final Animation<EdgeInsets> padding;

final Animation<BorderRadius> borderRadius;

final Animation<Color> color;

  // This function is called each time the controller "ticks" a new frame.
  // When it runs, all of the animation's values will have been
  // updated to reflect the controller's current value.

Widget _buildAnimation(BuildContext context, Widget child) {
    return Container(
      padding: padding.value,
      alignment: Alignment.bottomCenter,
      child: Opacity(
        opacity: opacity.value,
        child: Container(
          width: width.value,
          height: height.value,
          decoration: BoxDecoration(
            color: color.value,
            border: Border.all(
              color: Colors.indigo[300],
              width: 3.0,
            ),
            borderRadius: borderRadius.value,
          ),
        ),
      ),
    );
  }