android_frameworks_base/docs/html/guide/topics/nfc/index.jd

page.title=Near Field Communication
@jd:body

  <div id="qv-wrapper">
    <div id="qv">
      <h2>Near Field Communication quickview</h2>

      <ol>
        <li><a href="#api">API Overview</a></li>

        <li><a href="#manifest">Declaring Android Manifest Elements</a></li>

        <li>
          <a href="#dispatch">The Tag Dispatch System</a>

          <ol>
            <li><a href="#foreground-dispatch">Using the foreground dispatch system</a></li>

            <li><a href="#intent-dispatch">Using the intent dispatch system</a></li>
          </ol>
        </li>

        <li><a href="#ndef">NDEF messages</a></li>

        <li><a href="#read">Reading an NFC tag</a></li>

        <li><a href="#write">Writing to an NFC tag</a></li>

        <li><a href="#p2p">Peer to Peer Data Exchange</a></li>
      </ol>
    </div>
  </div>

  <p>Near Field Communication (NFC) is a set of short-range wireless technologies, similar to RFID.
  It typically requires a distance of 4 cm or less and operates at 13.56mhz and at rates ranging
  from 106 kbit/s to 848 kbit/s. NFC communication always involves an initiator and a target. The
  initiator actively generates an RF field that can power a passive target. This enables NFC
  targets to take very simple form factors such as tags, stickers or cards that do not require
  power. NFC peer-to-peer communication is also possible, where both devices are powered.</p>

  <p>Compared to other wireless technologies such as Bluetooth or WiFi, NFC provides much lower
  bandwidth and range, but provides low-cost, un-powered targets and do not require discovery or
  pairing. Users interact with NFC tags with just a tap. Targets can range in complexity. Simple
  tags just offer read and write capabilities, sometimes with one-time programmable areas to make
  the card read-only. More complex tags offer math operations, and have cryptographic hardware to
  authenticate access to a sector. The most sophisticated tags contain operating environments,
  allowing complex interactions with applets that are running on the tag.</p>

  <p>An Android device with NFC hardware typically acts as an initiator. This mode is also known as
  NFC reader/writer. The device actively looks for NFC tags and starts activities to handle them in
  this mode. In Android 2.3.3, devices also have some limited peer-to-peer support.</p>

  <h2 id="api">API Overview</h2>

  <p>The {@link android.nfc} package contain the high-level classes to interact with the local
  device's NFC adapter, to represent discovered tags, and to use the NDEF data format.</p>

  <table>
    <tr>
      <th>Class</th>

      <th>Description</th>
    </tr>

    <tr>
      <td>{@link android.nfc.NfcManager}</td>

      <td>A high level manager class that enumerates the NFC adapters on this Android device. Since
      most Android devices only have one NFC adapter, you can just use the static helper {@link
      android.nfc.NfcAdapter#getDefaultAdapter()} for most situations.</td>
    </tr>

    <tr>
      <td>{@link android.nfc.NfcAdapter}</td>

      <td>Represents the local NFC adapter and defines the Intents that are used in the tag
      dispatch system. It provides methods to register for foreground tag dispatching and
      foreground NDEF pushing. Foreground NDEF push is the only peer-to-peer support that is
      currently provided in Android.</td>
    </tr>

    <tr>
      <td>{@link android.nfc.NdefMessage} and {@link android.nfc.NdefRecord}</td>

      <td>NDEF is an NFC Forum defined data structure, designed to efficiently store data on NFC
      tags, such as Text, URLs, and other MIME types. An {@link android.nfc.NdefMessage} acts as a
      container for the data that you want to transmit or read. One {@link android.nfc.NdefMessage}
      object contains zero or more {@link android.nfc.NdefRecord}s. Each NDEF Record has a type
      such as Text, URL, Smart Poster, or any MIME type. The type of the first NDEF Record in the
      NDEF message is used to dispatch a tag to an Activity.</td>
    </tr>

    <tr>
      <td>{@link android.nfc.Tag}</td>

      <td>Represents a passive NFC target. These can come in many form factors such as a tag, card,
      FOB, or an even more complex device doing card emulation. When a tag is discovered, a {@link
      android.nfc.Tag} object is created and wrapped inside an Intent. The dispatch system sends
      the Intent to a compatible Activity <code>startActivity()</code>. You can use the {@link
      android.nfc.Tag#getTechList getTechList()} method to determine the technologies supported by
      this tag and create the corresponding {@link android.nfc.tech.TagTechnology} object with one
      of classes provided by {@link android.nfc.tech}.</td>
    </tr>
  </table>

