future you won't need this bit of code.