Topic Messaging on iOS

Based on the publish/subscribe model, FCM topic messaging allows you to send a message to multiple devices that have opted in to a particular topic. You compose topic messages as needed, and FCM handles routing and delivering the message reliably to the right devices.

For example, users of a local weather forecasting app could opt in to a "severe weather alerts" topic and receive notifications of storms threatening specified areas. Users of a sports app could subscribe to automatic updates in live game scores for their favorite teams.

Some things to keep in mind about topics:

  • Topic messaging supports unlimited topics and subscriptions for each app.
  • Topic messaging is best suited for content such as news, weather, or other publicly available information.
  • Topic messages are optimized for throughput rather than latency. For fast, secure delivery to single devices or small groups of devices, target messages to tokens, not topics.
  • If you need to send messages to multiple devices per user, consider Device Group messaging for those use cases.

Subscribe the client app to a topic

Client apps can subscribe to any existing topic, or they can create a new topic. When a client app subscribes to a new topic name (one that does not already exist for your Firebase project), a new topic of that name is created in FCM and any client can subsequently subscribe to it.

The FIRMessaging class handles topic messaging functionality. To subscribe to a topic, call subscribeToTopic:topic from your application's main thread (FCM is not thread-safe):

[[FIRMessaging messaging] subscribeToTopic:@"/topics/news"];
NSLog(@"Subscribed to news topic");

This makes an asynchronous request to the FCM backend and subscribes the client to the given topic. If the subscription request fails initially, FCM retries until it can subscribe to the topic successfully. Each time the app starts, FCM makes sure that all requested topics have been subscribed.

To unsubscribe, call unsubscribeFromTopic:topic, and FCM unsubscribes from the topic in the background.

Manage topic subscriptions on the server

You can take advantage of Instance ID APIs to perform basic topic management tasks from the server side. Given the registration token(s) of client app instances, you can do the following:

Receive and handle topic messages

FCM delivers topic messages in the same way as other downstream messages.

Implement AppDelegate application:didReceiveRemoteNotification: to handle notifications received when the client app is in the foreground, and all data messages that are sent to the client. The message is a dictionary of keys and values.

Objective-C

- (void)application:(UIApplication *)application didReceiveRemoteNotification:(NSDictionary *)userInfo {
  // If you are receiving a notification message while your app is in the background,
  // this callback will not be fired till the user taps on the notification launching the application.
  // TODO: Handle data of notification

  // Print message ID.
  if (userInfo[kGCMMessageIDKey]) {
    NSLog(@"Message ID: %@", userInfo[kGCMMessageIDKey]);
  }

  // Print full message.
  NSLog(@"%@", userInfo);
}

- (void)application:(UIApplication *)application didReceiveRemoteNotification:(NSDictionary *)userInfo
    fetchCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler {
  // If you are receiving a notification message while your app is in the background,
  // this callback will not be fired till the user taps on the notification launching the application.
  // TODO: Handle data of notification

  // Print message ID.
  if (userInfo[kGCMMessageIDKey]) {
    NSLog(@"Message ID: %@", userInfo[kGCMMessageIDKey]);
  }

  // Print full message.
  NSLog(@"%@", userInfo);

  completionHandler(UIBackgroundFetchResultNewData);
}

Swift

func application(_ application: UIApplication, didReceiveRemoteNotification userInfo: [AnyHashable: Any]) {
  // If you are receiving a notification message while your app is in the background,
  // this callback will not be fired till the user taps on the notification launching the application.
  // TODO: Handle data of notification

  // Print message ID.
  if let messageID = userInfo[gcmMessageIDKey] {
    print("Message ID: \(messageID)")
  }

  // Print full message.
  print(userInfo)
}

func application(_ application: UIApplication, didReceiveRemoteNotification userInfo: [AnyHashable: Any],
                 fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
  // If you are receiving a notification message while your app is in the background,
  // this callback will not be fired till the user taps on the notification launching the application.
  // TODO: Handle data of notification

  // Print message ID.
  if let messageID = userInfo[gcmMessageIDKey] {
    print("Message ID: \(messageID)")
  }

  // Print full message.
  print(userInfo)

  completionHandler(UIBackgroundFetchResult.newData)
}

Build send requests

From the server side, sending messages to a Firebase Cloud Messaging topic is very similar to sending messages to an individual device or to a user group. The app server sets the to key with a value like /topics/yourTopic. Developers can choose any topic name that matches the regular expression: "/topics/[a-zA-Z0-9-_.~%]+".

To send to combinations of multiple topics, the app server sets the condition key to a boolean condition that specifies the target topics. For example, to send messages to devices that subscribed to TopicA and either TopicB or TopicC:

'TopicA' in topics && ('TopicB' in topics || 'TopicC' in topics)

FCM first evaluates any conditions in parentheses, and then evaluates the expression from left to right. In the above expression, a user subscribed to any single topic does not receive the message. Likewise, a user who does not subscribe to TopicA does not receive the message. These combinations do receive it:

  • TopicA and TopicB
  • TopicA and TopicC

Conditions for topics support two operators per expression, and parentheses are supported.

For more detail about app server keys, see the reference information for your chosen connection server protocol, HTTP or XMPP. Examples in this page show how to send data messages to topics in both HTTP and XMPP.

HTTP POST request

Send to a single topic:

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

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

Send to devices subscribed to topics "dogs" or "cats":

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

{
  "condition": "'dogs' in topics || 'cats' in topics",
  "data": {
    "message": "This is a Firebase Cloud Messaging Topic Message!",
   }
}

HTTP response

//Success example:
{
  "message_id": "1023456"
}

//failure example:
{
  "error": "TopicsMessageRateExceeded"
}

XMPP message

Send to a single topic:

<message id="">
  <gcm xmlns="google:mobile:data">
  {
      "to": "/topics/foo-bar",
      "message_id": "m-1366082849205" ,
      "data": {
          "message":"This is a Firebase Cloud Messaging Topic Message!"
      }
  }
  </gcm>
</message>

Send to devices subscribed to topics "dogs" or "cats":

<message id="">
  <gcm xmlns="google:mobile:data">
  {
    "condition": "'dogs' in topics || 'cats' in topics",
    "data": {
      "message": "This is a Firebase Cloud Messaging Topic Message!",
     }
  }
  </gcm>
</message>

XMPP response

//Success example:
{
  "message_id": "1023456"
}

//failure example:
{
  "error": "TopicsMessageRateExceeded"
}

Expect up to 30 seconds of delay before the FCM Connection Server returns a success or failure response to the topic send requests. Make sure to set the app server's timeout value in the request accordingly.

For the full list of message options, see the reference information for your chosen connection server protocol, HTTP or XMPP.

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