Android Beam
Android Beam
Android Beam is a new Near Field Communication (NFC) technology in Android 4 that allows applications
to share information over NFC when in close proximity.
Android Beam works by pushing messages over NFC when two devices are in range. An Activity on one
device creates a message and specifies an Activity (or Activities) that can handle pushing it. When the
specified Activity is in the foreground and the devices are in range, Android Beam will push the message to
the second device. On the receiving device, an Intent is invoked containing the message data.
Android supports two ways of setting messages with Android Beam:
SetNdefPushMessage - Before Android Beam is initiated, an application can call
SetNdefPushMessage to specify an NdefMessage to push over NFC, and the Activity that is pushing
it. This mechanism is best used when a message doesn’t change while an application is in use.
SetNdefPushMessageCallback - When Android Beam is initiated, an application can handle a
callback to create an NdefMessage. This mechanism allows for message creation to be delayed until
devices are in range. It supports scenarios where the message may vary based upon what’s
happening in the application.
In either case, to send data with Android Beam, an application sends an NdefMessage, packaging the
data in several NdefRecords. Let’s take a look at the key points that must be addressed before we can
trigger Android Beam. First, we’ll work with the callback style of creating an NdefMessage.
Creating a Message
We can register callbacks with an NfcAdapter in the Activity’s OnCreate method. For example,
assuming an NfcAdapter named mNfcAdapter is declared as a class variable in the Activity, we can
write the following code to create the callback that will construct the message:
mNfcAdapter = NfcAdapter.GetDefaultAdapter (this);
mNfcAdapter.SetNdefPushMessageCallback (this, this);
The Activity, which implements NfcAdapter.ICreateNdefMessageCallback, is passed to the
SetNdefPushMessageCallback method above. When Android Beam is initiated, the system will call
CreateNdefMessage, from which the Activity can construct an NdefMessage as shown below:
public NdefMessage CreateNdefMessage (NfcEvent evt)
{
DateTime time = DateTime.Now;
var text = ("Beam me up!\n\n" + "Beam Time: " +
time.ToString ("HH:mm:ss"));
NdefMessage msg = new NdefMessage (
new NdefRecord[]{ CreateMimeRecord (
"application/com.example.android.beam",
Encoding.UTF8.GetBytes (text)) });
} };
return msg;
}
public NdefRecord CreateMimeRecord (String mimeType, byte [] payload)
{
byte [] mimeBytes = Encoding.UTF8.GetBytes (mimeType);
NdefRecord mimeRecord = new NdefRecord (
NdefRecord.TnfMimeMedia, mimeBytes, new byte [0], payload);
return mimeRecord;
}
Receiving a Message
On the receiving side, the system invokes an Intent with the ActionNdefDiscovered action, from which
we can extract the NdefMessage as follows:
IParcelable [] rawMsgs = intent.GetParcelableArrayExtra
(NfcAdapter.ExtraNdefMessages);
NdefMessage msg = (NdefMessage) rawMsgs [0];
For a complete code example that uses Android Beam, shown running in the screenshot below, see the
Android Beam demo in the Xamarin.Android samples.
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