How to register for alert push notification in iOS

Issue #452

Use UserNotifications framework

import FirebaseMessaging
import UserNotifications

final class PushHandler: NSObject {
    private let center = UNUserNotificationCenter.current()
    private let options: UNAuthorizationOptions = [.alert]

    func setup() {
        Messaging.messaging().delegate = self
    }

    func register() {
        center.requestAuthorization(options: options, completionHandler: { granted, error in
            print(granted, error)
            self.getSettings()
        })
    }

    func getSettings() {
        center.getNotificationSettings(completionHandler: { settings in
            guard
                case let .authorized = settings.authorizationStatus,
                case let .enabled = settings.alertSetting,
                settings.alertStyle != .none
            else {
                return
            }

            // TODO
        })
    }

    func handle(userInfo: [AnyHashable: Any]) {
        
    }
}

extension PushHandler: MessagingDelegate {
    func messaging(_ messaging: Messaging, didReceiveRegistrationToken fcmToken: String) {
        print(fcmToken)
    }

    func messaging(_ messaging: Messaging, didReceive remoteMessage: MessagingRemoteMessage) {
        print(remoteMessage)
    }
}

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    func application(
        _ application: UIApplication,
        didReceiveRemoteNotification userInfo: [AnyHashable: Any],
        fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
        completionHandler(.noData)

        Deps.pushHandler.handle(userInfo: userInfo)
    }
}

A test message from Firebase looks like

[AnyHashable("google.c.a.e"): 1, AnyHashable("google.c.a.ts"): 1570701857, AnyHashable("aps"): {
    alert =     {
        body = Test;
        title = Test;
    };
}, AnyHashable("google.c.a.udt"): 0, AnyHashable("google.c.a.c_l"): Test, AnyHashable("gcm.message_id"): 1570701857965182, AnyHashable("google.c.a.c_id"): 1257698497812622975, AnyHashable("gcm.n.e"): 1]