Availability calendar with range select

import 'package:flutter/material.dart';

class AvailabilityCalendar extends StatefulWidget {
  const AvailabilityCalendar({super.key, required this.blocked, required this.onRange});
  final Set<DateTime> blocked; // dateOnly values that cannot be booked
  final ValueChanged<DateTimeRange> onRange;

  @override
  State<AvailabilityCalendar> createState() => _AvailabilityCalendarState();
}

class _AvailabilityCalendarState extends State<AvailabilityCalendar> {
  late DateTime _month = DateUtils.dateOnly(DateTime.now()).copyWith(day: 1);
  DateTime? _start, _end;

  bool _isBlocked(DateTime d) =>
      d.isBefore(DateUtils.dateOnly(DateTime.now())) || widget.blocked.contains(d);

  bool _rangeClear(DateTime a, DateTime b) {
    for (var d = a; !d.isAfter(b); d = DateUtils.addDaysToDate(d, 1)) {
      if (_isBlocked(d)) return false;
    }
    return true;
  }

  void _tap(DateTime d) {
    setState(() {
      if (_start == null || _end != null) {
        _start = d; // begin a new selection
        _end = null;
      } else if (d.isBefore(_start!) || !_rangeClear(_start!, d)) {
        _start = d; // earlier day or blocked gap in between: restart here
      } else {
        _end = d;
        widget.onRange(DateTimeRange(start: _start!, end: d));
      }
    });
  }

  bool _inRange(DateTime d) => _start != null && !d.isBefore(_start!) && !d.isAfter(_end ?? _start!);

  @override
  Widget build(BuildContext context) {
    final loc = MaterialLocalizations.of(context);
    final scheme = Theme.of(context).colorScheme;
    final days = DateUtils.getDaysInMonth(_month.year, _month.month);
    final lead = DateUtils.firstDayOffset(_month.year, _month.month, loc);
    return Column(children: [
      Row(mainAxisAlignment: MainAxisAlignment.spaceBetween, children: [
        IconButton(onPressed: () => setState(() => _month = DateUtils.addMonthsToMonthDate(_month, -1)), icon: const Icon(Icons.chevron_left)),
        Text(loc.formatMonthYear(_month), style: Theme.of(context).textTheme.titleMedium),
        IconButton(onPressed: () => setState(() => _month = DateUtils.addMonthsToMonthDate(_month, 1)), icon: const Icon(Icons.chevron_right)),
      ]),
      GridView.count(
        crossAxisCount: 7,
        shrinkWrap: true,
        physics: const NeverScrollableScrollPhysics(),
        children: [
          for (var i = 0; i < lead; i++) const SizedBox.shrink(),
          for (var n = 1; n <= days; n++)
            Builder(builder: (context) {
              final d = DateTime(_month.year, _month.month, n);
              final blocked = _isBlocked(d);
              final selected = _inRange(d);
              return InkResponse(
                onTap: blocked ? null : () => _tap(d),
                child: Container(
                  margin: const EdgeInsets.all(2),
                  alignment: Alignment.center,
                  decoration: BoxDecoration(
                    color: selected ? scheme.primary : null,
                    borderRadius: BorderRadius.circular(10),
                  ),
                  child: Text('$n', style: TextStyle(
                    decoration: blocked ? TextDecoration.lineThrough : null,
                    color: selected ? scheme.onPrimary : (blocked ? scheme.outline : null),
                  )),
                ),
              );
            }),
        ],
      ),
    ]);
  }
}