  <p>The {@link android.nfc.tech} package contains classes to query properties and perform I/O
  operations on a tag. The classes are divided to represent different NFC technologies that can be
  available on a Tag:</p>

  <table>
    <tr>
      <th>Class</th>

      <th>Description</th>
    </tr>

    <tr>
      <td>{@link android.nfc.tech.TagTechnology}</td>

      <td>The interface that all Tag Technology classes must implement.</td>
    </tr>

    <tr>
      <td>{@link android.nfc.tech.NfcA}</td>

      <td>Provides access to NFC-A (ISO 14443-3A) properties and I/O operations.</td>
    </tr>

    <tr>
      <td>{@link android.nfc.tech.NfcB}</td>

      <td>Provides access to NFC-B (ISO 14443-3B) properties and I/O operations.</td>
    </tr>

    <tr>
      <td>{@link android.nfc.tech.NfcF}</td>

      <td>Provides access to NFC-F (JIS 6319-4) properties and I/O operations.</td>
    </tr>

    <tr>
      <td>{@link android.nfc.tech.NfcV}</td>

      <td>Provides access to NFC-V (ISO 15693) properties and I/O operations.</td>
    </tr>

    <tr>
      <td>{@link android.nfc.tech.IsoDep}</td>

      <td>Provides access to ISO-DEP (ISO 14443-4) properties and I/O operations.</td>
    </tr>

    <tr>
      <td>{@link android.nfc.tech.Ndef}</td>

      <td>Provides access to NDEF data and operations on NFC Tags that have been formatted as
      NDEF.</td>
    </tr>

    <tr>
      <td>{@link android.nfc.tech.NdefFormatable}</td>

      <td>Provides a format operations for tags that may be NDEF formatable.</td>
    </tr>

    <tr>
      <td>{@link android.nfc.tech.MifareClassic}</td>

      <td>Provides access to MIFARE Classic properties and I/O operations. Not all Android devices
      provide implementations for this class.</td>
    </tr>

    <tr>
      <td>{@link android.nfc.tech.MifareUltralight}</td>

      <td>Provides access to MIFARE Ultralight properties and I/O operations. Not all Android
      devices provide implementations for this class.</td>
    </tr>
  </table>

  <h2 id="manifest">Declaring Android Manifest elements</h2>

  <p>Before you can access a device's NFC hardware and properly handle NFC intents, declare these
  items in your <code>AndroidManifest.xml</code> file:</p>

  <ol>
    <li>The NFC <code>&lt;uses-permission&gt;</code> element to access the NFC hardware:
      <pre>
&lt;uses-permission android:name="android.permission.NFC" /&gt;
</pre>
    </li>

    <li>The minimum SDK version that your application can support. API level 9 only supports
    limited tag dispatching with {@link android.nfc.NfcAdapter#ACTION_TAG_DISCOVERED}, and only
    gives access to NDEF messages via the {@link android.nfc.NfcAdapter#EXTRA_NDEF_MESSAGES} extra.
    No other tag properties or I/O operations are accessible. API level 10 adds comprehensive
    reader/writer support, so you probably want to use this for more functionality.
      <pre class="pretty-print">
&lt;uses-sdk android:minSdkVersion="9|10"/&gt;
</pre>
    </li>

    <li>The uses-feature element so that your application can show up in the Android Market for
    devices that have NFC hardware:
      <pre>
&lt;uses-feature android:name="android.hardware.nfc" android:required="true" /&gt;
</pre>
    </li>

    <li>The NFC intent filter to tell the Android system your Activity can handle NFC data. Specify
    one or more of these three intent filters:
      <pre>
&lt;intent-filter&gt;
  &lt;action android:name="android.nfc.action.NDEF_DISCOVERED"/&gt;
  &lt;data android:mimeType="<em>mime/type</em>" /&gt;
&lt;/intent-filter&gt;

&lt;intent-filter&gt;
  &lt;action android:name="android.nfc.action.TECH_DISCOVERED"/&gt;
  &lt;meta-data android:name="android.nfc.action.TECH_DISCOVERED"
                android:resource="@xml/<em>nfc_tech_filter</em>.xml" /&gt;
&lt;/intent-filter&gt;

&lt;intent-filter&gt;
  &lt;action android:name="android.nfc.action.TAG_DISCOVERED"/&gt;
&lt;/intent-filter&gt;
</pre>

