Stay in current activity upon NFC reception

Question

I have an application that seems to work fine and can transmit data via NFC perfectly well. I have a main activity, an activity to transmit the data, and a different activity to receive data.

The sender activity works great, but when the receiver gets the NFC intent, it restarts the app back to the main activity.

I'm not exactly sure why this is. I would like it to decline any pushes unless the user is already in that activity, and if they are, to stay in that activity and handle the NFC intent.

Here is the manifest:

<activity android:name=".MainActivity">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>
<activity android:name=".Timer" />
<activity android:name=".AddSlaves"
          android:label="Add Slave Devices"
          android:launchMode="singleTask">
    <intent-filter>
        <action android:name="android.nfc.action.NDEF_DISCOVERED" />
        <category android:name="android.intent.category.DEFAULT" />
        <data android:mimeType="text/plain" />
    </intent-filter>
</activity>
<activity android:name=".JoinSrv"
          android:launchMode="singleTask">
    <intent-filter>
        <action android:name="android.nfc.action.NDEF_DISCOVERED" />
        <category android:name="android.intent.category.DEFAULT"/>
        <data android:mimeType="text/plain" />
    </intent-filter>
</activity>

Here is the sender class:

public class JoinSrv extends Activity implements NfcAdapter.OnNdefPushCompleteCallback, NfcAdapter.CreateNdefMessageCallback {
    //The array lists to hold our messages
    private ArrayList<String> messagesToSendArray = new ArrayList<>();
    private ArrayList<String> messagesReceivedArray = new ArrayList<>();

    //Text boxes to add and display our messages
    private NfcAdapter mNfcAdapter;

    //Save our Array Lists of Messages for if the user navigates away
    @Override
    public void onSaveInstanceState(@NonNull Bundle savedInstanceState) {
        super.onSaveInstanceState(savedInstanceState);
        savedInstanceState.putStringArrayList("messagesToSend", messagesToSendArray);
        savedInstanceState.putStringArrayList("lastMessagesReceived", messagesReceivedArray);
    }

    //Load our Array Lists of Messages for when the user navigates back
    @Override
    public void onRestoreInstanceState(@NonNull Bundle savedInstanceState) {
        super.onRestoreInstanceState(savedInstanceState);
        messagesToSendArray = savedInstanceState.getStringArrayList("messagesToSend");
        messagesReceivedArray = savedInstanceState.getStringArrayList("lastMessagesReceived");
    }

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


        //Check if NFC is available on device
        mNfcAdapter = NfcAdapter.getDefaultAdapter(this);
        if (mNfcAdapter != null) {
            //Handle some NFC initialization here
        } else {
            Toast.makeText(this, "NFC not available on this device",
                    Toast.LENGTH_SHORT).show();
        }

        //Check if NFC is available on device
        mNfcAdapter = NfcAdapter.getDefaultAdapter(this);
        if (mNfcAdapter != null) {
            //This will refer back to createNdefMessage for what it will send
            mNfcAdapter.setNdefPushMessageCallback(this, this);

            //This will be called if the message is sent successfully
            mNfcAdapter.setOnNdefPushCompleteCallback(this, this);
        }
    }

    @Override
    public NdefMessage createNdefMessage(NfcEvent event) {
        //This will be called when another NFC capable device is detected.
        //We'll write the createRecords() method in just a moment
        NdefRecord[] recordsToAttach = createRecords();
        //When creating an NdefMessage we need to provide an NdefRecord[]
        return new NdefMessage(recordsToAttach);
    }

    @Override
    public void onNdefPushComplete(NfcEvent event) {
        //This is called when the system detects that our NdefMessage was
        //Successfully sent.
        messagesToSendArray.clear();
    }

    public NdefRecord[] createRecords() {
        NdefRecord[] records = new NdefRecord[1];
        //To Create Messages Manually if API is less than
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN) {

            byte[] payload = "192.168.1.100".
                    getBytes(Charset.forName("UTF-8"));
            NdefRecord record = new NdefRecord(
                    NdefRecord.TNF_WELL_KNOWN,      //Our 3-bit Type name format
                    NdefRecord.RTD_TEXT,            //Description of our payload
                    new byte[0],                    //The optional id for our Record
                    payload);                       //Our payload for the Record

            records[1] = record;

        }
        //Api is high enough that we can use createMime, which is preferred.
        else {

                byte[] payload = "192.168.1.100".
                        getBytes(Charset.forName("UTF-8"));

                NdefRecord record = NdefRecord.createMime("text/plain",payload);
                records[1] = record;

