main.dart

class _MainPageState extends State<MainPage> with SingleTickerProviderStateMixin {
  ...
  
  @override
  void initState() {
    super.initState();
    animationController = AnimationController(vsync: this, duration: Duration(milliseconds: 300));
    animationController.addStatusListener((status) {
      if (status == AnimationStatus.completed && isPlaying) {
        if (notes[currentNoteIndex].state != NoteState.tapped) {
          //game over
          setState(() {
            isPlaying = false;
            notes[currentNoteIndex].state = NoteState.missed;
          });
          animationController.reverse().then((_) => _showFinishDialog()); //<--Show dialog when reverse animaiton finishes
        } else if (currentNoteIndex == notes.length - 5) {
          //song finished
          _showFinishDialog(); //<--Show dialog when song is finished
        } else {
          setState(() => ++currentNoteIndex);
          animationController.forward(from: 0);
        }
      }
    });
  }

  void _showFinishDialog() {
    showDialog( //<--Show a dialog
      context: context,
      builder: (context) {
        return AlertDialog(
          title: Text("Score: $points"), //<--With number of points
          actions: <Widget>[
            FlatButton(                 //<-- and restart button
              onPressed: () => Navigator.of(context).pop(),
              child: Text("RESTART"),
            ),
          ],
        );
      },
    );
  }
}