As part of the NFC intent received by your activity, you will also receive a tag-handle (Tag
object) in an intent extra:
Tag tag = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);
Depending on the type of tag, you can then get an instance of the specific tag technology. For instance, if it's an NDEF tag, you can get:
Ndef ndefTag = Ndef.get(tag);
Then you can connect to the tag using the connect() method:
ndefTag.connect();
After that you can check if the tag is still "connected" to the device by periodically trying to read from the tag:
try {
ndefTag.getNdefMessage();
} catch (IOException e) {
// if you receive an IOException, contact to the tag has been lost
}
Note that this will only work if your activity is in the foreground all the time and the screen remains on.