Skill: Firebase Cloud Messaging (FCM)

Overview

Firebase Cloud Messaging is a cross-platform push layer that, on iOS, sits on top of APNs — FCM still requires an APNs key and device token, but gives you a unified API, topic subscriptions, and easier multi-platform targeting. Use it when you need cross-platform parity or topic/segment messaging. The same iOS notification handling (permissions, foreground, tap routing) applies; FCM just manages token mapping and delivery.

Use Cases

• Cross-platform (iOS + Android) apps wanting one messaging backend.
• Topic-based broadcast (e.g. "sports-news" subscribers).
• Teams already invested in Firebase.

Best Practices

• Configure APNs auth key in Firebase; FCM delegates actual delivery to APNs on iOS.
• Use the FCM registration token (not the raw APNs token) for sending; handle messaging(_:didReceiveRegistrationToken:) refresh.
• Prefer topic subscriptions for broadcast instead of managing huge token lists.
• Keep the standard UNUserNotificationCenter handling for foreground + taps.
• Don't put secrets/PII in data/notification payloads.

Anti-Patterns

• ❌ Forgetting the APNs key, so iOS pushes silently fail.
• ❌ Sending to stale FCM tokens; not handling token refresh.
• ❌ Managing per-device sends when a topic would do.
• ❌ Trusting data payload contents as authenticated/sensitive.
• ❌ Mixing FCM and direct APNs handling inconsistently.

Checklist

• [ ] APNs auth key configured in Firebase.
• [ ] FCM registration token used and refreshed.
• [ ] Topics used for broadcast where appropriate.
• [ ] Foreground + tap handling implemented.
• [ ] No sensitive data in payloads.

Swift Examples

import FirebaseMessaging
import UserNotifications

final class FCMManager: NSObject, MessagingDelegate, UNUserNotificationCenterDelegate {
    func configure() {
        Messaging.messaging().delegate = self
        UNUserNotificationCenter.current().delegate = self
    }

    // FCM token refresh — upload to your server
    func messaging(_ messaging: Messaging, didReceiveRegistrationToken fcmToken: String?) {
        guard let fcmToken else { return }
        Task { await DeviceTokenService.shared.upload(fcmToken) }
    }

    func subscribe(toTopic topic: String) {
        Messaging.messaging().subscribe(toTopic: topic)
    }

    func userNotificationCenter(_ center: UNUserNotificationCenter,
                                didReceive response: UNNotificationResponse) async {
        let info = response.notification.request.content.userInfo
        if let link = info["deep_link"] as? String, let url = URL(string: link) {
            await DeepLinkRouter.shared.handle(url)
        }
    }
}

Common Interview Questions

• How does FCM relate to APNs on iOS?
• FCM registration token vs APNs device token?
• When are topics better than token lists?
• How do you handle FCM token refresh?
• Trade-offs of FCM vs direct APNs?

AI Implementation Notes

• Remember FCM still needs an APNs key; use and refresh the FCM registration token.
• Reuse the standard notification handling and deep link router.
• Prefer topics for broadcast; keep payloads non-sensitive.
• Related: apns.md, deep_links.md.