如何检测 NFC 标签被移除

Question

I am new at NFC technology. I am trying to read and write Mifare Classic 4K tags. I succeeded reading and writing. I can detect the tag on onNewIntent action.

My problem is that I could't detect when an NFC tag was removed (similar to onNewIntent when detecting a tag).

How can I solve this problem?

Answer 1

There is no event that notifies your app if a tag is removed from the reader. A typical approach to detect removal is to actively poll the tag (eg by reading an NDEF message or a data block) until you receive IOException indicating that communication with the tag was lost.

---

UPDATE

As of Android API level 24 this is no longer true. There is now a method

NfcAdapter.ignore(tag, debounceMs, tagRemovedListener, handler);

that can be used to register a callback for getting notifications upon tag removal. This method may have some issue though, so carefully read the documentation on side-effects/corner cases.

Answer 2

You can check by calling isoDep.isConnected(); in IsoDep