This document describes advanced NFC topics, such as working with various tag technologies,
writing to NFC tags, and foreground dispatching, which allows an application in the foreground to
handle intents even when other applications filter for the same ones.
Work with supported tag technologies
When working with NFC tags and Android-powered devices, the main format you use to read
and write data on tags is NDEF. When a device scans a tag with NDEF data, Android provides support
in parsing the message and delivering it in an
NdefMessage
when
possible. There are cases, however, when you scan a tag that does not contain
NDEF data or when the NDEF data could not be mapped to a MIME type or URI.
In these cases, you need to open communication directly with the tag and read and write to it with
your own protocol (in raw bytes). Android provides generic support for these use cases with the
android.nfc.tech
package, which is described in
Table 1
. You can
use the
getTechList()
method to determine the technologies
supported by the tag and create the corresponding
TagTechnology
object with one of classes provided by
android.nfc.tech
Table 1.
Supported tag technologies
Class
|
Description
|
TagTechnology
|
The interface that all tag technology classes must implement.
|
NfcA
|
Provides access to NFC-A (ISO 14443-3A) properties and I/O operations.
|
NfcB
|
Provides access to NFC-B (ISO 14443-3B) properties and I/O operations.
|
NfcF
|
Provides access to NFC-F (JIS 6319-4) properties and I/O operations.
|
NfcV
|
Provides access to NFC-V (ISO 15693) properties and I/O operations.
|
IsoDep
|
Provides access to ISO-DEP (ISO 14443-4) properties and I/O operations.
|
Ndef
|
Provides access to NDEF data and operations on NFC tags that have been formatted as
NDEF.
|
NdefFormatable
|
Provides a format operations for tags that may be NDEF formattable.
|
The following tag technologies are not required to be supported by Android-powered devices.
Table 2.
Optional supported tag technologies
Class
|
Description
|
MifareClassic
|
Provides access to MIFARE Classic properties and I/O operations, if this Android device
supports MIFARE.
|
MifareUltralight
|
Provides access to MIFARE Ultralight properties and I/O operations, if this Android
device supports MIFARE.
|
Work with tag technologies and the ACTION_TECH_DISCOVERED intent
When a device scans a tag that has NDEF data on it, but could not be mapped to a MIME or URI,
the tag dispatch system tries to start an activity with the
ACTION_TECH_DISCOVERED
intent. The
ACTION_TECH_DISCOVERED
is also used when a tag
with non-NDEF data is scanned. Having this fallback allows you to work with the data on the tag
directly if the tag dispatch system could not parse it for you. The basic steps when working with
tag technologies are as follows:
- Filter for an
ACTION_TECH_DISCOVERED
intent specifying the
tag technologies that you want to handle. See
Filtering for NFC
intents
for more information. In general, the tag dispatch system tries to start a
ACTION_TECH_DISCOVERED
intent when an NDEF message
cannot be mapped to a MIME type or URI, or if the tag scanned did not contain NDEF data. For
more information on how this is determined, see
The Tag Dispatch System
.
- When your application receives the intent, obtain the
Tag
object from
the intent:
Kotlin
var tagFromIntent: Tag = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG)
Java
Tag tagFromIntent = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);
- Obtain an instance of a
TagTechnology
, by calling one of the
get
factory methods of the classes in the
android.nfc.tech
package. You can
enumerate the supported technologies of the tag by calling
getTechList()
before calling a
get
factory method. For example, to obtain an instance
of
MifareUltralight
from a
Tag
, do the following:
Kotlin
MifareUltralight.get(intent.getParcelableExtra(NfcAdapter.EXTRA_TAG))
Java
MifareUltralight.get(intent.getParcelableExtra(NfcAdapter.EXTRA_TAG));
Read and write to tags
Reading and writing to an NFC tag involves obtaining the tag from the intent and
opening communication with the tag. You must define your own protocol stack to read and write data
to the tag. Keep in mind, however, that you can still read and write NDEF data when working
directly with a tag. It is up to you how you want to structure things. The
following example shows how to work with a MIFARE Ultralight tag.
