从标签获取NDEF消息会冻结Android应用

Question

I'm trying to read an NDEF message from an NFC tag. The detection of the tag works correctly. For reading the NDEF data from the tag I'm running a TimerTask. The task polls the NDEF message from the tag with getNdefMessage() every 900 ms and updates the UI.

The procedure works perfect until I remove the phone. Than the app freezes without a logcat error message.

Does anyone have an idea why this happens?

package com.example.infcdemo;

import java.io.IOException;
import java.util.Timer;

import android.app.Activity;
import android.app.PendingIntent;
import android.content.Intent;
import android.content.IntentFilter;
import android.nfc.NfcAdapter;
import android.nfc.Tag;
import android.nfc.tech.Ndef;
import android.nfc.tech.NdefFormatable;
import android.nfc.tech.NfcA;
import android.os.Bundle;
import android.os.Handler;
import android.widget.TextView;

public class MainActivity extends Activity
{
    protected NfcAdapter mAdapter;
    protected PendingIntent mPendingIntent;
    protected IntentFilter mIntentfilter;
    protected String[][] mTechLists;
    protected IntentFilter[] mFilters;
    protected NfcA nfca = null;
    protected Intent mIntent = null;
    protected Tag mTag;
    private boolean nfc_initialized = false;

    Tag myTag = null;
    Ndef _ndef = null;

    Timer _incomingMessageTimer;


    @Override
    protected void onCreate(Bundle savedInstanceState) 
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        if (nfc_initialized == false) 
        {
            // Initialize NFC Specific
            mAdapter = NfcAdapter.getDefaultAdapter(this);
            mPendingIntent = PendingIntent.getActivity(this, 0,
                    new Intent(this, getClass())
                            .addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP), 1);
            IntentFilter ndef = new IntentFilter(
                    NfcAdapter.ACTION_TAG_DISCOVERED);
            mTechLists = new String[][] { new String[] { NfcA.class.getName(), Ndef.class.getName(),
                    NdefFormatable.class.getName() } };
            mFilters = new IntentFilter[] { ndef, };
            nfc_initialized = true;
        }
    }

    public void updateIncomingMessage(String msg)
    {
        TextView txtView = (TextView) findViewById(R.id.txtReceive);
        txtView.setText(msg);
    }

    @Override
    protected void onStart()
    {
        super.onStart();

        _incomingMessageTimer = new Timer();
        _incomingMessageTimer.schedule(new MessageReceiveTimer(new Handler(), this),0,900);
    }

    @Override
    protected void onStop()
    {
        super.onStop();

        _incomingMessageTimer.cancel();
        _incomingMessageTimer.purge();

    }

    @Override
    public void onNewIntent(Intent intent) 
    {
        mIntent = intent;

        String action = intent.getAction();
        if (NfcAdapter.ACTION_TAG_DISCOVERED.equals(action)) 
        {
            myTag = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);

            if(_ndef != null)
            {
                try 
                {
                    _ndef.close();
                    _ndef = null;
                } 
                catch (IOException e) 
                {
                    e.printStackTrace();
                }
            }

            if(_ndef == null)
            {
                _ndef = Ndef.get(myTag);

                try 
                {
                    _ndef.connect();
                } 
                catch (IOException e) 
                {
                    e.printStackTrace();
                }
            }
        } 
    }
    @Override
    public void onPause()
    {
        super.onPause();
        mAdapter.disableForegroundDispatch(this);
    }

    @Override
    public void onResume() 
    {
        super.onResume();
        mAdapter.enableForegroundDispatch(this, mPendingIntent, mFilters,
            mTechLists);
    }
}

//TimerTask Class

package com.example.infcdemo;
import java.util.TimerTask;

import android.nfc.NdefMessage;
import android.os.Handler;

public class MessageReceiveTimer extends TimerTask
{
    Handler _handler;
    MainActivity _mainActivity;

    MessageReceiveTimer(Handler handler, MainActivity mainActivity)
    {
        super();
        _handler = handler;
        _mainActivity = mainActivity;
    }

    @Override
    public void run() 
    {
        _handler.post(new Runnable() 
        {
            @Override
            public void run() 
            {
                try 
                {
                    if(_mainActivity._ndef != null)
                    {
                        NdefMessage ndefMsg = null;

                        if(_mainActivity._ndef.isConnected())
                            ndefMsg = _mainActivity._ndef.getNdefMessage();
                        else
                            ndefMsg = null;

                        byte[] ndefRecord = ndefMsg.getRecords()[0].getPayload();
                        String strMsg = new String(ndefRecord, "US-ASCII"); 
                        _mainActivity.updateIncomingMessage(strMsg);
                    }
                } 
                catch (Exception e)
                {
                    return;
                }
            }
        });
    }
}

Answer 1

The reason your app blocks when you remove a tag is that you execute IO operations on the tag in your app's main thread (UI thread).

You first create a timer that processes its scheduled tasks in a separate thread

_incomingMessageTimer = new Timer();
                        ^^^^^^^^^^^
_incomingMessageTimer.schedule(new MessageReceiveTimer(new Handler(), this), 0, 900);
                      ^^^^^^^^

However, when the TimerTask executes (on the Timer thread)

public class MessageReceiveTimer extends TimerTask {
    @Override
    public void run() {
        // code excecution happens in the Timer thread
    }
}

you immediately return control to the main (UI) thread by posting a Runnable on its message queue:

_handler.post(new Runnable() {
    @Override
    public void run() {
        // code execution happens in the Handler's thread
    }
});

The Handler's thread is the main (UI) thread in your case as you created the handler on the main thread:

_incomingMessageTimer.schedule(new MessageReceiveTimer(new Handler(), this),0,900);
                                                       ^^^^^^^^^^^^^

Consequently the IO operation ( _mainActivity._ndef.getNdefMessage() ) will block the main thread. Note that the Android documentation explicitly says that this method " must not be called from the main application thread " (see here ). The same applies to the connect() method , btw.