How to send silent push when app is closed or in the backgroud? (Android)

[benevbright]

Hi, I'm using this plugin long time but have not figured out how to send a silent push on Android.

min SDK : 21
First of all, weird thing is that

• I don't have to use Service as creator said on the readme. I can receive push notification all the time even when app is closed.
• And It works fine if I comment all codes in the PushNotificationGcmListener.cs except
  CrossPushNotification.PushNotificationListener.OnMessage(values, DeviceType.Android);
  which includes feature handling silent push.

My app receives push notifications when app is closed and is in the background.
https://developers.google.com/cloud-messaging/downstream
like this document said,

When your app is in the background, Android directs messages with notification to the system tray. A user click on a notification opens the app launcher by default.

But the problem is that I can't handle silent push when my app is in the background or closed. I got push notification that doesn't have a title, it's empty message but it goes off!

Where can I try to avoid to receive my silent push message?