Kotlin
package com.example.android.nfc
import android.nfc.Tag
import android.nfc.tech.MifareUltralight
import java.io.IOException
import java.nio.charset.Charset
class MifareUltralightTagTester {
fun writeTag(tag: Tag, tagText: String) {
MifareUltralight.get(tag)?.use { ultralight ->
ultralight.connect()
Charset.forName("US-ASCII").also { usAscii ->
ultralight.writePage(4, "abcd".toByteArray(usAscii))
ultralight.writePage(5, "efgh".toByteArray(usAscii))
ultralight.writePage(6, "ijkl".toByteArray(usAscii))
ultralight.writePage(7, "mnop".toByteArray(usAscii))
}
}
}
fun readTag(tag: Tag): String? {
return MifareUltralight.get(tag)?.use { mifare ->
mifare.connect()
val payload = mifare.readPages(4)
String(payload, Charset.forName("US-ASCII"))
}
}
}
Java
package com.example.android.nfc;
import android.nfc.Tag;
import android.nfc.tech.MifareUltralight;
import android.util.Log;
import java.io.IOException;
import java.nio.charset.Charset;
public class MifareUltralightTagTester {
private static final String TAG = MifareUltralightTagTester.class.getSimpleName();
public void writeTag(Tag tag, String tagText) {
MifareUltralight ultralight = MifareUltralight.get(tag);
try {
ultralight.connect();
ultralight.writePage(4, "abcd".getBytes(Charset.forName("US-ASCII")));
ultralight.writePage(5, "efgh".getBytes(Charset.forName("US-ASCII")));
ultralight.writePage(6, "ijkl".getBytes(Charset.forName("US-ASCII")));
ultralight.writePage(7, "mnop".getBytes(Charset.forName("US-ASCII")));
} catch (IOException e) {
Log.e(TAG, "IOException while writing MifareUltralight...", e);
} finally {
try {
ultralight.close();
} catch (IOException e) {
Log.e(TAG, "IOException while closing MifareUltralight...", e);
}
}
}
public String readTag(Tag tag) {
MifareUltralight mifare = MifareUltralight.get(tag);
try {
mifare.connect();
byte[] payload = mifare.readPages(4);
return new String(payload, Charset.forName("US-ASCII"));
} catch (IOException e) {
Log.e(TAG, "IOException while reading MifareUltralight message...", e);
} finally {
if (mifare != null) {
try {
mifare.close();
}
catch (IOException e) {
Log.e(TAG, "Error closing tag...", e);
}
}
}
return null;
}
}
Use the foreground dispatch system
The foreground dispatch system allows an activity to intercept an intent and claim
priority over other activities that handle the same intent. Using this system 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:
- Add the following code in the
onCreate()
method of your activity:
- Create a mutable
PendingIntent
object so the Android system can populate it
with the details of the tag when it is scanned.
Kotlin
val intent = Intent(this, javaClass).apply {
addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP)
}
var pendingIntent: PendingIntent = PendingIntent.getActivity(this, 0, intent,
PendingIntent.FLAG_MUTABLE)
Java
PendingIntent pendingIntent = PendingIntent.getActivity(
this, 0, new Intent(this, getClass()).addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP),
PendingIntent.FLAG_MUTABLE);
- 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 it matches, then your application handles the intent. If it does
not match, the foreground dispatch system falls back to the intent dispatch system.
Specifying a
null
array of intent filters and technology filters, specifies
that you want to filter for all tags that fallback to the
TAG_DISCOVERED
intent. The code snippet below handles all MIME types for
NDEF_DISCOVERED
. You
should only handle the ones that you need.
Kotlin
val ndef = IntentFilter(NfcAdapter.ACTION_NDEF_DISCOVERED).apply {
try {
addDataType("*/*") /* Handles all MIME based dispatches.
You should specify only the ones that you need. */
} catch (e: IntentFilter.MalformedMimeTypeException) {
throw RuntimeException("fail", e)
}
}
intentFiltersArray = arrayOf(ndef)
Java
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, };
- Set up an array of tag technologies that your application wants to handle. Call the
Object.class.getName()
method to obtain the class of the technology that you
want to support.
Kotlin
techListsArray = arrayOf(arrayOf<String>(NfcF::class.java.name))
Java
techListsArray = new String[][] { new String[] { NfcF.class.getName() } };
- Override the following activity lifecycle callbacks and add logic to enable and disable the
foreground dispatch when the activity loses (
onPause()
)
and regains (
onResume()
) focus.
enableForegroundDispatch()
must be called from
the main thread and only when the activity is in the foreground (calling in
onResume()
guarantees this). You also need to implement the
onNewIntent
callback to process the data from the scanned NFC
tag.
Kotlin
public override fun onPause() {
super.onPause()
adapter.disableForegroundDispatch(this)
}
public override fun onResume() {
super.onResume()
adapter.enableForegroundDispatch(this, pendingIntent, intentFiltersArray, techListsArray)
}
public override fun onNewIntent(intent: Intent) {
val tagFromIntent: Tag = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG)
// do something with tagFromIntent
}
Java
public void onPause() {
super.onPause();
adapter.disableForegroundDispatch(this);
}
public void onResume() {
super.onResume();
adapter.enableForegroundDispatch(this, pendingIntent, intentFiltersArray, techListsArray);
}
public void onNewIntent(Intent intent) {
Tag tagFromIntent = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);
// do something with tagFromIntent
}