      <p>The three intent filters are prioritized and behave in specific ways. Declare only the
      ones that your Activity needs to handle. For more information on how to handle these filters,
      see the section about <a href="#dispatch">The Tag Dispatch System</a>.</p>
    </li>
  </ol>

  <p>View the <a href=
  "../../../resources/samples/NFCDemo/AndroidManifest.html">AndroidManifest.xml</a> from the
  NFCDemo sample to see a complete example.</p>

  <h2 id="dispatch">The Tag Dispatch System</h2>

  <p>When an Android device scans an NFC tag, the desired behavior is to have the most appropriate
  Activity handle the intent without asking the user what appplication to use. Because devices scan
  NFC tags at a very short range, it is likely that making users manually select an Activity forces
  them to move the device away from the tag and break the connection. You should develop your
  Activity to only handle the NFC tags that your Activity cares about to prevent the Activity
  Chooser from appearing. Android provides two systems to help you correctly identify an NFC tag
  that your Activity should handle: the Intent dispatch system and the foreground Activity dispatch
  system.</p>

  <p>The intent dispatch system checks the intent filters of all the Activities along with the
  types of data that the Activities support to find the best Activity that can handle the NFC tag.
  If multiple Activities specify the same intent filter and data to handle, then the Activity
  Chooser is presented to the user as a last resort.</p>

  <p>The foreground dispatch system allows an Activity application to override the intent dispatch
  system and have priority when an NFC tag is scanned. The Activity handling the request must be
  running in the foreground of the device. When an NFC tag is scanned and matches the intent and
  data type that the foreground dispatch Activity defines, the intent is immediately sent to the
  Activity even if another Activity can handle the intent. If the Activity cannot handle the
  intent, the foreground dispatch system falls back to the intent dispatch system.</p>

  <h3 id="intent-dispatch">Using the intent dispatch system</h3>

  <p>The intent dispatch system specifies three intents that each have a priority. The intents that
  start when a device scans a tag depend on the type of tag scanned. In general, the intents are
  started in the following manner:</p>

  <ul>
    <li>
      <code>android.nfc.action.NDEF_DISCOVERED</code>: This intent starts when a tag that contains
      an NDEF payload is scanned. This is the highest priority intent. The Android system does not
      let you specify this intent generically to handle all data types. You must specify
      <code>&lt;data&gt;</code> elements in the <code>AndroidManifest.xml</code> along with this
      intent to correctly handle NFC tags that start this intent. For example, to handle a
      <code>NDEF_DISCOVERED</code> intent that contains plain text, specify the following filter in
      your <code>AndroidManifest.xml</code> file:
      <pre>
&lt;intent-filter&gt;
    &lt;action android:name="android.nfc.action.NDEF_DISCOVERED"/&gt;
    &lt;data android:mimeType="text/plain" /&gt;
&lt;/intent-filter&gt;
</pre>

      <p>If the <code>NDEF_DISCOVERED</code> intent is started, the <code>TECH_DISCOVERED</code>
      and <code>TAG_DISCOVERED</code> intents are not started. This intent does not start if an
      unknown tag is scanned or if the tag does not contain an NDEF payload.</p>
    </li>

    <li><code>android.nfc.action.TECH_DISCOVERED</code>: If the <code>NDEF_DISCOVERED</code> intent
    does not start or is not filtered by any Activity on the device, this intent starts if the tag
    is known. The <code>TECH_DISCOVERED</code> intent requires that you specify the technologies
    that you want to support in an XML resource file. For more information, see the section about
    <a href="#technology-resources">Specifying tag technologies to handle</a>.</li>

    <li><code>android.nfc.action.TAG_DISCOVERED</code>: This intent starts if no Activities handle
    the <code>NDEF_DISCOVERED</code> and <code>TECH_DISCOVERED</code> intents or if the tag that is
    scanned is unknown.</li>
  </ul>

