Flutter TableCalendar . SharedPreferences - flutter

Question

I want to save the table calendar data.
First of all, I succeeded in creating data for each date, but I don't know how to use sharedPreferences.
I don't know what it is even if I googled it and applied it in various ways.
https://medium.flutterdevs.com/display-dynamic-events-at-calendar-in-flutter-22b69b29daf6
I think the code that's closest to the answer is this person's method
If you apply it and make some changes, you will encounter various bugs.
I spent almost a week on this for three hours. It's so hard that I just want to solve it today.
I have to work out, and I have a lot of other assignments
Because of the desire to keep solving this problem, it doesn't get out of the chair.
My personality is getting dirty after meeting a problem that hasn't been solved for a week.
I beg you.
class DiaryService extends ChangeNotifier {
DateTime focusedDay = DateTime.now();
DateTime selectedDay = DateTime.now();
TextEditingController createFieldController = TextEditingController();
TextEditingController updateFieldController = TextEditingController();
List<Diary> diaryList = [];
Map<DateTime, List<Diary>> diaryMap = {};
List<Diary> getByDate(DateTime date) {
return diaryMap[date] ?? [];
}
void create(String text, DateTime selectedDate) {
diaryList.add(Diary(text: text, createdAt: DateTime.now(), selectedDate:
selectedDate));
diaryMap[selectedDate] = diaryList.where((element) {
return element.selectedDate == selectedDate;
}).toList();
print(selectedDate.day);
notifyListeners();
}
void update(DateTime createdAt, String newContent) {
int t = diaryList.indexWhere((element) => element.createdAt == createdAt);
diaryList.removeWhere((element) => element.createdAt == createdAt);
diaryList.insert(t, Diary(text: newContent, createdAt: createdAt,
selectedDate: selectedDay));
addListInMap();
notifyListeners();
}
void delete(DateTime createdAt) {
diaryList.removeWhere((element) => element.createdAt == createdAt);
addListInMap();
print(getByDate);
print(diaryMap);
notifyListeners();
}
void addListInMap() {
diaryMap[selectedDay] = diaryList.where((element) => element.selectedDate
== selectedDay).toList();
}
void encodeData() async {
String dd = Diary.encode(diaryList);
SharedPreferences prefs = await SharedPreferences.getInstance();
prefs.setString("diaryList", dd);
notifyListeners();
}
Future<List<Diary>> decodeData() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
String getData = prefs.getString("diaryList") ?? "[]";
print(getData);
diaryList = Diary.decode(getData);
notifyListeners();
return diaryList;
}
}