UNUserNotificationCenter.current().getDeliveredNotifications only ever returns an empty array

Question

Basically, I'm simply trying to print the notifications that my app has delivered, but doing something like:

UNUserNotificationCenter.current().getDeliveredNotifications { (notifications) in
    print(notifications)
}

just displays an empty array in the Xcode console, even when the notification center contains several delivered notifications. (print(notifications.count) returns 0)

I can use getPendingNotificationRequests to successfully grab the notifications that are scheduled yet undelivered, update them, and even remove them with removePendingNotificationRequests(withIdentifiers:). But any attempt to access/modify/remove anything that's been delivered does not work.

Am I missing something simple here? Did something change recently with the way iOS 10+ accesses the delivered notifications?

Answer 1

It was empty when I used it on a UNNotificationServiceExtension, it turned out that I needed to use a Semaphore in order to prevent the extension to return 0 results:

        let semaphore = DispatchSemaphore(value: 0)
        let center = UNUserNotificationCenter.current()
        center.getDeliveredNotifications { notifications in
            defer {
                semaphore.signal()
            }

            let relatedNotificationIdentifiers = notifications.filter { notification in
                notification.request.content.userInfo["callId"] as? String == callId
                    && notification.request.identifier != request.identifier
            }.map(\.request.identifier)

            // This call is async
            center.removeDeliveredNotifications(withIdentifiers: relatedNotificationIdentifiers)

            // ...so this call needs to be done with a slight delay because otherwise
            // it will be killed before it is done removing the notifications
            DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) {
                contentHandler(bestAttemptContent)
            }
        }

        semaphore.wait()

Also, in the end it needed some wait to execute removeDeliveredNotifications which is async under the hood as well.

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