Flutter - adding a new event to TableCalendar widget

add_event.dart

import 'dart:math';

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

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'TableCalendar Demo',
      theme: ThemeData(primarySwatch: Colors.indigo),
      home: HomeScreen(),
    );
  }
}

class HomeScreen extends StatefulWidget {
  @override
  _HomeScreenState createState() => _HomeScreenState();
}

class _HomeScreenState extends State<HomeScreen> {
  CalendarController _calendarController;
  Map<DateTime, List> _events;

  @override
  void initState() {
    super.initState();
    _calendarController = CalendarController();

    _events = {
      DateTime.utc(2019, 11, 5, 12): ['Event 1'],
      DateTime.utc(2019, 11, 6, 12): ['Event 2'],
    };
  }

  @override
  void dispose() {
    _calendarController.dispose();
    super.dispose();
  }

  // Adds a new random event to currently selected day
  void _addEvent() {
    final random = Random();
    final randomValue = random.nextInt(99) + 1;

    final day = _calendarController.selectedDay;
    final event = 'Random event $randomValue';

    setState(() {
      _events.update(
        day,
        (previousEvents) => previousEvents..add(event),
        ifAbsent: () => [event],
      );
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('TableCalendar Add Event Demo')),
      body: Center(
        child: Column(
          children: <Widget>[
            TableCalendar(
              calendarController: _calendarController,
              events: _events,
            ),
            RaisedButton(
              onPressed: _addEvent,
              child: Text('Add event'),
            ),
            RaisedButton(
              onPressed: () => print(_calendarController.visibleEvents[_calendarController.selectedDay]),
              child: Text('Print selected events'),
            ),
            RaisedButton(
              onPressed: () => print(_events),
              child: Text('Print all events'),
            ),
          ],
        ),
      ),
    );
  }
}