NFC

Overview

ExNfc polls the NFC antenna on the back of the phone and reports tags that come close.

Is the NFC controller up?

ExNfc.state()

:polling means it is waiting for a tag.

ExNfc.list_devices()

Detect tags

Run this cell, then hold a tag or a contactless card flat against the back of the phone, over the lower half of the battery. The antenna is there, not at the top of the phone. It listens for 30 seconds.

ExNfc.subscribe()
frame = Kino.Frame.new() |> Kino.render()

deadline = System.monotonic_time(:millisecond) + 30_000

# One tap produces a burst of arrivals a few milliseconds apart, so a tag
# counts as back only after it has been out of sight for a second.
# last_seen maps each UID to the time it was last detected.
listen = fn listen, last_seen ->
  now = System.monotonic_time(:millisecond)

  if now < deadline do
    receive do
      {ExNfc, :tag_arrived, tag} ->
        seen_at = Map.get(last_seen, tag.uid_hex)

        if seen_at == nil or now - seen_at >= 1_000 do
          Kino.Frame.append(frame, Kino.Markdown.new("Tag `#{tag.uid_hex}` (#{tag.protocol})"))
        end

        listen.(listen, Map.put(last_seen, tag.uid_hex, now))

      {ExNfc, _other, _} ->
        listen.(listen, last_seen)
    after
      deadline - now -> :ok
    end
  end
end

listen.(listen, %{})
ExNfc.unsubscribe()

Each tag is reported once, with its UID and protocol. Take it away for a second and tap again to see it reported again.