Not able to format NFC card using NdefFormatable

问题:

I am using NdefFormatable class to format my NFC card. The code is like:

NdefFormatable formatable=NdefFormatable.get(tag);

But i get formatable reference as null. I checked the code that is run inside the get(tag) method. It checks:

if (!tag.hasTech(TagTechnology.NDEF_FORMATABLE)) return null;

I ran "tag.getTechList()" for my tag. I got:

android.nfc.tech.NfcA
android.nfc.tech.Ndef

I was able to format this card using Trigger application.

What can be done to format this.

回答1:

The NdefFormatable and the Ndef tag technologies are mutually exclusive. Either your tag is not yet NDEF formatted (i.e. it is not yet prepared to store NDEF messages) or it is already prepared to store NDEF messages (and already contains (at least an empty) an NDEF message).

So NdefFormatable is only used for writing an initial NDEF message to a tag.

As your tag already lists the android.nfc.tech.Ndef technology, it is already prepared to store an NDEF message and does not need further formatting. You can simply overwrite (given that the tag is not read-only) any existing NDEF message by using the writeNdefMessage() method of the Ndef object. E.g. to "format" the tag to an empty NDEF message, you could do something like:

Ndef ndefTag = Ndef.get(tag);
ndefTag.writeNdefMessage(new NdefMessage(new NdefRecord(NdefRecord.TNF_EMPTY, null, null, null)));

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Update: On devices with the libnfc-nci NFC stack, it seems that NdefFormatable and Ndef tag technologies are no longer mutually exclusive. Formatted tags will (sometimes?) be enumerated as both NdefFormatable and Ndef. In thoses cases, you would still typically try to use the Ndef first and only if that fails use the NdefFormatable object.

回答2:

Your device cannot format the tag to NDEF. Otherwise you would have gotten android.nfc.tech.NdefFormatable in the tech list.

But you can still read and write in NDEF format to the tag.

I am guessing Trigger just writes an empty message to the Ndef tag.