xamarin/xamarin.forms 在锁屏电源唤醒时保持后台运行

PARTIAL_WAKE_LOCK:保持CPU 运转，屏幕和键盘灯有可能是关闭的。
SCREEN_DIM_WAKE_LOCK：保持CPU 运转，允许保持屏幕显示但有可能是灰的，允许关闭键盘灯
SCREEN_BRIGHT_WAKE_LOCK：保持CPU 运转，允许保持屏幕高亮显示，允许关闭键盘灯
FULL_WAKE_LOCK：保持CPU 运转，保持屏幕高亮显示，键盘灯也保持亮度
ACQUIRE_CAUSES_WAKEUP：强制使屏幕亮起，这种锁主要针对一些必须通知用户的操作.
ON_AFTER_RELEASE：当锁被释放时，保持屏幕亮起一段时间**

方法（一）

 [BroadcastReceiver(Enabled = true)]
     public class AlarmService : BroadcastReceiver
     {
         //public event EventHandler WakeLockStatusChanged;
         public override void OnReceive(Context context, Intent intent)
         {
             PowerManager pm = (PowerManager)context.GetSystemService(Context.PowerService);
             PowerManager.WakeLock wl = pm.NewWakeLock(WakeLockFlags.Partial, "MyWakelockTag");
             wl.Acquire();
             //if (wl.IsHeld)
             //{
             //    wl.Release();
             //}
         }
     }

  private void InitBroadcastReceiver()
         {
             _alarmService = new AlarmService();
             Application.Context.RegisterReceiver(_alarmService, new IntentFilter(ConnectivityManager.ConnectivityAction));
         }

  protected override void OnStart()
         {
             base.OnStart();
             InitBroadcastReceiver();
         }

         protected override void OnStop()
         {
             base.OnStop();

             if (_alarmService != null)
             {
                 Application.Context.UnregisterReceiver(_alarmService);
                 _alarmService = null;
             }

         }

方法（二）

    //获取电源锁，保持该服务在屏幕熄灭时仍然获取CPU时，保持运行
         private void AcquireWakeLock()
         {
             if (null == _wakeLock)
             {
                 PowerManager pm = (PowerManager)GetSystemService(Context.PowerService);
                 _wakeLock = pm.NewWakeLock(WakeLockFlags.Partial | WakeLockFlags.OnAfterRelease, "MyWakelockTag");
                 if (_wakeLock != null)
                 {
                     _wakeLock.Acquire();
                 }
             }
         }

         //释放设备电源锁
         private void ReleaseWakeLock()
         {
             if (null != _wakeLock || _wakeLock.IsHeld)
             {
                 _wakeLock.Release();
                 _wakeLock = null;
             }
         }  

参考资料：
https://docs.microsoft.com/zh-cn/dotnet/api/Android.OS.PowerManager.NewWakeLock?view=xamarin-android-sdk-9#Android_OS_PowerManager_NewWakeLock_Android_OS_WakeLockFlags_System_String_

https://stackoverflow.com/questions/54532221/prevent-app-from-going-on-onpause-when-turning-screen-off-using-partial-wakelock/54548392?r=SearchResults#54548392

http://www.itgo.me/a/x9010099145016859770/xamarin-forms-broadcast-receiver-not-working-when-app-is-closed