iOS SDK setup

import UserNotifications
import OneSignalExtension

class NotificationService: UNNotificationServiceExtension {
    var contentHandler: ((UNNotificationContent) -> Void)?
    var receivedRequest: UNNotificationRequest!
    var bestAttemptContent: UNMutableNotificationContent?

    // Note this extension only runs when `mutable_content` is set
    // Setting an attachment or action buttons automatically sets the property to true
    override func didReceive(_ request: UNNotificationRequest, withContentHandler contentHandler: @escaping (UNNotificationContent) -> Void) {
        self.receivedRequest = request
        self.contentHandler = contentHandler
        self.bestAttemptContent = (request.content.mutableCopy() as? UNMutableNotificationContent)

        if let bestAttemptContent = bestAttemptContent {
            // DEBUGGING: Uncomment the 2 lines below to check this extension is executing
//            print("Running NotificationServiceExtension")
//            bestAttemptContent.body = "[Modified] " + bestAttemptContent.body

            OneSignalExtension.didReceiveNotificationExtensionRequest(self.receivedRequest, with: bestAttemptContent, withContentHandler: self.contentHandler)
        }
    }

    override func serviceExtensionTimeWillExpire() {
        // Use this as an opportunity to deliver your "best attempt" at modified content, otherwise the original push payload will be used.
        if let contentHandler = contentHandler, let bestAttemptContent =  bestAttemptContent {
            OneSignalExtension.serviceExtensionTimeWillExpireRequest(self.receivedRequest, with: self.bestAttemptContent)
            contentHandler(bestAttemptContent)
        }
    }
}

// In application(_:didFinishLaunchingWithOptions:), BEFORE OneSignal.initialize()
UNUserNotificationCenter.current().delegate = self

func application(_ application: UIApplication,
                 didRegisterForRemoteNotificationsWithDeviceToken deviceToken: Data) {
    OneSignal.Notifications.didRegisterForRemoteNotifications(application, deviceToken: deviceToken)
}

func application(_ application: UIApplication,
                 didFailToRegisterForRemoteNotificationsWithError error: Error) {
    OneSignal.Notifications.handleDidFailRegisterForRemoteNotification(error as NSError)
}

func application(_ application: UIApplication,
                 didReceiveRemoteNotification userInfo: [AnyHashable: Any],
                 fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
    OneSignal.Notifications.receiveRemoteNotification(application,
                                                      userInfo: userInfo,
                                                      completionHandler: completionHandler)
}

func userNotificationCenter(_ center: UNUserNotificationCenter,
                            willPresent notification: UNNotification,
                            withCompletionHandler completionHandler: @escaping (UNNotificationPresentationOptions) -> Void) {
    OneSignal.Notifications.handleWillPresentNotificationInForeground(
        withPayload: notification.request.content.userInfo
    ) { notif in
        if notif != nil {
            if #available(iOS 14.0, *) {
                completionHandler([.banner, .list, .sound])
            } else {
                completionHandler([.alert, .sound])
            }
        } else {
            completionHandler([])
        }
    }
}

func userNotificationCenter(_ center: UNUserNotificationCenter,
                            didReceive response: UNNotificationResponse,
                            withCompletionHandler completionHandler: @escaping () -> Void) {
    OneSignal.Notifications.handleNotificationResponse(response)
    completionHandler()
}

OneSignal.Notifications.setBadgeCount(5)