import 'package:flutter/material.dart';
import 'package:sticky_headers/sticky_headers.dart';

Map _months = {
  1:'January',
  2:'February',
  3:'March',
  4:'April',
  5:'May',
  6:'June',
  7:'July',
  8:'August',
  9:'September',
  10:'October',
  11:'November',
  12:'December',
};

abstract class BasePage extends StatefulWidget {
  BasePage({Key? key}) : super(key: key);
}

abstract class BaseState<Page extends BasePage> extends State<Page> {
  String screenName();
}
class CalendarPage extends BasePage {
  CalendarPage({Key? key}) : super(key: key);
  @override
  _CalendarPageState createState() => _CalendarPageState();
}


class _CalendarPageState extends BaseState<CalendarPage>{
  _CalendarPageState();


  Key centerKey = ValueKey<String>('center');
  var date = DateTime.now();


 Widget monthBuilder (BuildContext context, int plusMinus){
    int count = (date.month - 13) * -1;
    if(plusMinus==-1){count=date.month-1;}
    return SliverList(
      key: plusMinus==1 ? centerKey:null,
      delegate: SliverChildBuilderDelegate(
        (BuildContext context, int index) {
          int month=date.month+index;
          if(plusMinus==-1){month=date.month-1+(-1*index);}
          return StickyHeader(
            header: Container(
              color: Colors.brown,
              padding: EdgeInsets.symmetric(vertical: 12.0),
              alignment: Alignment.center,
              child: Text('${_months[month]}',
                style: Theme.of(context).textTheme.headline2,
              ),
            ),
            content: ListView.builder(
              shrinkWrap: true,
              itemCount: DateTime(date.year,month+1,0).day,//check last day of month for days,
              itemBuilder: (context, int index) {
                return Column(children: [
                  Divider(),
                  TextButton( onPressed: () {  },
                  child: Text('Data'),),
                ]);
              },
            ),
          );
        },
        childCount: count,
      )
    );
  }

  @override
  String screenName() => "Calendar ${date.year}";

  @override
  Widget build(BuildContext context) {
    return CustomScrollView(
        center: centerKey,
        slivers: [
          monthBuilder(context, -1),
          monthBuilder(context, 1),
        ],
      );
  }
}