Wait for completion handler to finish - Swift

Do not wait, use a completion handler, for convenience with an enum:

enum AuthResult {
    case success(Bool), failure(Error)
}

func checkAvailabilty(completion: @escaping (AuthResult) -> ()) {

    //
    // other checking
    //
    UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .sound], completionHandler: { (granted, error) in
        if error != nil {
            completion(.failure(error!))
        } else {
            completion(.success(granted))
        }

    })
}

And call it:

checkAvailabilty { result in
    switch result {
    case .success(let granted) : 
      if granted {
         print("access is granted")
      } else {
         print("access is denied")
      }
    case .failure(let error): print(error)
    }
}

Yeah. So as you figured what is happening here is that the function returns before the completion handler gets called. So what you want to do is pass an asynchronous callback to the checkAvailability function so it will callback once the completion handler is fired.

    func checkAvailability(callback: @escaping (Bool) -> Void) {

    //
    // other checking
    //

        UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .sound], completionHandler: { (granted, error) in
            if granted {
                callback(true)
            } else {
                callback(false)
            }
        })
    }

you would call this function like so...

    checkAvailability(callback: { (isAvailable) -> Void in
        if isAvailable {
            // notifications are available
        } else {
            // present alert
        }
    })

Keep in mind that when you go to present the alert you may need to explicitly dispatch the call to the main thread since the completion handler may callback on a different thread. In which case this is how you would want to call the function and present the alert...

    checkAvailability(callback: { (isAvailable) -> Void in
        if isAvailable {
            // notifications are available
        } else {
            DispatchQueue.main.async {
                // present alert
            }
        }
    })