Date and time slot picker

import 'package:flutter/material.dart';

typedef SlotLoader = Future<Set<TimeOfDay>> Function(DateTime day); // free slots

class SlotPicker extends StatefulWidget {
  const SlotPicker({super.key, required this.load, required this.onChanged, this.days = 14});
  final SlotLoader load;
  final ValueChanged<DateTime?> onChanged;
  final int days;

  @override
  State<SlotPicker> createState() => _SlotPickerState();
}

class _SlotPickerState extends State<SlotPicker> {
  static final _all = [for (var h = 9; h < 18; h++) TimeOfDay(hour: h, minute: 0)];
  late final DateTime _today = DateUtils.dateOnly(DateTime.now());
  late DateTime _day = _today;
  TimeOfDay? _slot;
  Future<Set<TimeOfDay>>? _free;

  @override
  void initState() {
    super.initState();
    _free = widget.load(_day);
  }

  void _pickDay(DateTime d) {
    setState(() {
      _day = d;
      _slot = null;
      _free = widget.load(d);
    });
    widget.onChanged(null);
  }

  @override
  Widget build(BuildContext context) {
    final loc = MaterialLocalizations.of(context);
    return Column(crossAxisAlignment: CrossAxisAlignment.start, children: [
      SizedBox(
        height: 72,
        child: ListView.separated(
          scrollDirection: Axis.horizontal,
          itemCount: widget.days,
          separatorBuilder: (_, _) => const SizedBox(width: 8),
          itemBuilder: (_, i) {
            final d = _today.add(Duration(days: i));
            return ChoiceChip(
              selected: DateUtils.isSameDay(d, _day),
              onSelected: (_) => _pickDay(d),
              label: Column(mainAxisAlignment: MainAxisAlignment.center, children: [
                Text(loc.narrowWeekdays[d.weekday % 7]),
                Text('${d.day}', style: const TextStyle(fontWeight: FontWeight.bold)),
              ]),
            );
          },
        ),
      ),
      const SizedBox(height: 12),
      FutureBuilder<Set<TimeOfDay>>(
        future: _free,
        builder: (context, snap) {
          if (!snap.hasData) return const LinearProgressIndicator();
          return Wrap(spacing: 8, runSpacing: 8, children: [
            for (final t in _all)
              ChoiceChip(
                label: Text(loc.formatTimeOfDay(t)),
                selected: _slot == t,
                // Past times today are also unavailable.
                onSelected: snap.data!.contains(t) && !_isPast(t)
                    ? (_) {
                        setState(() => _slot = t);
                        widget.onChanged(DateTime(_day.year, _day.month, _day.day, t.hour, t.minute));
                      }
                    : null,
              ),
          ]);
        },
      ),
    ]);
  }

  bool _isPast(TimeOfDay t) =>
      DateUtils.isSameDay(_day, _today) &&
      DateTime(_day.year, _day.month, _day.day, t.hour, t.minute).isBefore(DateTime.now());
}