Notification Service Extensions

import Foundation
import UserNotifications

final class NotificationService: UNNotificationServiceExtension {
    private var handler: ((UNNotificationContent) -> Void)?
    private var original: UNNotificationContent?
    private var bestAttempt: UNMutableNotificationContent?
    private var task: Task<Void, Never>?
    private let lock = NSLock()
    private var completed = false

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

        guard let content = bestAttempt else {
            finish(original)
            return
        }

        task = Task { [weak self] in
            guard let self else { return }
            await self.modify(content)
            self.finish(content)
        }
    }

    override func serviceExtensionTimeWillExpire() {
        finish(bestAttempt ?? original)
    }

    private func finish(_ content: UNNotificationContent?) {
        lock.lock()
        guard !completed, let content, let handler else {
            lock.unlock()
            return
        }
        completed = true
        self.handler = nil
        task?.cancel()
        task = nil
        lock.unlock()
        handler(content)
    }

    private func modify(_ content: UNMutableNotificationContent) async {
        // Keep downloads, decryption, and attachment creation bounded.
    }
}