Swift: NFCError Code=202 "Session is invalidated unexpectedly"

Question

I'm trying to use NFC. I followed those steps:

• Enabled NFC in the AppID configuration

• Created a provisioning profile and installed it

• Added NFC capability to the target

• Added the privacy description in the plist file

After this I imported CoreNFC and implemented those code:

@available(iOS 11.0, *)    
    extension EventPreviewViewController: NFCNDEFReaderSessionDelegate {
            func readerSession(_ session: NFCNDEFReaderSession, didInvalidateWithError error: Error) {
                let alert = UIAlertController.withOkButton(andTitle: NSLocalizedString("TitleWarning"), andText: NSLocalizedString("ErrorNFCInvalidate"), okHandler: nil)
                self.present(alert, animated: true, completion: nil)
            }

            func readerSession(_ session: NFCNDEFReaderSession, didDetectNDEFs messages: [NFCNDEFMessage]) {
                // TODO
            }
        }


class EventPreviewViewController: UITableViewController {
@available(iOS 11.0, *)
var nfcSession: NFCNDEFReaderSession {
        return NFCNDEFReaderSession(delegate: self, queue: nil, invalidateAfterFirstRead: true)
    }

    @IBAction func startAccess(_ sender: UIButton) {
    if #available(iOS 11.0, *) {
                    nfcSession.begin()
                } else {
                    let alert = UIAlertController.withOkButton(andTitle: NSLocalizedString("TitleWarning"), andText: NSLocalizedString("ErrorNFCUnsupported"), okHandler: nil)
                    self.present(alert, animated: true, completion: nil)
                }
    }
}

Why I keep getting "Error Domain=NFCError Code=202 "Session is invalidated unexpectedly" UserInfo={NSLocalizedDescription=Session is invalidated unexpectedly}"?

Answer 1

update for ios13 / swift 5.1

a) original sample from Apple does sometimes fails the same error.

(https://developer.apple.com/documentation/corenfc/building_an_nfc_tag-reader_app)

b) if fails (seems a be stupid.. anyway.. works..) reboot device. It happens ;(

Answer 2

I am not sure but below line causing this error Session is invalidated unexpectedly

When I was worked with CoreNFC, I was faced similar kind of issue. Fix it by defining as property

let nfcSession = NFCNDEFReaderSession(delegate: self, queue: DispatchQueue(label: "queueName", attributes: .concurrent), invalidateAfterFirstRead: true)

I suggest you need to define nfcSession as property.

var nfcSession: NFCNDEFReaderSession?

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
    self.nfcSession = NFCNDEFReaderSession(delegate: self, queue: DispatchQueue.global(qos: .background), invalidateAfterFirstRead: false)
    self.nfcSession?.begin()
    return true
}

Update:

You can define a property for iOS 11 like below.

@available(iOS 10.0, *)
    var session: NFCNDEFReaderSession?