Tag

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public final class Tag
extends Object implements Parcelable

java.lang.Object
↳	android.nfc.Tag

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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 NfcAdapter.EXTRA_TAG extra in an Intent via Context.startActivity. 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 Context.startActivity is called with NfcAdapter.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 NfcAdapter.ACTION_NDEF_DISCOVERED for more detail. If the tag does not contain NDEF data, or if no activity is registered for NfcAdapter.ACTION_NDEF_DISCOVERED with a matching data URI or MIME type then dispatch moves to stage 3.

3. Tag Technology dispatch

Context.startActivity is called with NfcAdapter.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 NfcAdapter.ACTION_TECH_DISCOVERED for more detail.

4. Fall-back dispatch

If no activity has been matched then Context.startActivity is called with NfcAdapter.ACTION_TAG_DISCOVERED. This is intended as a fall-back mechanism. See NfcAdapter.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 int CONTENTS_FILE_DESCRIPTOR Descriptor bit used with describeContents(): indicates that the Parcelable object's flattened representation includes a file descriptor. int PARCELABLE_WRITE_RETURN_VALUE Flag for use with writeToParcel(Parcel, int): the object being written is a return value, that is the result of a function such as "Parcelable someFunction()", "void someFunction(out Parcelable)", or "void someFunction(inout 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 Object clone() Creates and returns a copy of this object. boolean equals(Object obj) Indicates whether some other object is "equal to" this one. void finalize() Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. final Class<?> getClass() Returns the runtime class of this Object. int hashCode() Returns a hash code value for the object. final void notify() Wakes up a single thread that is waiting on this object's monitor. final void notifyAll() Wakes up all threads that are waiting on this object's monitor. String toString() Returns a string representation of the object. final void wait(long timeoutMillis, int nanos) Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed. final void wait(long timeoutMillis) Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed. final void wait() Causes the current thread to wait until it is awakened, typically by being notified or interrupted.
From interface android.os.Parcelable abstract int describeContents() Describe the kinds of special objects contained in this Parcelable instance's marshaled representation. abstract void writeToParcel(Parcel dest, int flags) Flatten this object in to a Parcel.

Fields

Public methods