NFCNDEFReaderSessionDelegate.swift

extension ViewController: NFCNDEFReaderSessionDelegate {
  func readerSession(_ session: NFCNDEFReaderSession, didInvalidateWithError error: Error) {
    /*
    Example of error message:
       "Session is invalidated by user cancellation" --- User has cancelled the NFC reader dialog
       "Session is invalidated due to maximum session timeout" --- Time out for scanning a NFC tag is exceeded.
       "Feature not supported" --- Device does not support the NFC feature (iPhone 6S or older) or app is running at simulator
    */
    print("The session was invalidated: \(error.localizedDescription)")
  }
  
  func readerSession(_ session: NFCNDEFReaderSession, didDetectNDEFs messages: [NFCNDEFMessage]) {
    // Step 7: Retrieving the NFC data from the NFCNDEFMessage list
    var result = ""
    messages.forEach { (nfcndefMessage) in
      nfcndefMessage.records.forEach({ (nfcndefPayload) in
        result += String.init(data: nfcndefPayload.payload.advanced(by: 3), encoding: .utf8)!
      })
    }
    
    // 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
    }
  }
}