gradient_animation_vertical.dart

class GradientAnimation extends StatefulWidget {
  @override
  _GradientAnimationState createState() => _GradientAnimationState();
}

class _GradientAnimationState extends State<GradientAnimation> with SingleTickerProviderStateMixin {
  AnimationController _controller;
  final _max = 1.2;
  bool _covered = false;

  @override
  void initState() {
    _controller = AnimationController(
      vsync: this,
      upperBound: _max,
      duration: const Duration(milliseconds: 1500),
    );
    super.initState();
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    final size = MediaQuery.of(context).size;
    return Scaffold(
      extendBody: true,
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.animation),
        onPressed: () {
          if (!_covered) {
            _controller.forward();
          } else {
            _controller.reverse();
          }
          _covered = !_covered;
        },
      ),
      body: Stack(
        children: [
          Container(
            color: Colors.black,
          ),
          AnimatedBuilder(
            animation: _controller,
            builder: (context, _) {
              final value = _controller.value;
              return Positioned(
                bottom: 0,
                left: 0,
                width: size.width,
                height: size.height * value,
                child: Transform.translate(
                  offset: Offset(0.0, 100),
                  child: Container(
                    decoration: BoxDecoration(
                      color: Colors.white,
                      borderRadius: BorderRadius.vertical(
                        top: Radius.circular(size.width / _max * (_max - value)),
                      ),
                      boxShadow: [
                        BoxShadow(
                          blurRadius: 60,
                          spreadRadius: 70,
                          offset: Offset(0.0, 1.0),
                          color: Colors.white,
                        )
                      ],
                    ),
                  ),
                ),
              );
            },
          ),
        ],
      ),
    );
  }
}