iOS Notifications While App is Open

You first need a class that acts as the delegate for the userNotificationCenter.

class AppNotificationDelegate: NSObject, UNUserNotificationCenterDelegate {
    func userNotificationCenter(_ center: UNUserNotificationCenter, willPresent notification: UNNotification, withCompletionHandler completionHandler: @escaping (UNNotificationPresentationOptions) -> Void) {
        // Customize the appearance of the notification, for example:
        completionHandler([.banner, .sound])

Then you need to create an instance of this on your @main, and then tell the notification center what the delegate is.

let notificationDelegate = POSTAppNotificationDelegate()
init() {
    let notificationCenter = UNUserNotificationCenter.current()
    notificationCenter.delegate = notificationDelegate