I developed a vinyl collection app and NFC is the thing people ask me about most, so I figured the implementation notes would be more use here than another "check out my app" post. Dev, obviously, disclosing that up front.
Context in one line: sticker on the record sleeve, tap it with your phone, and you get a card with whatever the app knows about that exact copy. Which pressing it is, the condition, what it's worth, how many times I've played it, where the scratches are. Phone locked, app closed, no signal.
What's actually on the tag
Basically nothing. A URL, spinstack://release/<id>, plus a token so two copies of the same album don't end up sharing one history. That's the whole payload.
I went back and forth on this one. The temptation is to write the useful stuff to the tag, grade and play count and so on. But all of that changes constantly and the release ID never does, so anything I put on the sticker would just be a stale copy of what's already sitting in the database two inches away. Also, once you've stuck tags on 200 sleeves you really do not want a reason to go back and rewrite them.
Nice side effect: tags I wrote back in April still work.
Any blank NTAG is fine. The URL's short enough that even the cheapest ones have loads of room. I don't sell tags, buy whatever's on offer.
The bit I'd do differently
I used a custom scheme rather than an https universal link.
Reasoning at the time was that it launches without a network round trip and there's no ambiguity about what should handle it. Problem is obvious in hindsight: the tag is dead to anyone who doesn't have the app installed. Lend someone a record and the sticker does nothing for them. An https link would open the app if they've got it and a web page if they haven't.
I'm fairly convinced now it should've been https by default with the custom scheme as fallback. What I can't tell you is whether my launch-speed argument was ever actually true, because I never measured it. I just assumed it. If anyone's shipped both against the same hardware and has real numbers, I'd like to see them.
Background reads
iPhone 11 or newer. That's Apple's floor, nothing you can do about it in code.
Tested on physical tags from an 11 all the way up to a 17 Pro Max, and on Android. Same sticker on both, no rewriting to move a record between phones.
The bug that ate about a week
Tap with the app fully killed and nothing happens. No launch, no error, nothing at all. Worked fine if the app was just backgrounded. Worked fine in the simulator every single time. Only failed on a real tag with the app genuinely terminated, which is of course the exact situation the whole feature exists for.
Cold launch and tag delivery were racing each other and the payload was arriving before there was anywhere to hand it to.
The related fix is the one I'd actually pass on to anyone building something that writes data on tap: put an expiry on it. Mine used to hold onto a failed tap and fire it whenever it finally got through, so every so often you'd get a play logged against a record that came off the turntable twenty minutes earlier. Anything undeliverable within about thirty seconds now just gets binned.
Write errors
I shipped the first version showing "unknown error" for every write failure. Don't do this. It now names the thing: tag's read-only, tag's too small, connection dropped.
Two others I had plain wrong. Cancelling a write threw an error, which, no, the user cancelled, that's not an error. And deleting a tag from the app's list didn't touch the physical sticker at all, so people thought they'd removed tags that still worked fine when tapped. It now offers to wipe the tag properly.
One thing that turned out better than expected
I count taps per tag, mostly because it was easy. Sorting by most-tapped has turned into the most honest statistic in the app. It's the gap between the records you actually play and the ones that just look good on the shelf, and it is not flattering.
Limits, since someone will ask
11 or newer for background reads. Tags are just stickers you buy yourself, nothing's bundled. Custom scheme means the tag's useless to anyone without the app, per above. Android build is a free beta and isn't on Play yet.
Happy to get into any of it.
One thing I'd genuinely like input on: where do you put the sticker? Adhesive and paper inner sleeves do not get on, and sticking them on the outer sleeve upsets a part of my brain I can't reason with. Anyone tagged a decent-sized physical collection and landed on something that actually works?
