Example code used to demonstrate the use of the library file, alarm_manager.dart, working with the plugin, android_alarm_manager.

android_alarm_manager.dart

// Copyright 2017 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// ignore_for_file: public_member_api_docs

import 'dart:math' show Random, pow;
import 'package:flutter/material.dart';

import 'alarm_manager.dart' show AlarmManager;
import 'package:shared_preferences/shared_preferences.dart'
    show SharedPreferences;

/// The [SharedPreferences] key to access the alarm fire count.
const String countKey = 'count';

/// Global [SharedPreferences] object.
SharedPreferences prefs;

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

class AlarmManagerExampleApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        title: 'Flutter Demo',
        home: FutureBuilder<bool>(
            future: initSettings(),
            initialData: false,
            builder: (_, snapshot) {
              if (snapshot.connectionState == ConnectionState.done) {
                return _AlarmHomePage(title: 'Flutter Demo Home Page');
              } else {
                return Container(
                    child: Center(child: CircularProgressIndicator()));
              }
            }));
  }

  Future<bool> initSettings() async {
    bool init = await AlarmManager.init(
      exact: true,
      alarmClock: true,
      wakeup: true,
    );
    prefs = await SharedPreferences.getInstance();
    if (!prefs.containsKey(countKey)) prefs.setInt(countKey, 0);
    return init;
  }
}

class _AlarmHomePage extends StatefulWidget {
  _AlarmHomePage({Key key, this.title}) : super(key: key);
  final String title;
  @override
  _AlarmHomePageState createState() => _AlarmHomePageState();
}

class _AlarmHomePageState extends State<_AlarmHomePage> {
  @override
  Widget build(BuildContext context) {
    final textStyle = Theme.of(context).textTheme.headline4;
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'Alarm fired $_counter times',
              style: textStyle,
            ),
            Row(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                Text(
                  'Total alarms fired: ',
                  style: textStyle,
                ),
                Text(
                  prefs.getInt(countKey).toString(),
                  key: ValueKey('BackgroundCountText'),
                  style: textStyle,
                ),
              ],
            ),
            RaisedButton(
              child: Text(
                'Schedule OneShot Alarm',
              ),
              key: ValueKey('RegisterOneShotAlarm'),
              onPressed: () async {
                await AlarmManager.oneShot(
                  const Duration(seconds: 5),
                  // Ensure we have a unique alarm ID.
                  Random().nextInt(pow(2, 31)),
                  (int id) => _incrementCounter(),
                );
              },
            ),
          ],
        ),
      ),
    );
  }

  int _counter = 0;
  Future<void> _incrementCounter() async {
    int currentCount = prefs.getInt(countKey);
    await prefs.setInt(countKey, currentCount + 1);
    // Ensure we've loaded the updated count from the background isolate.
    await prefs.reload();
    setState(() {
      _counter++;
    });
    print('Increment counter!');
  }
}

not working when kill app

Hello there, @Kinasr. If you like, you can detail the problem and post an issue.