// Each animation defined here transforms its value during the subset
    // of the controller's duration defined by the animation's interval.
    // For example the opacity animation transforms its value during
    // the first 10% of the controller's duration.

opacity = Tween<double>(
      begin: 0.0,
      end: 1.0,
    ).animate(
      CurvedAnimation(
        parent: controller,
        curve: Interval(
          0.0, 0.100,
          curve: Curves.ease,
        ),
      ),
    ),

    // ... Other tween definitions ...

    super(key: key);

final AnimationController controller;

final Animation<double> opacity;

final Animation<double> width;

final Animation<double> height;

final Animation<EdgeInsets> padding;