Can I send a silent push notification from a Firebase cloud function?

Question

Is it possible to send a silent APNs (iOS) remote notification from a Firebase Cloud Function? If so, how can this be done? I want to send data to iOS app instances when the app is not in the foreground, without the user seeing a notification.

I currently send a notification that can be seen by users:

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);

exports.sendNotifications = functions.database.ref('/events/{pushId}').onWrite(event => {
  const id = event.params.pushId

  const payload = {
    notification: {
      title: 'An event has occurred!',
      body: 'Please respond to this event.',
      event_id: id
    }
  };

  return admin.messaging().sendToTopic("events", payload);
});

I would like to be able to send that id to the app without a visual notification.

Answer 1

I figured out how to modify my code to successfully send a silent notification. My problem was that I kept trying to put content_available in the payload, when it really should be in options. This is my new code:

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);

exports.sendNotifications = functions.database.ref('/events/{pushId}').onWrite(event => {
  const id = event.params.pushId

  const payload = {
    data: {
      title: 'An event has occurred!',
      body: 'Please respond to this event.',
      event_id: id
    }
  };

  const options = {
    content_available: true
  }

  return admin.messaging().sendToTopic("events", payload, options);
});

I successfully received the silent notification on my iOS device after implementing application:didReceiveRemoteNotification:fetchCompletionHandler and userNotificationCenter:willPresent:withCompletionHandler.