AlarmManager only works in near future

Question

I am working on a Xamarin app for Android, iOS and Windows Phone. This app should show local notifications at specified times. For Android I'm using AlamManager. The notifications work on iOS, WinPhone and on the Android Emulators. On my device (Samsung Galaxy S2) only notifications in the near future (1 min, 5 min, 20 min) work. Notifications in about 45 minutes or more don't appear. Notifications in about 30 minutes sometimes appear and sometimes not. Notifications are made in a broadcast receiver. Is there a problem with my code or the alarmManager? Can't find anything. Thanks for your help.

Here the alarmManager is set:

public class NotificationService : NotificationServiceBase
{
    private readonly AlarmManager _alarmManager;
    private readonly long _dayInMilliseconds = AlarmManager.IntervalDay;
    private PendingIntent _currentPendingIntent;

    public NotificationService()
    {
        _alarmManager = (AlarmManager) Forms.Context.GetSystemService(Context.AlarmService);
    }

    public override void CancelNotification()
    {
        _alarmManager.Cancel(_currentPendingIntent);
    }

    protected override void ChangeNotificationTime(TaskEntity task)
    {
        var settings = _settingsBusinessLayer.GetSettings();
        NotificationSetup(settings, task);
    }

    protected override async void ChangeNotificationSettings(SettingsEntity settings)
    {
        if (!settings.IsReminderOn)
        {
            CancelNotification();
            return;
        }

        var task = await _taskBusinessLayer.GetTaskForCurrentMonthAsync(true, true);
        NotificationSetup(settings, task);
    }

    private void NotificationSetup(SettingsEntity settings, TaskEntity task)
    {
        _currentPendingIntent = GetPendingIntent();
        _alarmManager.Cancel(_currentPendingIntent);

        if (settings.ReminderRepetitive == ReminderRepetitive.Daily)
        {
            SetDailyAlarm(settings);
        }
        else
        {
            SetSingleAlarm(settings, task);
        }
    }

    private void SetDailyAlarm(SettingsEntity settings)
    {
        long alarmTimeInMilliseconds;

        if (settings.RemindOn < DateTime.Now.TimeOfDay)
        {
            alarmTimeInMilliseconds = (long) CalculateAlarmTimeFromNow
                (DateTime.Now.AddDays(1), 0, settings.RemindOn).TotalMilliseconds;
        }
        else
        {
            alarmTimeInMilliseconds = (long) CalculateAlarmTimeFromNow
                (DateTime.Now, 0, settings.RemindOn).TotalMilliseconds;
        }

        var alarmTime = SystemClock.ElapsedRealtime() + alarmTimeInMilliseconds;
        _alarmManager.SetRepeating(AlarmType.ElapsedRealtime, alarmTime, _dayInMilliseconds, _currentPendingIntent);
    }

    private void SetSingleAlarm(SettingsEntity settings, TaskEntity task)
    {
        try
        {
            var alarmTimeInMilliseconds = (long) CalculateAlarmTimeFromNow
                (task.DueDate, Convert.ToUInt32(settings.RemindDaysBefore), settings.RemindOn).TotalMilliseconds;
            var alarmTime = SystemClock.ElapsedRealtime() + alarmTimeInMilliseconds;
            _alarmManager.Set(AlarmType.ElapsedRealtime, alarmTime, _currentPendingIntent);
        }
        catch (AlarmTimeIsInPastException)
        {
        }
    }

    private PendingIntent GetPendingIntent()
    {
        var alarmIntent = new Intent(Forms.Context, typeof (NotificationReceiver));
        return PendingIntent.GetBroadcast(Forms.Context, 0, alarmIntent,
            PendingIntentFlags.UpdateCurrent);
    }
}

Broadcast receiver, where the notification is made.

[BroadcastReceiver]
public class NotificationReceiver : BroadcastReceiver
{
    public override void OnReceive(Context context, Intent intent)
    {
        var taskBusinessLayer = DependencyInjection.Container.Resolve<ITaskBusinessLayer>();
        var task = taskBusinessLayer.GetTaskForCurrentMonthAsync(true, true).Result;
        var message = NotificationServiceBase.CreateNotificationMessageRemainingTime(task);
        var title = AppResources.NotificationTitle;
        var notIntent = new Intent(context, typeof (MainActivity));
        var contentIntent = PendingIntent.GetActivity(context, 0, notIntent, PendingIntentFlags.CancelCurrent);

        var builder = new Notification.Builder(context)
            .SetContentIntent(contentIntent)
            .SetContentTitle(title)
            .SetContentText(message)
            .SetSmallIcon(Resource.Drawable.icon)
            .SetDefaults(NotificationDefaults.Sound | NotificationDefaults.Vibrate)
            .SetPriority((int) NotificationPriority.Max)
            .SetAutoCancel(true)
            .SetWhen(JavaSystem.CurrentTimeMillis());

        var notification = builder.Build();
        var notificationManager = NotificationManager.FromContext(context);

        const int notificationId = 0;
        notificationManager.Notify(notificationId, notification);
    }

}

Answer 1

Alarm delivery beginning from KITKAT is inexact. So you probably need to change in your SetSingleAlarm method this:

_alarmManager.Set(AlarmType.ElapsedRealtime, alarmTime, _currentPendingIntent);

to this:

if (Build.VERSION.SdkInt < BuildVersionCodes.Kitkat)
{
    _alarmManager.Set(AlarmType.ElapsedRealtime, alarmTime, _currentPendingIntent);
}
else
{
    _alarmManager.SetExact(AlarmType.ElapsedRealtime, alarmTime, _currentPendingIntent);
}