Push notifications (APNs)

How it works

• Token auth (.p8), preferred: one key for all your apps and both environments, never expires. The server signs a JWT (ES256; kid = key id, iss = team id, iat), reuses it and refreshes every 20–60 min. Certificate (.p12): one per app, expires yearly.
• Authorization (UNUserNotificationCenter): .alert .sound .badge. .provisional (iOS 12): no prompt, lands quietly in Notification Center, user decides later. .criticalAlert: ignores mute/Focus, needs an Apple-approved entitlement. Time-sensitive is an interruption level (passive · active · time-sensitive · critical, iOS 15) set in the payload + capability; it breaks through Focus. Check with getNotificationSettings.
• Alert push: push-type alert, priority 10. Background (silent): only "content-available":1, push-type background, priority 5, needs the remote-notification background mode. The app wakes in didReceiveRemoteNotification(…) for ~30 s and must call its fetchCompletionHandler.
• Service Extension (NSE): separate process, runs only for a visible alert with "mutable-content":1; ~30 s in didReceive(_:withContentHandler:) to decrypt, rewrite or attach media (UNNotificationAttachment). serviceExtensionTimeWillExpire() ships the best attempt. Content Extension: custom expanded UI, picked by UNNotificationExtensionCategory.
• Categories: register UNNotificationCategory + UNNotificationAction (.foreground, .destructive, text input) at launch. Payload "category" picks one; the tap arrives as response.actionIdentifier.

Example

// didFinishLaunching: set the delegate BEFORE it returns
let c = UNUserNotificationCenter.current(); c.delegate = self
c.requestAuthorization(options: [.alert, .sound]) { _, _ in }
app.registerForRemoteNotifications()  // token even if denied
func application(_ a: UIApplication,
  didRegisterForRemoteNotificationsWithDeviceToken t: Data) {
  api.upload(t.map { String(format: "%02x", $0) }.joined()) }
func userNotificationCenter(_ c: UNUserNotificationCenter,
  willPresent n: UNNotification) async
  -> UNNotificationPresentationOptions { [.banner, .sound] }
func userNotificationCenter(_ c: UNUserNotificationCenter,
  didReceive r: UNNotificationResponse) async {
  router.open(r.notification.request.content.userInfo) }

Payload (≤ 4 KB; VoIP 5 KB)

{"aps":{"alert":{"title":"Door","body":"Opened"},
  "sound":"default","badge":3,"category":"DOOR",
  "mutable-content":1,"interruption-level":"time-sensitive"},
 "route":"/door/7"}                 // own keys beside "aps"
{"aps":{"content-available":1}}     // silent

Test: xcrun simctl push booted <bundle-id> x.apns, or drag a .apns file onto the Simulator; on a device, Apple’s Push Notifications Console or curl --http2.

Interview traps

• Device token ≠ .p8 key: the token addresses the install, the key authenticates the server.
• Works from Xcode, fails in TestFlight: Xcode builds use the sandbox host, TestFlight/App Store use production. Tokens differ, so you get BadDeviceToken.
• Silent pushes are best-effort: throttled (a few per hour), coalesced, delayed in Low Power Mode, and never delivered to an app the user force-quit.
• NSE “never runs”: no mutable-content, no visible alert, or it crashed on its tight memory limit (the original is shown).
• Foreground: nothing shows unless willPresent returns options.
• The token can change (restore, reinstall, new device). Re-register every launch. token.description is not the hex string.
• Permission is only for alerts. The token and silent pushes work without it.

Remember

Ask · Register · Upload · Send · Deliver. The key names the sender; the token names the phone.

Likely questions

1. .p8 vs .p12? — one JWT key for all apps, no expiry vs a yearly per-app cert.
2. Silent push? — content-available, priority 5, throttled wake.
3. Image in a push? — mutable-content + an NSE attaches it.
4. Where is a tap handled? — didReceive → userInfo → router.