Firebase 云消息传递 (FCM) 主题消息传递允许您向已选择加入特定主题的多个设备发送消息.基于发布/订阅模型，主题消息支持每个应用程序的无限订阅，即您的组附加到特定主题，如新闻组、体育组等.

Firebase Cloud Messaging (FCM) topic messaging allows you to send a message to multiple devices that have opted in to a particular topic. Based on the publish/subscribe model, topic messaging supports unlimited subscriptions for each app i.e your group is attached to specific topic like news group,sports group etc.

FirebaseMessaging.getInstance().subscribeToTopic("news");

取消订阅unsubscribeFromTopic("news")

从服务器端，您需要设置特定主题，即一组用户，如下所示:

From Server side you need to set up for specif topic i.e a group of user like this:

https://fcm.googleapis.com/fcm/send
Content-Type:application/json
Authorization:key=AIzaSyZ-1u...0GBYzPu7Udno5aA

{
  "to": "/topics/news",
  "data": {
    "message": "This is a Firebase Cloud Messaging Topic Message!",
   }
}

"/topics/news" 这将向订阅新闻主题的人群发送通知

"/topics/news" This will send notification to group of people who have subsribe the news topic