iOS 푸시 알림 면접 2026: APNs, 토큰, 트러블슈팅

import UIKit
import UserNotifications

@main
class AppDelegate: UIResponder, UIApplicationDelegate {

    func application(
        _ application: UIApplication,
        didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
    ) -> Bool {
        // Request notification authorization
        UNUserNotificationCenter.current().requestAuthorization(
            options: [.alert, .badge, .sound]
        ) { granted, error in
            guard granted else { return }
            // Register with APNs
            DispatchQueue.main.async {
                application.registerForRemoteNotifications()
            }
        }
        return true
    }
}

extension AppDelegate {

    // Callback when APNs provides the token
    func application(
        _ application: UIApplication,
        didRegisterForRemoteNotificationsWithDeviceToken deviceToken: Data
    ) {
        // Convert token to hexadecimal string
        let tokenString = deviceToken.map { String(format: "%02.2hhx", $0) }.joined()
        print("Device Token: \(tokenString)")

        // Send to backend server
        sendTokenToServer(tokenString)
    }

    // Callback on registration failure
    func application(
        _ application: UIApplication,
        didFailToRegisterForRemoteNotificationsWithError error: Error
    ) {
        print("Failed to register: \(error.localizedDescription)")
    }

    private func sendTokenToServer(_ token: String) {
        // Implement HTTP request to backend
    }
}

enum APNsError: Int {
    case badDeviceToken = 400
    case unregistered = 410
    case payloadTooLarge = 413
    case tooManyRequests = 429
    case internalServerError = 500

    var shouldRemoveToken: Bool {
        // Remove token only if device is no longer registered
        return self == .unregistered || self == .badDeviceToken
    }
}

struct APNsResponse {
    let statusCode: Int
    let deviceToken: String

    func handleError() {
        guard let error = APNsError(rawValue: statusCode) else { return }

        if error.shouldRemoveToken {
            // Remove token from database
            TokenRepository.shared.remove(deviceToken)
        }
    }
}

func application(
    _ application: UIApplication,
    didReceiveRemoteNotification userInfo: [AnyHashable: Any],
    fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void
) {
    // Check if this is a silent notification
    guard let aps = userInfo["aps"] as? [String: Any],
          aps["content-available"] as? Int == 1 else {
        completionHandler(.noData)
        return
    }

    // Perform background task
    performBackgroundTask { result in
        switch result {
        case .success:
            completionHandler(.newData)
        case .failure:
            completionHandler(.failed)
        }
    }
}

import UserNotifications

class NotificationService: UNNotificationServiceExtension {

    var contentHandler: ((UNNotificationContent) -> Void)?
    var bestAttemptContent: UNMutableNotificationContent?

    override func didReceive(
        _ request: UNNotificationRequest,
        withContentHandler contentHandler: @escaping (UNNotificationContent) -> Void
    ) {
        self.contentHandler = contentHandler
        bestAttemptContent = request.content.mutableCopy() as? UNMutableNotificationContent

        guard let bestAttemptContent = bestAttemptContent else {
            contentHandler(request.content)
            return
        }

        // Modify content
        if let imageURLString = bestAttemptContent.userInfo["image-url"] as? String,
           let imageURL = URL(string: imageURLString) {
            downloadImage(from: imageURL) { attachment in
                if let attachment = attachment {
                    bestAttemptContent.attachments = [attachment]
                }
                contentHandler(bestAttemptContent)
            }
        } else {
            contentHandler(bestAttemptContent)
        }
    }

    override func serviceExtensionTimeWillExpire() {
        // Called when time limit (30 seconds) is exceeded
        if let contentHandler = contentHandler,
           let bestAttemptContent = bestAttemptContent {
            contentHandler(bestAttemptContent)
        }
    }

    private func downloadImage(
        from url: URL,
        completion: @escaping (UNNotificationAttachment?) -> Void
    ) {
        URLSession.shared.downloadTask(with: url) { localURL, _, error in
            guard let localURL = localURL, error == nil else {
                completion(nil)
                return
            }

            let tempDirectory = FileManager.default.temporaryDirectory
            let fileName = url.lastPathComponent
            let destinationURL = tempDirectory.appendingPathComponent(fileName)

            try? FileManager.default.moveItem(at: localURL, to: destinationURL)

            let attachment = try? UNNotificationAttachment(
                identifier: fileName,
                url: destinationURL,
                options: nil
            )
            completion(attachment)
        }.resume()
    }
}

struct PushNotificationDebugger {

    static func checkNotificationStatus() async {
        let center = UNUserNotificationCenter.current()
        let settings = await center.notificationSettings()

        print("=== Push Notification Status ===")
        print("Authorization: \(settings.authorizationStatus.description)")
        print("Alert: \(settings.alertSetting.description)")
        print("Badge: \(settings.badgeSetting.description)")
        print("Sound: \(settings.soundSetting.description)")
        print("Notification Center: \(settings.notificationCenterSetting.description)")
    }
}

extension UNAuthorizationStatus {
    var description: String {
        switch self {
        case .notDetermined: return "Not Determined"
        case .denied: return "Denied"
        case .authorized: return "Authorized"
        case .provisional: return "Provisional"
        case .ephemeral: return "Ephemeral"
        @unknown default: return "Unknown"
        }
    }
}

struct APNsTestPayload {

    static let silentNotification = """
    {
        "aps": {
            "content-available": 1
        },
        "custom-data": "test"
    }
    """

    static let richNotification = """
    {
        "aps": {
            "alert": {
                "title": "New message",
                "body": "Message content"
            },
            "mutable-content": 1,
            "sound": "default"
        },
        "image-url": "https://example.com/image.jpg"
    }
    """
}

actor PushTokenManager {

    private var pendingToken: String?
    private var retryCount = 0
    private let maxRetries = 3

    func registerToken(_ token: String) async {
        pendingToken = token
        await sendTokenWithRetry()
    }

    private func sendTokenWithRetry() async {
        guard let token = pendingToken else { return }

        do {
            try await APIClient.shared.registerPushToken(token)
            pendingToken = nil
            retryCount = 0
        } catch {
            retryCount += 1
            if retryCount < maxRetries {
                // Retry with exponential backoff
                let delay = UInt64(pow(2.0, Double(retryCount))) * 1_000_000_000
                try? await Task.sleep(nanoseconds: delay)
                await sendTokenWithRetry()
            }
        }
    }
}

struct TokenStorage {

    private static let tokenKey = "com.app.pushToken"

    static func save(_ token: String) {
        UserDefaults.standard.set(token, forKey: tokenKey)
    }

    static func retrieve() -> String? {
        UserDefaults.standard.string(forKey: tokenKey)
    }

    static func hasTokenChanged(_ newToken: String) -> Bool {
        guard let savedToken = retrieve() else { return true }
        return savedToken != newToken
    }
}