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Android Reference



Class Tag

  • All Implemented Interfaces:

    public final class Tag
    extends Object
    implements Parcelable
    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(android.content.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 Context.startActivity(android.content.Intent) 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(android.content.Intent) 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 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(android.content.Intent) 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.

    • Constructor Detail

      • Tag

        public Tag(byte[] id,
           int[] techList,
           Bundle[] techListExtras,
           int serviceHandle,
           INfcTag tagService)
        Hidden constructor to be used by NFC service and internal classes.
    • Method Detail

      • createMockTag

        public static Tag createMockTag(byte[] id,
                        int[] techList,
                        Bundle[] techListExtras)
        Construct a mock Tag.

        This is an application constructed tag, so NfcAdapter methods on this Tag may fail with IllegalArgumentException since it does not represent a physical Tag.

        This constructor might be useful for mock testing.

        id - The tag identifier, can be null
        techList - must not be null
        freshly constructed tag
      • getServiceHandle

        public int getServiceHandle()
        For use by NfcService only.
      • getId

        public 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.

        ID as byte array, never null
      • getTechList

        public 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.

        an array of fully-qualified TagTechnology class-names.
      • rediscover

        public Tag rediscover()
                       throws IOException
        Rediscover the technologies available on this tag.

        The technologies that are available on a tag may change due to operations being performed on a tag. For example, formatting a tag as NDEF adds the Ndef technology. The rediscover method reenumerates the available technologies on the tag and returns a new Tag object containing these technologies.

        You may not be connected to any of this Tag's technologies when calling this method. This method guarantees that you will be returned the same Tag if it is still in the field.

        May cause RF activity and may block. Must not be called from the main application thread. A blocked call will be canceled with IOException by calling #close from another thread.

        Does not remove power from the RF field, so a tag having a random ID should not change its ID.

        the rediscovered tag object.
        IOException - if the tag cannot be rediscovered
      • hasTech

        public boolean hasTech(int techType)
      • getTechExtras

        public Bundle getTechExtras(int tech)
      • getTagService

        public INfcTag getTagService()
      • toString

        public String toString()
        Human-readable description of the tag, for debugging.
        toString in class Object
        a printable representation of this object.
      • describeContents

        public int describeContents()
        Description copied from interface: Parcelable
        Describe the kinds of special objects contained in this Parcelable's marshalled representation.
        Specified by:
        describeContents in interface Parcelable
        a bitmask indicating the set of special object types marshalled by the Parcelable.
      • setConnectedTechnology

        public void setConnectedTechnology(int technology)
        For internal use only.
      • getConnectedTechnology

        public int getConnectedTechnology()
        For internal use only.
      • setTechnologyDisconnected

        public void setTechnologyDisconnected()
        For internal use only.


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RFC (standard status)

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