main.dart

class _MainPageState extends State<MainPage> with SingleTickerProviderStateMixin {
  List<Note> notes = initNotes();
  int currentNoteIndex = 0;
  AnimationController animationController;

  @override
  void initState() {
    super.initState();
    animationController =
        AnimationController(vsync: this, duration: Duration(milliseconds: 300)); //<--- Init the AnimationController
    animationController.addStatusListener((status) {
      if (status == AnimationStatus.completed) { //<--- Whenever animation finishes
        if (currentNoteIndex == notes.length - 4) {
          //song finished
        } else {                                //<--- If the song didn't end
          setState(() => ++currentNoteIndex);   //<--- Increase currentNoteIndex
          animationController.forward(from: 0); //<--- And start over the animation
        }
      }
    });
    animationController.forward(); //<--- start the animation for the first time
  }

  @override
  void dispose() {
    animationController.dispose(); //<--- remember to dispose the controller
    super.dispose();
  }
  
  _drawLine(int lineNumber) {
    return Expanded(
      child: Line(
        lineNumber: lineNumber,
        currentNotes: notes.sublist(currentNoteIndex, currentNoteIndex + 4), //<--- use currentNoteIndex
      ),
    );
  }
  ...
}