Background NFC Tag reading on iOS - No usable data found

Question

I'm trying to implement background NFC tag reading for an iOS app. The NFC tag I am using has specific requirements as to the format that must be written - specifically, only the second record can be a URI, and it must be 56 characters long.

I've constructed and written the following NDEF message:

        let message = NFCNDEFMessage(records: [
            NFCNDEFPayload(
                format: .nfcExternal,
                type: "-- omitted --".data(using: .utf8)!,
                identifier: "".data(using: .utf8)!,
                payload: "-- omitted --".data(using: .utf8)!
            ),
            NFCNDEFPayload(
                format: .nfcWellKnown,
                type: "U".data(using: .utf8)!,
                identifier: "".data(using: .utf8)!,
                payload: "https://apple.com?                                      ".data(using: .utf8)!
            ),
            NFCNDEFPayload(
                format: .nfcWellKnown,
                type: "T".data(using: .utf8)!,
                identifier: "".data(using: .utf8)!,
                payload: "-- omitted --".data(using: .utf8)!
            )
        ])

Now when I touch an iOS device to the phone, I get a prompt saying (original in parens, assumed English translation first):

NFC-TAG DETECTED (NFC-TAG GEDETECTEERD)

No usable data found (Green bruikbare gegevens gevonden)

I have tried replacing the whitespace with ?_______... or with Universal Links but I always get the same response. When I try reading other NFC tags that I have not written to I don't get the prompt, so I know the new message is activating background reading, but I can't figure out why it can't be read.

My only guess is that iOS is not decoding the .utf8 byte array, but I don't know how else to store/encode a message to an NFC tag

Answer 1

I replaced:

NFCNDEFPayload(
   format: .nfcWellKnown,
   type: "U".data(using: .utf8)!,
   identifier: "".data(using: .utf8)!,
   payload: "https://apple.com?                                      ".data(using: .utf8)!
),

with:

NFCNDEFPayload.wellKnownTypeURIPayload(string: "https://apple.com?_____________________________________________")!

I don't know why

1. I needed a longer string (63 characters)
2. Why that string was still only 56 bytes
3. What is different in the constructor
4. Why I did in fact need the underscores, as whitespace did not register as a background tag

Perhaps someone else can yet explain this solution. However, it works!

Answer 2

So https://developer.apple.com/documentation/corenfc/adding_support_for_background_tag_reading says

Note
Background tag reading doesn’t support custom URL schemes. Use universal links instead.

With

NFCNDEFPayload(
   format: .nfcWellKnown,
   type: "U".data(using: .utf8)!,
   identifier: "".data(using: .utf8)!,
   payload: "https://apple.com?                                      ".data(using: .utf8)!
),

I don't think you are using one of the supported URL schema from the linked docs, you are using a "Custom URL scheme" that it says it does not support.

Using the help function NFCNDEFPayload.wellKnownTypeURIPayload this probably decodes your string in to a supported URL scheme.

The docs are not clear and references the NDEF spec for URI's which is at https://github.com/haldean/ndef/blob/master/docs/NFCForum-TS-RTD_URI_1.0.pdf

So my guess is that to do the more manual create of the NFCNDEFPayload you need to format it correctly as

NFCNDEFPayload(
   format: .nfcWellKnown,
   type: "U".data(using: .utf8)!,
   identifier: "4".data(using: .utf8)!,
   payload: "apple.com?".data(using: .utf8)!
),

As 4 is the decimal code for the https:// prefix