Tag Class

Definition

[Android.Runtime.Register("android/nfc/Tag", DoNotGenerateAcw=true)]
public sealed class Tag : Java.Lang.Object, Android.OS.IParcelable, IDisposable, Java.Interop.IJavaPeerable

[<Android.Runtime.Register("android/nfc/Tag", DoNotGenerateAcw=true)>]
type Tag = class
    inherit Object
    interface IParcelable
    interface IJavaObject
    interface IDisposable
    interface IJavaPeerable

Inheritance

Object

Object

Tag

Attributes

RegisterAttribute

Implements

IParcelable IJavaObject IJavaPeerable IDisposable

Remarks

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 <em>not</em> 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.

<h3>Tag Dispatch</h3> When a tag is discovered, a Tag object is created and passed to a single activity via the NfcAdapter#EXTRA_TAG extra in an android.content.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.

<h4>1. Foreground activity dispatch</h4> A foreground activity that has called NfcAdapter#enableForegroundDispatch NfcAdapter.enableForegroundDispatch() is given priority. See the documentation on NfcAdapter#enableForegroundDispatch NfcAdapter.enableForegroundDispatch() for its usage. <h4>2. NDEF data dispatch</h4> 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. <h4>3. Tag Technology dispatch</h4> 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. <h4>4. Fall-back dispatch</h4> 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.

<h3>NFC Tag Background</h3> 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.

Java documentation for android.nfc.Tag.

Portions of this page are modifications based on work created and shared by the Android Open Source Project and used according to terms described in the Creative Commons 2.5 Attribution License.

Properties

Methods

Explicit Interface Implementations

Extension Methods

Applies to