需求:

1. 允许左右切换日历月份并且监听
2. 如果是当天则被圈起来显示红色
3. 点击可选择，选择的事件展示绿色背景
4. 切换月份时已选择的日期被重置
5. 显示当前时间，并且判断显示是上午还是下午

import 'package:camera/app_allocation/app_style.dart';
import 'package:flutter/material.dart';
import 'package:intl/intl.dart';

import '../generated/l10n.dart';

class CalendarScreenUtils extends StatefulWidget {
  const CalendarScreenUtils({super.key});

  @override
  State<CalendarScreenUtils> createState() => _CalendarScreenUtilsState();
}

class _CalendarScreenUtilsState extends State<CalendarScreenUtils> {
  DateTime selectedDate = DateTime.now();

  //被手动选中的日期
  //赋值默认选择当天，不赋值则不选择
  DateTime? manuallySelectedDate;

 //头部显示的星期
  final weekdays = [S().sun, S().mon, S().tue, S().wed, S().thu, S().fri, S().sat];
  
  //每个月对应的天数
  late int daysInMonth;
  
  late DateTime firstDay;
  
  //将周日调整为第一天
  late int startingWeekday; 
  //是否当天
  bool isToday = true;
  //是否被选中
  bool isSelected = false;

  @override
  Widget build(BuildContext context) {
    //该处数据每次遍历都需要用到，不能放到init里
    daysInMonth = DateTime(selectedDate.year, selectedDate.month + 1, 0).day;
    firstDay = DateTime(selectedDate.year, selectedDate.month, 1);
    startingWeekday = (firstDay.weekday - 7) % 7;
    
    return Scaffold(
      appBar: AppBar(
        title: const Text('Calendar App'),
      ),
      body: Column(
        children: <Widget>[
          Padding(
            padding: const EdgeInsets.symmetric(horizontal: 16.0),
            child: Row(
              mainAxisAlignment: MainAxisAlignment.spaceBetween,
              children: <Widget>[
                IconButton(
                  icon: const Icon(Icons.arrow_back),
                  onPressed: () {
                    setState(() {
                      manuallySelectedDate = null;
                      selectedDate = DateTime(selectedDate.year, selectedDate.month - 1, selectedDate.day);
                    });
                  },
                ),
                Text(
                  DateFormat.yMMMM().format(selectedDate),
                  style: const TextStyle(fontSize: 20),
                ),
                IconButton(
                  icon: const Icon(Icons.arrow_forward),
                  onPressed: () {
                    setState(() {
                      manuallySelectedDate = null;
                      selectedDate = DateTime(selectedDate.year, selectedDate.month + 1, selectedDate.day);
                    });
                  },
                ),
              ],
            ),
          ),
          Row(
            mainAxisAlignment: MainAxisAlignment.spaceEvenly,
            children: weekdays.map((day) {
              return Text(day, style: const TextStyle(fontSize: 16, fontWeight: FontWeight.bold));
            }).toList(),
          ),
          
        Expanded(child: calendarGrid()),
          calendarGrid(),
          Text(
            'Time: ${DateFormat('h:mm a').format(DateTime.now())}',
            style: const TextStyle(fontSize: 16),
          ),
        ],
      ),
    );
  }

  calendarGrid() {
    return CustomScrollView(
        shrinkWrap: true,//收缩
        slivers: <Widget>[
          SliverGrid(
            gridDelegate: const SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 7),
            delegate: SliverChildBuilderDelegate(
              (BuildContext context, int index) {
                if (index < startingWeekday) return Container();
                
                int day = index - startingWeekday + 1;
                DateTime currentDate = DateTime(selectedDate.year, selectedDate.month, day);
                
                isToday = currentDate.day == DateTime.now().day &&
                    currentDate.month == DateTime.now().month &&
                    currentDate.year == DateTime.now().year;
                    
                isSelected = currentDate.year == selectedDate.year &&
                    currentDate.month == selectedDate.month &&
                    currentDate.day == selectedDate.day &&
                    manuallySelectedDate != null;
                    
                return GestureDetector(
                  onTap: () {
                    // 处理日期点击事件
                    setState(() {
                      // 更新被选中的日期
                      selectedDate = currentDate;
                      manuallySelectedDate = currentDate;
                      print("选择的日期:$manuallySelectedDate");
                    });
                  },
                  child: UnconstrainedBox(
                    child: Container(
                      width: 32,
                      height: 32,
                      alignment: Alignment.center,
                      decoration: BoxDecoration(
                        shape: BoxShape.circle,
                        border: Border.all(color: isToday ? AppStyle().getColor.mainColor : Colors.transparent),
                        color: isSelected ? AppStyle().getColor.mainColor : Colors.transparent,
                      ),
                      child: Text(
                        day.toString(),
                        style: TextStyle(
                          color: isSelected ? Colors.white : (isToday ? Colors.red : Colors.black),
                          fontWeight: isSelected ? FontWeight.bold : FontWeight.normal,
                        ),
                      ),
                    ),
                  ),
                );
              },
              childCount: daysInMonth + startingWeekday,
            ),
          ),
        ],
     
    );
  }
}