Flutterでプッシュ通知を実装（Firebaseを使わず、flutter_local_notificationsを使う）

import UIKit
import Flutter

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {
    // flutter_local_notification ★ここから追記
    if #available(iOS 10.0, *) {
      UNUserNotificationCenter.current().delegate = self as? UNUserNotificationCenterDelegate
    }
    // ★ここまで追記
    GeneratedPluginRegistrant.register(with: self)
    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }
}

<activity
    android:showWhenLocked="true"
    android:turnScreenOn="true">

<?xml version="1.0" encoding="utf-8"?>
<resources xmlns:tools="http://schemas.android.com/tools"
    tools:keep="@drawable/*" />

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();

  // (通知スケジュールに使う)タイムゾーンを設定する
  tz.initializeTimeZones();
  final String? timeZoneName = await FlutterNativeTimezone.getLocalTimezone();
  tz.setLocalLocation(tz.getLocation(timeZoneName!));

  // flutter_local_notificationsの初期化
  const AndroidInitializationSettings initializationSettingsAndroid =
      AndroidInitializationSettings('app_icon');
  const IOSInitializationSettings initializationSettingsIOS =
      IOSInitializationSettings(
    requestAlertPermission: true,
    requestBadgePermission: true,
    requestSoundPermission: true,
  );
  const InitializationSettings initializationSettings = InitializationSettings(
    android: initializationSettingsAndroid,
    iOS: initializationSettingsIOS,
  );
  await FlutterLocalNotificationsPlugin().initialize(initializationSettings);

  runApp(const MyApp());
}

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('flutter_local_notifications_sample'),
      ),
      body: Center(
        child: Column(
          children: [
            // 今すぐ通知
            TextButton(
              onPressed: () {
                NotificationsUtils.notifyNow();
              },
              child: const Text('今すぐ通知'),
            ),

            // 通知をスケジュール
            TextButton(
              onPressed: () {
                NotificationsUtils.scheduleNotifications(
                  DateTime.now().add(const Duration(seconds: 3)),
                );
              },
              child: const Text('通知をスケジュール'),
            ),

            // スケジュールされた通知をすべてキャンセル
            TextButton(
              onPressed: () {
                NotificationsUtils.cancelNotificationsSchedule();
              },
              child: const Text('スケジュールされた通知をすべてキャンセル'),
            ),
          ],
        ),
      ),
    );
  }
}

// 今すぐ通知する
static Future<void> notifyNow() async {
  final flnp = FlutterLocalNotificationsPlugin();
  flnp.show(
    0,
    '手動通知',
    'あなたがボタンをタップしました',
    const NotificationDetails(
      android: AndroidNotificationDetails(
        '1',
        '手動通知',
        'あなたがボタンをタップしたときに通知されます',
        importance: Importance.high,
        priority: Priority.high,
      ),
      iOS: IOSNotificationDetails(
        presentSound: true,
      ),
    ),
  );
}

// 通知をスケジュール
static Future<void> scheduleNotifications(DateTime dateTime,
    {DateTimeComponents? dateTimeComponents}) async {
  // 日時をTimeZoneを考慮した日時に変換する
  final scheduleTime = tz.TZDateTime.from(dateTime, tz.local);

  // 通知をスケジュールする
  final flnp = FlutterLocalNotificationsPlugin();
  await flnp.zonedSchedule(
    1,
    'スケジュール通知',
    'あなたがスケジュールした時間になりました',
    scheduleTime,
    const NotificationDetails(
      android: AndroidNotificationDetails(
        '2',
        'スケジュール通知',
        '設定した時刻に通知されます',
        importance: Importance.high,
        priority: Priority.high,
      ),
      iOS: IOSNotificationDetails(
        presentSound: true,
      ),
    ),
    uiLocalNotificationDateInterpretation:
        UILocalNotificationDateInterpretation.absoluteTime,
    androidAllowWhileIdle: true,
    matchDateTimeComponents: dateTimeComponents,
  );
}

// 通知をキャンセル
static Future<void> cancelNotificationsSchedule() async {
  final flnp = FlutterLocalNotificationsPlugin();
  await flnp.cancelAll();
}