即使从Xamarin的Android版本8.1中的缓存中清除，如何在后台启动服务？

Question

我创建了一个示例应用程序，它将在后台启动服务，即使从缓存中清除服务正在运行但适用于Android版本5.在Android版本8.1中不支持。

我的MainActivity.cs代码

protected override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);


        SetContentView(Resource.Layout.Main);

        Android.Support.V7.Widget.Toolbar toolbar = FindViewById<Android.Support.V7.Widget.Toolbar>(Resource.Id.toolbar);

        SetSupportActionBar(toolbar);

        //var alarmIntent = new Intent(this, typeof(AlarmReceiver));
        //alarmIntent.PutExtra("title", "Hello");
        //alarmIntent.PutExtra("message", "World!");

        Intent intent = new Intent(NOTIFICATION_CHANNEL_ID);
        intent.SetClass(this, typeof(AlarmReceiver));

        var pending = PendingIntent.GetBroadcast(this, 0, intent, PendingIntentFlags.UpdateCurrent);

        var alarmManager = GetSystemService(AlarmService).JavaCast<AlarmManager>();
        alarmManager.SetRepeating(AlarmType.ElapsedRealtime, DateTime.Now.Millisecond, 5 * 100, pending);

    }

我有AlarmReciever.cs类

 [BroadcastReceiver(Enabled = true)]
[IntentFilter(new[] { Android.Content.Intent.ActionBootCompleted })]
class AlarmReceiver : BroadcastReceiver
{
    Context context;
    public override void OnReceive(Context context, Intent intent)
    {
        this.context = context;
        Toast.MakeText(context, "Service is Running in background", ToastLength.Long).Show();

        if (Build.VERSION.SdkInt >= BuildVersionCodes.O)
        {
            context.StartForegroundService(new Intent(context, typeof(BackgroundService)));
        }
        else
        {
            context.StartService(new Intent(context, typeof(BackgroundService)));
        }
        //Intent background = new Intent(context, typeof(BackgroundService));
        //context.StartForegroundService(background);

    }
}

从缓存中清除应用程序时，我在我的服务类中调用OnTaskRemoved方法 -

 [Service]
public class BackgroundService : Service
{
    public override void OnCreate()
    {
        base.OnCreate();

    }

    public override StartCommandResult OnStartCommand(Intent intent, StartCommandFlags flags, int startId)
    {
        Log.Debug("service", "Service Started");
        //init the handler in oncreate
        System.Timers.Timer Timer1 = new System.Timers.Timer();
        Timer1.Start();
        Timer1.Interval = 3000;
        int a = 0;
        Timer1.Enabled = true;
        //Timer1.Elapsed += OnTimedEvent;
        Timer1.Elapsed += (object sender, System.Timers.ElapsedEventArgs e) =>
        {
            Timer1.Stop();
            Timer1.Start();
            a++;
            //Delete time since it will no longer be used.
            Timer1.Dispose();
        };
        Timer1.Start();

        return StartCommandResult.Sticky;
    }

    public override IBinder OnBind(Intent intent)
    {
        return null;
    }

    public override void OnTaskRemoved(Intent rootIntent)
    {
        Intent restartServiceIntent = new Intent(this, this.Class);
        restartServiceIntent.SetPackage(this.PackageName);
        Log.Debug("service", "Service Restarted");

        PendingIntent restartServicePendingIntent = PendingIntent.GetService(this, 1, restartServiceIntent, PendingIntentFlags.OneShot);
        AlarmManager alarmService = (AlarmManager)this.GetSystemService(Context.AlarmService);

        alarmService.SetRepeating(AlarmType.RtcWakeup, SystemClock.CurrentThreadTimeMillis(), 30 * 1000, restartServicePendingIntent);
        NotificationManager notificationManager =
            (NotificationManager)GetSystemService(NotificationService);

        Notification.Builder builder = new Notification.Builder(this);
        Intent notificationIntent = new Intent(this, typeof(MainActivity));
        PendingIntent contentIntent = PendingIntent.GetActivity(this, 0, notificationIntent, 0);

        //set
        builder.SetContentIntent(contentIntent);
        builder.SetSmallIcon(Resource.Mipmap.launcher_foreground);
        builder.SetContentText("Service Running in background");
        builder.SetContentTitle("Service Restarted");
        builder.SetAutoCancel(true);
        builder.SetDefaults(NotificationDefaults.All);


        notificationManager.Notify(1, builder.Build());

        base.OnTaskRemoved(rootIntent);
    }
}

这个代码适用于Android版本5.但是当我清除应用程序缓存并在Android版本8.1中测试时，应用程序停止了（没有点击OnTaskRemoved方法）。

任何人都可以帮助我出错的地方。谢谢你。

Answer 1

Android API级别26（8.0）对后台服务能够执行的操作以及持续时间实施了更严格的限制，尤其是当他们各自的应用程序不在前台时。关于这些限制究竟是什么，有一些Android documentation。

官方推荐TL; DR是 - 如果您要运行的服务是“用户可见的”，那么考虑将您的服务设为Foreground Service。 Foreground Services要求您在该服务运行时显示通知（想一想您的音频播放器在播放音乐时显示的通知）。如果没有，那么它可能由scheduling a Job处理。