Check if NFC tag is near

问题

Is it possible to force android to check if an NFC tag is near? I'm only able to read the tag when android detects it, I would like to force it to check if a tag is near at a specific moment

回答1:

What you want to do is in general not possible. However if you can live with a dirty hack the following will work (thanks to unspecified behaviour):

First disable the reader-mode of all supported tag types. This brings the NFC subsystem into a clean state, e.g. it makes sure that the NFC controller will have no connection to the tag.

Once done restore the reader-mode again. If a tag is present at that moment you will get the usual discovery action as an intent. It may take a second or two though.

Control of the reader-mode is possible using NfcAdapter.enableReaderMode and NfcAdapter.disableReaderMode

回答2:

I figured out a bit of a hack for this that works (for me, at least!)

First, when you initially detect the tag via the android.nfc.action.NDEF_DISCOVERED, get the tag as a field in your class and start a timer (this is in C#/Xamarin but the same should apply for Java):

_tag = Intent.GetParcelableExtra(NfcAdapter.ExtraTag) as Tag;

ReaderTimer = new Timer(2000);
ReaderTimer.Elapsed += TimerElapsed;
ReaderTimer.Start();

Now, every 2 seconds this will fire. It will attempt to reconnect to the tag. If it can't the tag is gone.:

    private void TimerElapsed(object sender, ElapsedEventArgs e)
    {
        if (_tag == null)
        {
            return;
        }

        Ndef ndef = Ndef.Get(_tag);

        if (ndef == null)
        {
            // NDEF is not supported by this Tag.
            return;
        }

        if (!ndef.IsConnected)
        {
            try
            {
                ndef.Close();
                ndef.Connect();
            }
            catch (Exception ex)
            {
                // could not reconnect
                // implies tag is not in proximity

                //do whatever needs to be done when NFC is disconnected

                ReaderTimer.Stop();
            }
        }
    }

I've tested this with API 14.

Unfortunately when the screen is turned off ndef.Connect() fails, so that is registered as a disconnect.

回答3:

Try this code and it works ! It will continuously check if the NFC tag is near to the phone or not.

@Override
protected void onNewIntent(Intent intent) {
    setIntent(intent);
    readFromIntent(intent);
    if(NfcAdapter.ACTION_TAG_DISCOVERED.equals(intent.getAction())){
        myTag = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);

    final Handler handler = new Handler();
    handler.postDelayed(new Runnable() {

        @Override
        public void run() {
        checkNFCStatus();
        handler.postDelayed(this,1000);
        }

    }, 1000);

    }
}

public void checkNFCStatus(){
    try {
        if(myTag != null) {
            Ndef ndefTag = Ndef.get(myTag);
            ndefTag.connect();
            if (ndefTag.isConnected()) {
                Log.d("network", "NFC connected");
            } else {
                Log.d("network", "NFC disconnected");
            }
            ndefTag.close();
        }

    } catch (IOException e) {
        e.printStackTrace();
        Log.d("network", "NFC disconnected");
    }
}

//If the connection is not closed, an exception will be thrown

来源：https://stackoverflow.com/questions/27017813/check-if-nfc-tag-is-near

标签

android

nfc