main.dart

class _MainPageState extends State<MainPage> with SingleTickerProviderStateMixin {
  ...
  void _onTap(Note note) {
    setState(() {
      note.state = NoteState.tapped; //<--set note state to tapped
    });
  }

  _drawLine(int lineNumber) {
    return Expanded(
      child: Line(
        lineNumber: lineNumber,
        currentNotes: notes.sublist(currentNoteIndex, currentNoteIndex + 5),
        onTileTap: _onTap, //<--specify onTap callback
        animation: animationController,
      ),
    );
  }
}