将纯文本写入NFC标签，意图问题

Question

I'm trying to make an application that writes a simple plain text message to an NFC tag. The user writes a message in the EditText box, then on button press, the app writes the text onto a tag. I've been following this tutorial: http://www.framentos.com/it/android-tutorial/2012/07/31/write-hello-world-into-a-nfc-tag-with-a/ However, when i try to put the NFC tag up next to it, the phone opens up another app, instead of using mine that is already open. I know the problem's with the intents, but i don't want to declare the intent filter in the manifest, because i don't want the app to open up when the tag is placed next to the phone, i want the app to be already open beforehand. Here's the code for the application:

import android.app.PendingIntent;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.nfc.FormatException;
import android.nfc.NdefMessage;
import android.nfc.NdefRecord;
import android.nfc.NfcAdapter;
import android.nfc.Tag;
import android.nfc.tech.Ndef;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import java.io.IOException;
import java.io.UnsupportedEncodingException;

public class MainActivity extends AppCompatActivity {

    NfcAdapter adapter;
    PendingIntent pendingIntent;
    IntentFilter writeTagFilters[];
    Tag mytag;
    Context ctx;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ctx=this;
        Button btnWrite = (Button) findViewById(R.id.button);
        final EditText message = (EditText)findViewById(R.id.edit_message);
        btnWrite.setOnClickListener(new View.OnClickListener()
        {
            @Override
            public void onClick(View v) {
                try {
                    if(mytag==null){
                        Toast.makeText(ctx, ctx.getString(R.string.error_detected), Toast.LENGTH_LONG).show();
                    }else{
                        write(message.getText().toString(),mytag);
                        Toast.makeText(ctx, ctx.getString(R.string.ok_writing), Toast.LENGTH_LONG ).show();
                    }
                } catch (IOException e) {
                    Toast.makeText(ctx, ctx.getString(R.string.error_writing), Toast.LENGTH_LONG ).show();
                    e.printStackTrace();
                } catch (FormatException e) {
                    Toast.makeText(ctx, ctx.getString(R.string.error_writing) , Toast.LENGTH_LONG ).show();
                    e.printStackTrace();
                }
            }
        });

        adapter = NfcAdapter.getDefaultAdapter(this);
        pendingIntent = PendingIntent.getActivity(this, 0, new Intent(this, getClass()).addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP), 0);
        IntentFilter tagDetected = new IntentFilter(NfcAdapter.ACTION_NDEF_DISCOVERED);
        tagDetected.addCategory(Intent.CATEGORY_DEFAULT);
        writeTagFilters = new IntentFilter[] { tagDetected };
    }

    private NdefRecord createRecord(String text) throws UnsupportedEncodingException {

        //create the message in according with the standard
        String lang = "en";
        byte[] textBytes = text.getBytes();
        byte[] langBytes = lang.getBytes("US-ASCII");
        int langLength = langBytes.length;
        int textLength = textBytes.length;

        byte[] payload = new byte[1 + langLength + textLength];
        payload[0] = (byte) langLength;

        // copy langbytes and textbytes into payload
        System.arraycopy(langBytes, 0, payload, 1, langLength);
        System.arraycopy(textBytes, 0, payload, 1 + langLength, textLength);

        NdefRecord recordNFC = new NdefRecord(NdefRecord.TNF_WELL_KNOWN, NdefRecord.RTD_TEXT, new byte[0], payload);
        return recordNFC;
    }

    private void write(String text, Tag tag) throws IOException, FormatException {

        NdefRecord[] records = { createRecord(text) };
        NdefMessage message = new NdefMessage(records);
        Ndef ndef = Ndef.get(tag);
        ndef.connect();
        ndef.writeNdefMessage(message);
        ndef.close();
    }

    @Override
    protected void onNewIntent(Intent intent){
        if(NfcAdapter.ACTION_NDEF_DISCOVERED.equals(intent.getAction())){
            mytag = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);
            Toast.makeText(this, this.getString(R.string.ok_detection) + mytag.toString(), Toast.LENGTH_LONG ).show();
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {


   return true;
    }

    return super.onOptionsItemSelected(item);
}

}

Please help, i don't know what i'm doing wrong! I'm just a beginner at NFC and Android programming!

Answer 1

You have to use enable forground dispatch : http://developer.android.com/guide/topics/connectivity/nfc/advanced-nfc.html#foreground-dispatch

The foreground dispatch system allows an activity to intercept an intent and claim priority over other activities that handle the same intent.

UPDATE1

It's difficult to write on all kind of tags, some of them have closed specifications. You could start by writing on Ndef and NdefFormatable tags.

For instance:

@Override 
protected void onPause() {
    super.onPause();
    mAdapter.disableForegroundDispatch(this);
}

@Override 
protected void onResume(){
  super.onResume();
  PendingIntent pendingIntent=PendingIntent.getActivity(this,0,new Intent(this,getClass()).addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP),0);
  IntentFilter ndef=new IntentFilter(NfcAdapter.ACTION_NDEF_DISCOVERED);
  try {
    ndef.addDataType("*/*");
  }
 catch (  MalformedMimeTypeException e) {
    Log.e(TAG,"Bad MIME type declared",e);
    return;
  }
  IntentFilter[] filters=new IntentFilter[]{ndef};
  String[][] techLists=new String[][]{new String[]{Ndef.class.getName()},new String[]{NdefFormatable.class.getName()}};
  mNfcAdapter.enableForegroundDispatch(this,pendingIntent,filters,techLists);
}

@Override 
protected void onNewIntent(Intent intent) {
    Tag tag = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);
    String[] techList = tag.getTechList();
    for (String tech : techList) {
          if (tech.equals(Ndef.class.getName())) {
              //write NDEF msg
          } else if (tech.equals(NdefFormatable.class.getName())) {
              //format and write NDEF msg
          }
    }
}