只需要重写FirebaseMessagingService的OnCreate方法。当你的应用程序在后台时被调用:

public override void OnCreate()
{
    // your code
    base.OnCreate();
}

重写FirebaseMessageService的handleIntent方法对我有用。

下面是c#代码(Xamarin)

public override void HandleIntent(Intent intent)
{
    try
    {
        if (intent.Extras != null)
        {
            var builder = new RemoteMessage.Builder("MyFirebaseMessagingService");

            foreach (string key in intent.Extras.KeySet())
            {
                builder.AddData(key, intent.Extras.Get(key).ToString());
            }

            this.OnMessageReceived(builder.Build());
        }
        else
        {
            base.HandleIntent(intent);
        }
    }
    catch (Exception)
    {
        base.HandleIntent(intent);
    }
}

这就是Java代码

public void handleIntent(Intent intent)
{
    try
    {
        if (intent.getExtras() != null)
        {
            RemoteMessage.Builder builder = new RemoteMessage.Builder("MyFirebaseMessagingService");

            for (String key : intent.getExtras().keySet())
            {
                builder.addData(key, intent.getExtras().get(key).toString());
            }

            onMessageReceived(builder.build());
        }
        else
        {
            super.handleIntent(intent);
        }
    }
    catch (Exception e)
    {
        super.handleIntent(intent);
    }
}

根据Firebase云消息文档-如果Activity在前台，那么onmessagerecreceived将被调用。如果Activity处于后台或关闭，则通知消息将显示在应用程序启动器活动的通知中心。 你可以调用你的自定义活动点击通知，如果你的应用程序是在后台调用rest服务api为firebase消息:

URL-https: / / fcm.googleapis.com/fcm/send

方法类型- POST

Header- Content-Type:application/json
Authorization:key=your api key

车身/有效载荷：

{ "notification": {
    "title": "Your Title",
    "text": "Your Text",
     "click_action": "OPEN_ACTIVITY_1" // should match to your intent filter
  },
    "data": {
    "keyname": "any value " //you can get this data as extras in your activity and this data is optional
    },
  "to" : "to_id(firebase refreshedToken)"
} 

在你的应用程序中，你可以添加以下代码在你的活动中被调用:

<intent-filter>
                <action android:name="OPEN_ACTIVITY_1" />
                <category android:name="android.intent.category.DEFAULT" />
            </intent-filter>

基于以下情况调用的onmessagerecreceived (RemoteMessage RemoteMessage)方法。

FCM响应带有通知和数据块:

{
  
"to": "device token list",
  "notification": {
    "body": "Body of Your Notification",
    "title": "Title of Your Notification"
  },
  "data": {
    "body": "Body of Your Notification in Data",
    "title": "Title of Your Notification in Title",
    "key_1": "Value for key_1",
    "image_url": "www.abc.com/xyz.jpeg",
    "key_2": "Value for key_2"
  }
}

应用程序在前台:

onmessagerreceived (RemoteMessage RemoteMessage)调用，在通知栏中显示LargeIcon和BigPicture。我们可以从通知和数据块中读取内容

后台应用:

onmessagerreceived (RemoteMessage RemoteMessage)未被调用，系统托盘将接收消息并从通知块中读取正文和标题，并在通知栏中显示默认消息和标题。

只有数据块的FCM响应:

在本例中，从json中删除通知块

{
  
"to": "device token list",
  "data": {
    "body": "Body of Your Notification in Data",
    "title": "Title of Your Notification in Title",
    "key_1": "Value for key_1",
    "image_url": "www.abc.com/xyz.jpeg",
    "key_2": "Value for key_2"
  }
}

调用onmessagerreceived()的解决方案

应用程序在前台:

onmessagerreceived (RemoteMessage RemoteMessage)调用，在通知栏中显示LargeIcon和BigPicture。我们可以从通知和数据块中读取内容

后台应用:

onmessagerreceived (RemoteMessage RemoteMessage)调用，系统托盘将不会收到消息，因为通知键不在响应中。在通知栏中显示LargeIcon和BigPicture

Code

 private void sendNotification(Bitmap bitmap,  String title, String 
    message, PendingIntent resultPendingIntent) {

    NotificationCompat.BigPictureStyle style = new NotificationCompat.BigPictureStyle();
    style.bigPicture(bitmap);

    Uri defaultSound = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION);

    NotificationManager notificationManager = (NotificationManager) mContext.getSystemService(Context.NOTIFICATION_SERVICE);
    String NOTIFICATION_CHANNEL_ID = mContext.getString(R.string.default_notification_channel_id);

    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        NotificationChannel notificationChannel = new NotificationChannel(NOTIFICATION_CHANNEL_ID, "channel_name", NotificationManager.IMPORTANCE_HIGH);

        notificationManager.createNotificationChannel(notificationChannel);
    }
    Bitmap iconLarge = BitmapFactory.decodeResource(mContext.getResources(),
            R.drawable.mdmlogo);
    NotificationCompat.Builder notificationBuilder = new NotificationCompat.Builder(mContext, NOTIFICATION_CHANNEL_ID)
            .setSmallIcon(R.drawable.mdmlogo)
            .setContentTitle(title)
            .setAutoCancel(true)
            .setSound(defaultSound)
            .setContentText(message)
            .setContentIntent(resultPendingIntent)
            .setStyle(style)
            .setLargeIcon(iconLarge)
            .setWhen(System.currentTimeMillis())
            .setPriority(Notification.PRIORITY_MAX)
            .setChannelId(NOTIFICATION_CHANNEL_ID);


    notificationManager.notify(1, notificationBuilder.build());


}

参考链接:

https://firebase.google.com/docs/cloud-messaging/android/receive

我也有同样的问题。使用“数据消息”而不是“通知”更容易。数据消息总是加载类onmessagerreceived。

在该类中，您可以使用notificationbuilder创建自己的通知。

例子:

 @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {
        sendNotification(remoteMessage.getData().get("title"),remoteMessage.getData().get("body"));
    }

    private void sendNotification(String messageTitle,String messageBody) {
        Intent intent = new Intent(this, MainActivity.class);
        intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
        PendingIntent pendingIntent = PendingIntent.getActivity(this,0 /* request code */, intent,PendingIntent.FLAG_UPDATE_CURRENT);

        long[] pattern = {500,500,500,500,500};

        Uri defaultSoundUri= RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION);

        NotificationCompat.Builder notificationBuilder = (NotificationCompat.Builder) new NotificationCompat.Builder(this)
                .setSmallIcon(R.drawable.ic_stat_name)
                .setContentTitle(messageTitle)
                .setContentText(messageBody)
                .setAutoCancel(true)
                .setVibrate(pattern)
                .setLights(Color.BLUE,1,1)
                .setSound(defaultSoundUri)
                .setContentIntent(pendingIntent);

        NotificationManager notificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
        notificationManager.notify(0 /* ID of notification */, notificationBuilder.build());
    }

值得强调的一点是，你必须使用数据消息-数据键-让onmessagerecreceived处理程序被调用，即使应用程序是在后台。你不应该在你的有效负载中有任何其他通知消息键，否则如果应用程序在后台，处理程序将不会被触发。

这里提到了(但在FCM文档中没有特别强调):

https://firebase.google.com/docs/cloud-messaging/concept-options#notifications_and_data_messages

使用您的应用服务器和FCM服务器API:只设置数据键。可以 要么可折叠，要么不可折叠。