  <h4 id="tech">Specifying tag technologies to handle</h4>

  <p>If your Activity declares the <code>android.nfc.action.TECH_DISCOVERED</code> intent in your
  <code>AndroidManifest.xml</code> file, you must create an XML resource file that specifies the
  technologies that your Activity supports. The following sample defines all of the technologies.
  Specifiying multiple technologies within the same list tells the system
  to filter tags that support all of the technologies. The example below never filters a tag
  because no tag supports all of the technologies at once.
  You can remove the ones that you do not need. Save this file (you can name it anything you wish)
  in the <code>&lt;project-root&gt;/res/xml</code> folder.</p>
  <pre>
&lt;resources xmlns:xliff="urn:oasis:names:tc:xliff:document:1.2"&gt;
    &lt;tech-list&gt;
        &lt;tech&gt;android.nfc.tech.IsoDep&lt;/tech&gt;
        &lt;tech&gt;android.nfc.tech.NfcA&lt;/tech&gt;        
        &lt;tech&gt;android.nfc.tech.NfcB&lt;/tech&gt;
        &lt;tech&gt;android.nfc.tech.NfcF&lt;/tech&gt;
        &lt;tech&gt;android.nfc.tech.NfcV&lt;/tech&gt;
        &lt;tech&gt;android.nfc.tech.Ndef&lt;/tech&gt;
        &lt;tech&gt;android.nfc.tech.NdefFormatable&lt;/tech&gt;
        &lt;tech&gt;android.nfc.tech.MifareClassic&lt;/tech&gt;
        &lt;tech&gt;android.nfc.tech.MifareUltralight&lt;/tech&gt;
    &lt;/tech-list&gt;
&lt;/resources&gt;
</pre>

You can also specify multiple filter lists. In this case, a tag must match all of the
technologies within one of the lists. The following example filters for
cards that support the NfcA and Ndef technology or support the
NfcB and Ndef technology.

<pre>
&lt;resources xmlns:xliff="urn:oasis:names:tc:xliff:document:1.2"&gt;
    &lt;tech-list&gt;
        &lt;tech&gt;android.nfc.tech.NfcA&lt;/tech&gt;        
        &lt;tech&gt;android.nfc.tech.Ndef&lt;/tech&gt;
    &lt;/tech-list&gt;
&lt;/resources&gt;

&lt;resources xmlns:xliff="urn:oasis:names:tc:xliff:document:1.2"&gt;
    &lt;tech-list&gt;
        &lt;tech&gt;android.nfc.tech.NfcB&lt;/tech&gt;        
        &lt;tech&gt;android.nfc.tech.Ndef&lt;/tech&gt;
    &lt;/tech-list&gt;
&lt;/resources&gt;
</pre>

  <p>In your <code>AndroidManifest.xml</code> file, specify the resource file that you just created
  in the <code>&lt;meta-data&gt;</code> element inside the <code>&lt;intent-filter&gt;</code>
  element like in the following example:</p>
  <pre>
&lt;intent-filter&gt;
    &lt;action android:name="android.nfc.action.TECH_DISCOVERED"/&gt;
    &lt;meta-data android:name="android.nfc.action.TECH_DISCOVERED"
        android:resource="@xml/nfc_tech_filter.xml" /&gt;
&lt;/intent-filter&gt;
</pre>

  <h3 id="foreground-dispatch">Using the foreground dispatch system</h3>

  <p>The foreground dispatch system allows an Activity to intercept an intent and claim priority
  over other Activities that handle the same intent. The system is easy to use and involves
  constructing a few data structures for the Android system to be able to send the appropriate
  intents to your application. To enable the foreground dispatch system:</p>

  <ol>
    <li>Add the following code in the onCreate() method of your Activity:

      <ol type="a">
        <li>Create a {@link android.app.PendingIntent} object so the Android system can populate it
        with the details of the tag when it is scanned
          <pre>
PendingIntent pendingIntent = PendingIntent.getActivity(
    this, 0, new Intent(this, getClass()).addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP), 0);
</pre>
        </li>

        <li>Declare intent filters to handle the intents that you want to intercept. The foreground
        dispatch system checks the specified intent filters with the intent that is received when
        the device scans a tag. If they match, then your application handles the intent. If it does
        not match, the foreground dispatch system falls back to the intent dispatch system.
        Specifying a <code>null</code> array of intent filters and for the technology filters,
        you receive a <code>TAG_DISCOVERED</code> intent for all tags discovered. Note that the
        snippet below handles all MIME types. You should only handle the ones that you need.
          <pre>
    IntentFilter ndef = new IntentFilter(NfcAdapter.ACTION_NDEF_DISCOVERED);
        try {
            ndef.addDataType("*/*");    /* Handles all MIME based dispatches. 
                                           You should specify only the ones that you need. */
        }
        catch (MalformedMimeTypeException e) {
            throw new RuntimeException("fail", e);
        }
        intentFiltersArray = new IntentFilter[] {
                ndef,
        };
</pre>
        </li>

        <li>Set up an array of tag technologies that your application wants to handle. Call the
        <code>Object.class.getName()</code> method to obtain the class of the technology that you
        want to support.
          <pre>

  techListsArray = new String[][] { new String[] { NfcF.class.getName() } };
  
