DirectlyShowingAlert.swift

// invalidateAfterFirstRead = true
var nfcSession = NFCNDEFReaderSession.init(delegate: self, queue: nil, invalidateAfterFirstRead: true)
nfcSession.begin()

extension ViewController: NFCNDEFReaderSessionDelegate {
  func readerSession(_ session: NFCNDEFReaderSession, didDetectNDEFs messages: [NFCNDEFMessage]) {
    // Handling of [NFCNDEFMessage] and return the result String
    var result = ...
    
    // Step 8: didDetectNDEFs callback is run in background thread. All UI updates must be handled carefully.
    DispatchQueue.main.async { [weak self] in
      self?.showAlert("Scanned NFC tag info: " + result) // Simply show an UIAlertController with message
    }
  }
}