こんにちわ。いせきです。

ずっと「通知が送れない！なんでだよ！！」と思っていました。
サンプルアプリでも試してみても全然ダメ。試行錯誤した結果　→アホすぎる理由だった。

誰もいないと思いますが、もしいた時に、すぐに解決できるようにしてもらえると嬉しいです！

初めに使用したコードです！これで試してもらえると！

//main.dart


import 'package:flutter/material.dart';
import 'package:flutter_local_notifications/flutter_local_notifications.dart';
import 'notification_api.dart';

final FlutterLocalNotificationsPlugin flutterLocalNotificationsPlugin =
    FlutterLocalNotificationsPlugin();

void main() async {
  WidgetsFlutterBinding.ensureInitialized();

  IOSInitializationSettings initializationSettingsIOS =
      const IOSInitializationSettings(
    requestSoundPermission: true,
    requestBadgePermission: true,
    requestAlertPermission: true,
  );
  final InitializationSettings initializationSettings = InitializationSettings(
    iOS: initializationSettingsIOS,
  );
  await flutterLocalNotificationsPlugin.initialize(
    initializationSettings,
  );

  runApp(const MyApp());
}

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: [
            TextButton(
              onPressed: () => NotificationApi.showNotification(
                title: 'bbb',
                body: 'aaa',
                payload: 'ccc',
              ),
              child: const Text('Simple Notification'),
            ),
          ],
        ),
      ),
    );
  }
}


//notification_api.dart

import 'package:flutter_local_notifications/flutter_local_notifications.dart';

class NotificationApi {
  static final _notifications = FlutterLocalNotificationsPlugin();

  static Future _notificationsDetails() async {
    return const NotificationDetails(
      android: AndroidNotificationDetails(
        'id',
        'name',
        importance: Importance.max,
      ),
      iOS: IOSNotificationDetails(),
    );
  }

  static Future showNotification({
    int id = 0,
    String? title,
    String? body,
    String? payload,
  }) async =>
      _notifications.show(
        id,
        title,
        body,
        await _notificationsDetails(),
        payload: payload,
      );
}

アホだった点の解説

間違っていコードがこちら。AppDelegate.swiftの中身です。
気づいている人が多いでしょう。（インデントはすみません。）

import UIKit
import Flutter

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

わかりましたか？ほんとに単純かつコピペの悪影響が出ていました（反省しています。）

return移行にコードを書いても意味がない。むしろダメ。反省しています。。

#正解のコード

import UIKit
import Flutter

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

     
  }
}