</pre>
        </li>
      </ol>
    </li>

    <li>Override the following Activity lifecycle callbacks and add logic to enable and disable the
    foreground dispatch when the Activity loses ({@link android.app.Activity#onPause onPause()})
    and regains ({@link android.app.Activity#onResume onResume()}) focus. {@link
    android.nfc.NfcAdapter#enableForegroundDispatch} must best called from the main thread and only
    when the activity is in the foreground (calling in {@link android.app.Activity#onResume
    onResume()} guarantees this). You also need to implement the {@link
    android.app.Activity#onNewIntent onNewIntent} callback to process the data from the scanned NFC
    tag.
      <pre>
public void onPause() {
    super.onPause();
    mAdapter.disableForegroundDispatch(this);
}   

public void onResume() {
    super.onResume();
    mAdapter.enableForegroundDispatch(this, pendingIntent, intentFiltersArray, techListsArray);
}

public void onNewIntent(Intent intent) {
    Tag tagFromIntent = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);
    //do something with tagFromIntent
}
</pre>
    </li>
  </ol>

  <p>See the <a href=
  "{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/nfc/ForegroundDispatch.html">ForegroundDispatch</a>
  sample from API Demos for the complete sample.</p>

  <h2 id="ndef">Working with Data on NFC tags</h2>

  <p>Data on NFC tags are encoded in raw bytes, so you must convert the bytes to something human
  readable if you are presenting the data to the user. When writing to NFC tags, you must write
  them in bytes as well. Android provides APIs to help write messages that conform to the NDEF
  standard, which was developed by the <a href="http://www.nfc-forum.org/specs/">NFC Forum</a> to
  standardized data on tags. Using this standard ensures that your data will be supported by all
  Android NFC devices if you are writing to tags. However, many tag technologies use their own
  standard for storing data and are supported by Android as well, but you have to implement your
  own protocol stack to read and write to these tags. You can find a full list of the supported
  technologies in {@link android.nfc.tech} and an overview of the technolgies in the {@link
  android.nfc.tech.TagTechnology} interface. This section is a brief overview of how to work with
  NDEF messages in the context of the Android system. It is not meant to be a complete discussion
  of the NDEF specification, but highlights the main things that you need to be aware of when
  working with NDEF messages in Android.</p>

  <p>To facilitate working with NDEF messages, Android provides the {@link android.nfc.NdefRecord}
  and {@link android.nfc.NdefMessage} to encapsulate the raw bytes that represent NDEF messages. An
  {@link android.nfc.NdefMessage} is the container for zero or more {@link
  android.nfc.NdefRecord}s. Each {@link android.nfc.NdefRecord} has its own unique type name
  format, record type, and ID to distinguish them from other records within the same {@link
  android.nfc.NdefMessage}. You can store different types of records of varying length in a single
  {@link android.nfc.NdefMessage}. The size constraint of the NFC tag determines how big your
  {@link android.nfc.NdefMessage} can be.</p>

  <p>Tags that support the {@link android.nfc.tech.Ndef} and {@link android.nfc.tech.NdefFormatable}
  technologies return and accept {@link android.nfc.NdefMessage}
  objects as parameters for read and write operations. You need to create your own logic to read
  and write bytes for other tag technologies in {@link android.nfc.tech}.</p>

  <p>You can download technical specifications for different types of NDEF message standards, such
  as plain text and Smart Posters, at the <a href="http://www.nfc-forum.org/specs/">NFC Forum</a>
  website. The NFCDemo sample application also declares sample <a href=
  "{@docRoot}resources/samples/NFCDemo/src/com/example/android/nfc/simulator/MockNdefMessages.html">
  plain text and SmartPoster NDEF messages.</a></p>

  <h2 id="read">Reading an NFC tag</h2>

  <p>When a device comes in proximity to an NFC tag, the appropriate intent is started on the
  device, notifying interested applications that a NFC tag was scanned. By previously declaring the
  appropriate intent filter in your <code>AndroidManifest.xml</code> file or using foreground
  dispatching, your application can request to handle the intent.</p>

