pskink gist for calendar

main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.light(),
      darkTheme: ThemeData.dark(),
      themeMode: ThemeMode.system,
      home: const Home(),
    );
  }
}

class Home extends StatelessWidget {
  const Home({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(body: PagedCalendar());
  }
}

class PagedCalendar extends StatefulWidget {
  @override
  State<PagedCalendar> createState() => _PagedCalendarState();
}

class _PagedCalendarState extends State<PagedCalendar> {
  late Map<int, Widget> _days;
  int month = 1;

  @override
  void initState() {
    super.initState();

    const padding = EdgeInsets.all(2);
    final style = ButtonStyle(
      foregroundColor: MaterialStateProperty.all(Colors.black87),
      textStyle: MaterialStateProperty.all(
          const TextStyle(fontWeight: FontWeight.bold)),
      side: MaterialStateProperty.all(
          const BorderSide(width: 3, color: Colors.black45)),
      padding: MaterialStateProperty.all(EdgeInsets.zero),
      shape: MaterialStateProperty.all(
          RoundedRectangleBorder(borderRadius: BorderRadius.circular(12))),
    );
    final disabledStyle = ButtonStyle(
      foregroundColor: MaterialStateProperty.all(Colors.black45),
      textStyle: MaterialStateProperty.all(
          const TextStyle(fontWeight: FontWeight.normal)),
      side: MaterialStateProperty.all(
          const BorderSide(width: 2, color: Colors.black12)),
      padding: MaterialStateProperty.all(EdgeInsets.zero),
      shape: MaterialStateProperty.all(
          RoundedRectangleBorder(borderRadius: BorderRadius.circular(12))),
    );
    _days = {
      for (int i = 1; i <= 31; i++) // current month
        i: OutlinedButton(
            onPressed: () => _onPressed(i),
            style: style,
            child: Text(i.toString())),
      for (int i = 1; i <= 31; i++) // other month
        100 + i: OutlinedButton(
            onPressed: null, style: disabledStyle, child: Text(i.toString())),
    };
    _days.forEach(
        (key, child) => _days[key] = Padding(padding: padding, child: child));
  }

  _onPressed(int day) {
    print('day: $day, month: $month');
  }

  @override
  Widget build(BuildContext context) {
    return PageView.builder(
        itemCount: 12,
        itemBuilder: (ctx, i) {
          const materialLocalizations = DefaultMaterialLocalizations();
          final year = DateTime.now().year;
          month = i + 1;
          final m = Month(
            name: materialLocalizations.formatMonthYear(DateTime(year, month)),
            length: DateUtils.getDaysInMonth(year, month),
            prevMonthLength: month == 1
                ? DateUtils.getDaysInMonth(year - 1, 12)
                : DateUtils.getDaysInMonth(year, month - 1),
            firstDayInWeek:
                DateUtils.firstDayOffset(year, month, materialLocalizations),
          );
          return Column(
            children: [
              Text(m.name, style: Theme.of(context).textTheme.headline5),
              Expanded(
                child: GridView.count(
                  crossAxisCount: 7,
                  children: [...fillGrid(m).map((key) => _days[key]!)],
                ),
              ),
            ],
          );
        });
  }

  Iterable<int> fillGrid(Month month) sync* {
    // prev month
    final idx = month.prevMonthLength - month.firstDayInWeek + 101;
    yield* Iterable.generate(month.firstDayInWeek, (i) => i + idx);

    // current month
    yield* Iterable.generate(month.length, (i) => i + 1);

    // next month
    final fillEnd = (month.firstDayInWeek + month.length) % 7;
    if (fillEnd > 0) {
      int day = 101;
      yield* Iterable.generate(7 - fillEnd, (i) => day++);
    }
  }
}

class Month {
  Month(
      {required this.name,
      required this.length,
      required this.prevMonthLength,
      required this.firstDayInWeek});

  final String name;
  final int length;
  final int prevMonthLength;
  final int firstDayInWeek;
}