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Tag

public final class Tag
extends Object implements Parcelable

java.lang.Object
   ↳ android.nfc.Tag


Represents an NFC tag that has been discovered.

Tag is an immutable object that represents the state of a NFC tag at the time of discovery. It can be used as a handle to TagTechnology classes to perform advanced operations, or directly queried for its ID via getId() and the set of technologies it contains via getTechList(). Arrays passed to and returned by this class are not cloned, so be careful not to modify them.

A new tag object is created every time a tag is discovered (comes into range), even if it is the same physical tag. If a tag is removed and then returned into range, then only the most recent tag object can be successfully used to create a TagTechnology.

Tag Dispatch

When a tag is discovered, a Tag object is created and passed to a single activity via the EXTRA_TAG extra in an Intent via startActivity(Intent). A four stage dispatch is used to select the most appropriate activity to handle the tag. The Android OS executes each stage in order, and completes dispatch as soon as a single matching activity is found. If there are multiple matching activities found at any one stage then the Android activity chooser dialog is shown to allow the user to select the activity to receive the tag.

The Tag dispatch mechanism was designed to give a high probability of dispatching a tag to the correct activity without showing the user an activity chooser dialog. This is important for NFC interactions because they are very transient -- if a user has to move the Android device to choose an application then the connection will likely be broken.

1. Foreground activity dispatch

A foreground activity that has called NfcAdapter.enableForegroundDispatch() is given priority. See the documentation on NfcAdapter.enableForegroundDispatch() for its usage.

2. NDEF data dispatch

If the tag contains NDEF data the system inspects the first NdefRecord in the first NdefMessage. If the record is a URI, SmartPoster, or MIME data startActivity(Intent) is called with ACTION_NDEF_DISCOVERED. For URI and SmartPoster records the URI is put into the intent's data field. For MIME records the MIME type is put in the intent's type field. This allows activities to register to be launched only when data they know how to handle is present on a tag. This is the preferred method of handling data on a tag since NDEF data can be stored on many types of tags and doesn't depend on a specific tag technology. See ACTION_NDEF_DISCOVERED for more detail. If the tag does not contain NDEF data, or if no activity is registered for ACTION_NDEF_DISCOVERED with a matching data URI or MIME type then dispatch moves to stage 3.

3. Tag Technology dispatch

startActivity(Intent) is called with ACTION_TECH_DISCOVERED to dispatch the tag to an activity that can handle the technologies present on the tag. Technologies are defined as sub-classes of TagTechnology, see the package android.nfc.tech. The Android OS looks for an activity that can handle one or more technologies in the tag. See ACTION_TECH_DISCOVERED for more detail.

4. Fall-back dispatch

If no activity has been matched then startActivity(Intent) is called with ACTION_TAG_DISCOVERED. This is intended as a fall-back mechanism. See ACTION_TAG_DISCOVERED.

NFC Tag Background

An NFC tag is a passive NFC device, powered by the NFC field of this Android device while it is in range. Tag's can come in many forms, such as stickers, cards, key fobs, or even embedded in a more sophisticated device.

Tags can have a wide range of capabilities. Simple tags just offer read/write semantics, and contain some one time programmable areas to make read-only. More complex tags offer math operations and per-sector access control and authentication. The most sophisticated tags contain operating environments allowing complex interactions with the code executing on the tag. Use TagTechnology classes to access a broad range of capabilities available in NFC tags.

Summary

Inherited constants

From interface android.os.Parcelable

Fields

public static final Creator<Tag> CREATOR

Public methods

int describeContents()

Describe the kinds of special objects contained in this Parcelable instance's marshaled representation.

byte[] getId()

Get the Tag Identifier (if it has one).

String[] getTechList()

Get the technologies available in this tag, as fully qualified class names.

String toString()

Human-readable description of the tag, for debugging.

void writeToParcel(Parcel dest, int flags)

Flatten this object in to a Parcel.

Inherited methods

From class java.lang.Object
From interface android.os.Parcelable

Fields

CREATOR

Added in API level 10
Creator<Tag> CREATOR

Public methods

describeContents

Added in API level 10
int describeContents ()

Describe the kinds of special objects contained in this Parcelable instance's marshaled representation. For example, if the object will include a file descriptor in the output of writeToParcel(Parcel, int), the return value of this method must include the CONTENTS_FILE_DESCRIPTOR bit.

Returns
int a bitmask indicating the set of special object types marshaled by this Parcelable object instance.

getId

Added in API level 10
byte[] getId ()

Get the Tag Identifier (if it has one).

The tag identifier is a low level serial number, used for anti-collision and identification.

Most tags have a stable unique identifier (UID), but some tags will generate a random ID every time they are discovered (RID), and there are some tags with no ID at all (the byte array will be zero-sized).

The size and format of an ID is specific to the RF technology used by the tag.

This function retrieves the ID as determined at discovery time, and does not perform any further RF communication or block.

Returns
byte[] ID as byte array, never null

getTechList

Added in API level 10
String[] getTechList ()

Get the technologies available in this tag, as fully qualified class names.

A technology is an implementation of the TagTechnology interface, and can be instantiated by calling the static get(Tag) method on the implementation with this Tag. The TagTechnology object can then be used to perform advanced, technology-specific operations on a tag.

Android defines a mandatory set of technologies that must be correctly enumerated by all Android NFC devices, and an optional set of proprietary technologies. See TagTechnology for more details.

The ordering of the returned array is undefined and should not be relied upon.

Returns
String[] an array of fully-qualified TagTechnology class-names.

toString

Added in API level 10
String toString ()

Human-readable description of the tag, for debugging.

Returns
String a string representation of the object.

writeToParcel

Added in API level 10
void writeToParcel (Parcel dest, 
                int flags)

Flatten this object in to a Parcel.

Parameters
dest Parcel: The Parcel in which the object should be written.
flags int: Additional flags about how the object should be written. May be 0 or PARCELABLE_WRITE_RETURN_VALUE.
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