  <p>The following method (slightly modified from the NFCDemo sample application), handles the
  <code>TAG_DISCOVERED</code> intent and iterates through an array obtained from the intent that
  contains the NDEF payload:</p>
  <pre>
NdefMessage[] getNdefMessages(Intent intent) {
    // Parse the intent
    NdefMessage[] msgs = null;
    String action = intent.getAction();
    if (NfcAdapter.ACTION_TAG_DISCOVERED.equals(action)) {
        Parcelable[] rawMsgs = intent.getParcelableArrayExtra(NfcAdapter.EXTRA_NDEF_MESSAGES);
        if (rawMsgs != null) {
            msgs = new NdefMessage[rawMsgs.length];
            for (int i = 0; i &lt; rawMsgs.length; i++) {
                msgs[i] = (NdefMessage) rawMsgs[i];
            }
        }
        else {
        // Unknown tag type
            byte[] empty = new byte[] {};
            NdefRecord record = new NdefRecord(NdefRecord.TNF_UNKNOWN, empty, empty, empty);
            NdefMessage msg = new NdefMessage(new NdefRecord[] {record});
            msgs = new NdefMessage[] {msg};
        }
    }        
    else {
        Log.e(TAG, "Unknown intent " + intent);
        finish();
    }
    return msgs;
}
</pre>

  <p>Keep in mind that the data that the device reads is in bytes, so you must implement your own
  logic if you need to present the data in a readable format to the user. The classes in
  <code>com.example.android.nfc.record</code> of the NFCDemo sample show you how to parse some
  common types of NDEF messages such as plain text or a SmartPoster.</p>

  <h2 id="write">Writing to an NFC tag</h2>

  <p>Writing to an NFC tag involves constructing your NDEF message in bytes and using the
  appropriate tag technology for the tag that you are writing to. The following code sample shows
  you how to write a simple text message to a {@link android.nfc.tech.NdefFormatable} tag:</p>
  <pre>
NdefFormatable tag = NdefFormatable.get(t);
Locale locale = Locale.US;
final byte[] langBytes = locale.getLanguage().getBytes(Charsets.US_ASCII);
String text = "Tag, you're it!";
final byte[] textBytes = text.getBytes(Charsets.UTF_8);
final int utfBit = 0;
final char status = (char) (utfBit + langBytes.length);
final byte[] data = Bytes.concat(new byte[] {(byte) status}, langBytes, textBytes);
NdefRecord record = NdefRecord(NdefRecord.TNF_WELL_KNOWN, NdefRecord.RTD_TEXT, new byte[0], data);
try {
    NdefRecord[] records = {text};
    NdefMessage message = new NdefMessage(records);
    tag.connect();
    tag.format(message);
}
catch (Exception e){
    //do error handling
}
</pre>

  <h2 id="p2p">Peer-to-peer data exchange</h2>

  <p>Support for simple peer-to-peer data exchange is supported by the foreground push feature,
  which is enabled with the {@link android.nfc.NfcAdapter#enableForegroundNdefPush} method. To use
  this feature:</p>

  <ul>
    <li>The Activity that is pushing the data must be in the foreground</li>

    <li>You must encapsulate the data that you are sending in an {@link android.nfc.NdefMessage}
    object</li>

    <li>The NFC device that is receiving the pushed data (the scanned device) must support the
    <code>com.android.npp</code> NDEF push protocol, which is optional for Android devices.</li>
</li>
  </ul>

  <p class="note">If your Activity enables the foreground push feature and is in the foreground,
  the standard intent dispatch system is disabled. However, if your Activity also enables
  foreground dispatching, then it can still scan tags that match the intent filters set in the
  foreground dispatching.</p>

  <p>To enable foreground dispatching:</p>

  <ol>
    <li>Create an NdefMessage that contains the NdefRecords that you want to push onto the other
    device.</li>

    <li>Implement the {@link android.app.Activity#onResume onResume()} and {@link
    android.app.Activity#onPause onPause()} callbacks in your Activity to appropriately handle the
    foreground pushing lifecycle. You must call {@link
    android.nfc.NfcAdapter#enableForegroundNdefPush} from the main thread and only when the
    activity is in the foreground (calling in {@link android.app.Activity#onResume onResume()}
    guarantees this).
      <pre>
public void onResume() {
    super.onResume();
    if (mAdapter != null)
        mAdapter.enableForegroundNdefPush(this, myNdefMessage);
}
public void onPause() {
    super.onPause();
    if (mAdapter != null)
        mAdapter.disableForegroundNdefPush(this);
}
</pre>
    </li>
  </ol>

  <p>When the Activity is in the foreground, you can now tap the device to another device and push
  the data to it. See the <a href=
  "../../../resources/samples/ApiDemos/src/com/example/android/apis/nfc/ForegroundNdefPush.html">ForegroundNdefPush</a>
  sample in API Demos for a simple example of peer-to-peer data exchange.</p>