bitwarden-mobile/src/iOS.Core/NFCReaderDelegate.cs

using CoreNFC;
using Foundation;
using System;
using System.Collections.Generic;
using System.Text.RegularExpressions;

namespace Bit.iOS.Core
{
    public class NFCReaderDelegate : NFCNdefReaderSessionDelegate
    {
        private Regex _otpPattern = new Regex("^.*?([cbdefghijklnrtuv]{32,64})$");
        private Action<bool, string> _callback;

        public NFCReaderDelegate(Action<bool, string> callback)
        {
            _callback = callback;
        }

        public override void DidDetect(NFCNdefReaderSession session, NFCNdefMessage[] messages)
        {
            var results = new List<string>();
            foreach (var message in messages)
            {
                foreach (var record in message.Records)
                {
                    try
                    {
                        results.Add(new NSString(record.Payload, NSStringEncoding.UTF8));
                    }
                    catch { }
                }
            }

            foreach (var result in results)
            {
                var matches = _otpPattern.Matches(result);
                if (matches.Count > 0 && matches[0].Groups.Count > 1)
                {
                    var otp = matches[0].Groups[1].ToString();
                    _callback.Invoke(true, otp);
                    return;
                }
            }

            _callback.Invoke(false, "No tags were read.");
        }

        public override void DidInvalidate(NFCNdefReaderSession session, NSError error)
        {
            _callback.Invoke(false, error?.LocalizedDescription);
        }
    }
}