price_tab.dart

class _PriceTabState extends State<PriceTab> with TickerProviderStateMixin {
  final double _initialPlanePaddingBottom = 16.0;
  final double _minPlanePaddingTop = 16.0;

  AnimationController _planeSizeAnimationController;
  AnimationController _planeTravelController;
  Animation _planeSizeAnimation;
  Animation _planeTravelAnimation;

  double get _planeTopPadding =>
      _minPlanePaddingTop +
      (1 - _planeTravelAnimation.value) * _maxPlaneTopPadding;

  double get _maxPlaneTopPadding =>
      widget.height - _initialPlanePaddingBottom - _planeSize;

  double get _planeSize => _planeSizeAnimation.value;

  @override
  void initState() {
    super.initState();
    _initSizeAnimations();
    _initPlaneTravelAnimations();
    _planeSizeAnimationController.forward();
  }

  Widget _buildPlane() {
    return AnimatedBuilder(
      animation: _planeTravelAnimation,
      child: Column(
        children: <Widget>[
          AnimatedPlaneIcon(animation: _planeSizeAnimation),
        ],
      ),
      builder: (context, child) => Positioned(
            top: _planeTopPadding,
            child: child,
          ),
    );
  }

  _initSizeAnimations() {
    _planeSizeAnimationController = AnimationController(
      duration: const Duration(milliseconds: 340),
      vsync: this,
    )..addStatusListener((status) {
        if (status == AnimationStatus.completed) {
          Future.delayed(Duration(milliseconds: 500),
              () => _planeTravelController.forward());
        }
      });
    [...]
  }

  _initPlaneTravelAnimations() {
    _planeTravelController = AnimationController(
      vsync: this,
      duration: const Duration(milliseconds: 400),
    );
    _planeTravelAnimation = CurvedAnimation(
      parent: _planeTravelController,
      curve: Curves.fastOutSlowIn,
    );
  }
}