CrossfadeTitle.dart

class CrossfadeTitle extends AnimatedWidget {
  final Widget primaryTitle;
  final Widget secondaryTitle;

  const CrossfadeTitle({
    Key key,
    Listenable listenable,
    this.primaryTitle,
    this.secondaryTitle,
  }) : super(key: key, listenable: listenable);

  @override
  Widget build(BuildContext context) {
    final Animation<double> animation = this.listenable;
    return DefaultTextStyle(
      style: Theme.of(context).primaryTextTheme.title,
      softWrap: false,
      overflow: TextOverflow.ellipsis,
      child: Stack(
        children: <Widget>[
          Opacity(
            opacity: CurvedAnimation(
              parent: ReverseAnimation(animation),
              curve: Interval(0.5, 1.0),
            ).value,
            child: secondaryTitle,
          ),
          Opacity(
            opacity: CurvedAnimation(
              parent: animation,
              curve: Interval(0.5, 1.0),
            ).value,
            child: primaryTitle,
          ),
        ],
      ),
    );
  }
}