Masking/Transition

transitionmaskingbuild.dart

@override
  Widget build(BuildContext context) {
    return AnimatedBuilder(
      animation: _controller,
      builder: (BuildContext context, Widget child) {
        return Scaffold(
            body: new Center(
                child: new Stack(
                  children: <Widget>[
                    new Center(
                        child: Container(
                          width: 200.0,
                          height: 200.0,
                          color: Colors.black12,
                        )),
                    new Center(
                        child: Container(
                          alignment: Alignment.bottomCenter,
                          width: transitionTween.value,
                          height: transitionTween.value,
                          decoration: BoxDecoration(
                            color: Colors.black12,
                            borderRadius: borderRadius.value,
                          ),
                        )),
                  ],
                )));
      },
    );
  }