        }
        records[messagesToSendArray.size()] =
                NdefRecord.createApplicationRecord(getPackageName());
        return records;
    }

    private void handleNfcIntent(Intent NfcIntent) {
        if (NfcAdapter.ACTION_NDEF_DISCOVERED.equals(NfcIntent.getAction())) {
            Parcelable[] receivedArray =
                    NfcIntent.getParcelableArrayExtra(NfcAdapter.EXTRA_NDEF_MESSAGES);

            if (receivedArray != null) {
                messagesReceivedArray.clear();
                NdefMessage receivedMessage = (NdefMessage) receivedArray[0];
                NdefRecord[] attachedRecords = receivedMessage.getRecords();

                for (NdefRecord record : attachedRecords) {
                    String string = new String(record.getPayload());
                    //Make sure we don't pass along our AAR (Android Application Record)
                    if (string.equals(getPackageName())) {
                        continue;
                    }
                    messagesReceivedArray.add(string);
                }
                Toast.makeText(this, "Received " + messagesReceivedArray.size() +
                        " Messages", Toast.LENGTH_LONG).show();
            } else {
                Toast.makeText(this, "Received Blank Parcel", Toast.LENGTH_LONG).show();
            }
        }
    }

    @Override
    public void onNewIntent(Intent intent) {
        handleNfcIntent(intent);
    }

    @Override
    public void onResume() {
        super.onResume();
        handleNfcIntent(getIntent());
    }
}

Here is the receiver class:

public class AddSlaves extends Activity implements NfcAdapter.OnNdefPushCompleteCallback, NfcAdapter.CreateNdefMessageCallback{
    //The array lists to hold our messages
    private ArrayList<String> messagesToSendArray = new ArrayList<>();
    private ArrayList<String> messagesReceivedArray = new ArrayList<>();

    //Text boxes to add and display our messages
    private EditText txtBoxAddMessage;
    private TextView txtReceivedMessages;
    private TextView txtMessagesToSend;
    private NfcAdapter mNfcAdapter;

    private  void updateTextViews() {
        txtReceivedMessages.setText("Messages Received:\n");
        //Populate our list of messages we have received
        if (messagesReceivedArray.size() > 0) {
            for (int i = 0; i < messagesReceivedArray.size(); i++) {
                txtReceivedMessages.append(messagesReceivedArray.get(i));
                txtReceivedMessages.append("\n");
            }
        }
    }

    //Save our Array Lists of Messages for if the user navigates away
    @Override
    public void onSaveInstanceState(@NonNull Bundle savedInstanceState) {
        super.onSaveInstanceState(savedInstanceState);
        savedInstanceState.putStringArrayList("lastMessagesReceived",messagesReceivedArray);
    }

    //Load our Array Lists of Messages for when the user navigates back
    @Override
    public void onRestoreInstanceState(@NonNull Bundle savedInstanceState) {
        super.onRestoreInstanceState(savedInstanceState);
        messagesReceivedArray = savedInstanceState.getStringArrayList("lastMessagesReceived");
    }

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

        txtReceivedMessages = (TextView) findViewById(R.id.txtMessagesReceived);
        Button btnAddMessage = (Button) findViewById(R.id.buttonAddMessage);


        //Check if NFC is available on device
        mNfcAdapter = NfcAdapter.getDefaultAdapter(this);
        if(mNfcAdapter != null) {
            //Handle some NFC initialization here
        }
        else {
            Toast.makeText(this, "NFC not available on this device",
                    Toast.LENGTH_SHORT).show();
        }

        //Check if NFC is available on device
        mNfcAdapter = NfcAdapter.getDefaultAdapter(this);
        if(mNfcAdapter != null) {
            //This will refer back to createNdefMessage for what it will send
            mNfcAdapter.setNdefPushMessageCallback(this, this);

            //This will be called if the message is sent successfully
            mNfcAdapter.setOnNdefPushCompleteCallback(this, this);
        }
    }

    @Override
    public NdefMessage createNdefMessage(NfcEvent event) {
        //This will be called when another NFC capable device is detected.
        return null;

    }

    @Override
    public void onNdefPushComplete(NfcEvent event) {
        //This is called when the system detects that our NdefMessage was
        //Successfully sent.
        messagesToSendArray.clear();
    }


    private void handleNfcIntent(Intent NfcIntent) {
        if (NfcAdapter.ACTION_NDEF_DISCOVERED.equals(NfcIntent.getAction())) {
            Parcelable[] receivedArray =
                    NfcIntent.getParcelableArrayExtra(NfcAdapter.EXTRA_NDEF_MESSAGES);

            if(receivedArray != null) {
                messagesReceivedArray.clear();
                NdefMessage receivedMessage = (NdefMessage) receivedArray[0];
                NdefRecord[] attachedRecords = receivedMessage.getRecords();

                for (NdefRecord record:attachedRecords) {
                    String string = new String(record.getPayload());
                    //Make sure we don't pass along our AAR (Android Application Record)
                    if (string.equals(getPackageName())) { continue; }
                    messagesReceivedArray.add(string);
                }
                Toast.makeText(this, "Received " + messagesReceivedArray.size() +
                        " Messages", Toast.LENGTH_LONG).show();
                updateTextViews();
            }
            else {
                Toast.makeText(this, "Received Blank Parcel", Toast.LENGTH_LONG).show();
            }
        }
    }


    @Override
    public void onNewIntent(Intent intent) {
        handleNfcIntent(intent);
    }

    @Override
    public void onResume() {
        super.onResume();
        updateTextViews();
        handleNfcIntent(getIntent());
    }
}

Answer 1

You have qute a few issues in your code of the sender activity:

1. You store messagesToSendArray, but you never actually fill this array list with data (i.e. messagesToSendArray.size() is always 0). Since you freshly create the NDEF message whenever createNdefMessage() is invoked, there is no need to save and restore messagesToSendArray.

2. You wrote that you want to send NDEF messages in one activity, but you want to receive NFC events in another activity. However, you registered your sender activity to receive NDEF_DISCOVERED events in the manifest. There is no need for the NDEF_DISCOVERED intent filter in the manifest if you do not want to receive and process these events.

3. Moreover, there is no need to handle the NDEF_DISCOVERED intent in your sender activity (i.e. you can safely remove the methods onNewIntent() and handleNfcIntent()).

4. On Android versions below Jelly Bean you create an NFC Forum Text record with an invalid structure. The Text RTD requres a payload that is encoded in the form (also see this post)

   +----------+---------------+--------------------------------------+
   | Status   | Language Code | Text                                 |
   | (1 byte) | (n bytes)     | (m bytes)                            |
   +----------+---------------+--------------------------------------+
   

   where Status equals to the length n of the Language Code if the Text is UTF-8 encoded and Language Code is an IANA language code (e.g. "en" for English). Consequently, the proper way to encode that record would be:

   public static NdefRecord createTextRecord(String language, String text) {
       byte[] languageBytes;
       byte[] textBytes;
       try {
           languageBytes = language.getBytes("US-ASCII");
           textBytes = text.getBytes("UTF-8");
       } catch (UnsupportedEncodingException e) {
           throw new AssertionError(e);
       }
   
       byte[] recordPayload = new byte[1 + (languageBytes.length & 0x03F) + textBytes.length];
   
       recordPayload[0] = (byte)(languageBytes.length & 0x03F);
       System.arraycopy(languageBytes, 0, recordPayload, 1, languageBytes.length & 0x03F);
       System.arraycopy(textBytes, 0, recordPayload, 1 + (languageBytes.length & 0x03F), textBytes.length);
   
       return new NdefRecord(NdefRecord.TNF_WELL_KNOWN, NdefRecord.RTD_TEXT, null, recordPayload);
   }
   

5. It's unclear to me why you create an NFC Forum Text record on Android versions below Jelly Bean while you create a MIME type record on Jelly Bean and above. You should be consistent and create the same record type on all platforms (see Method NdefRecord.createTextRecord("en" , "string") not working below API level 21):

   String text = "192.168.1.100";
   String language = "en";
   
   NdefRecord record;
   if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
       record = NdefRecord.createTextRecord(language, text);
   } else {
       record = createTextRecord(language, text);
   }
   

6. Finally, in createRecords() you create the array records as

   NdefRecord[] records = new NdefRecord[1];
   

   Consequently, the array has one element accessible at index 0. Hoever, you try to access element 1 later on:

   records[1] = record;
   

   This results in an IndexOutOfBounds exception. Since createRecords() is called by Android through the createNdefMessage() callback, the callback fails (due to the runtime exception) and Android will not use your NDEF message. Instead, Android will use a default NDEF message for your app. This default NDEF message contains an Android Application record that will cause your main activity to be called (since none of your other activities are registered to be started for the specific contents of the default NDEF message); see NFC tag detection is not calling onNewIntent and it's Launching From Main Activity. Consequently, you need to change the offset in records where you store your newly created NDEF record to 0:

   records[0] = record;
   

   Moreover, you need to remove the line

   records[messagesToSendArray.size()] =
       NdefRecord.createApplicationRecord(getPackageName());
   

   since this would then overwrite the previously stored NDEF record at index 0 (messagesToSendArray.size() is 0) with an Android Application record. Again, this would cause your main activity to be started since you did not register for that specific record type in your manifest.

Finally, if you want to decline pushes unless the user is inside the receiver activity, you should consider using the foreground dispatch system. In that case, you would complete remove all NDEF_DISCOVERED intent filters from your manifest and instead register each activity (receiver, sender, and main) with the foreground dispatch system. In the receiver, you would then receive the NDEF messages through onNewIntent(). In the sender and in the main activity, you would simply ignore and drop any received NDEF message. See Android app enable NFC only for one Activity for an example.