Sometimes we may wish to make use of NFC, a very short range communication technology, to transfer data or to interact with NFC tags.
We can query the NfcAdapter instance for the status of the hardware as well as to be notified when a new tag is detected:
- As with the other hardware features, we need permission to access NFC services:
[assembly: UsesPermission(Manifest.Permission.Nfc)]
- Additionally, we need to specify that we are going to be using the NFC device feature. If our app cannot run without NFC, we set the
Requiredproperty totrue:[assembly: UsesFeature( PackageManager.FeatureNfc, Required = true)]
However, if our app runs fine on a device without NFC, we set the
Requiredproperty tofalse:[assembly: UsesFeature( PackageManager.FeatureNfc, Required = false)]
- Now that we have permission, we can get the
NfcAdapterinstance:adapter = NfcAdapter.GetDefaultAdapter(this);
- If NFC is enabled and the app is in the foreground, we can start listening for NFC events using the
EnableForegroundDispatch()method:if (adapter != null && adapter.IsEnabled) { var tagDetected = new IntentFilter( NfcAdapter.ActionTagDiscovered); var pendingIntent = PendingIntent.GetActivity( this, 0, new Intent(this, GetType()), 0); adapter.EnableForegroundDispatch( this, pendingIntent, new[]{ tagDetected }, null); } - When the app goes into the background, we need to disable the listener:
adapter.DisableForegroundDispatch(this);
- As soon as the device detects an NFC device or tag, it will notify our app via the
OnNewIntent()method. We will receive the type of technology detected via theTagextra:protected override void OnNewIntent(Intent intent) { var tag = intent.GetParcelableExtra( NfcAdapter.ExtraTag) as Tag; if (tag != null) { string[] techs = tag.GetTechList(); } } - We can access the actual data in the tag or device message via the
NdefMessagesextra:var ndefMessages = intent.GetParcelableArrayExtra( NfcAdapter.ExtraNdefMessages); if (ndefMessages != null) { foreach (var msg in ndefMessages.Cast<NdefMessage>()) { foreach (var record in msg.GetRecords()) { byte[] typeBytes = record.GetTypeInfo(); byte[] payloadBytes = record.GetPayload() var type = Encoding.UTF8.GetString(typeBytes); var payload = Encoding.UTF8.GetString(payloadBytes); } } }
Like Bluetooth, NFC is a wireless communication technology for use over short distances. However, in the case of NFC, the distance is less than 10 centimeters. Unlike Bluetooth, which uses radio transmissions, NFC uses electromagnetic radio fields to communicate. More information on NFC and how it works can be found on the official NFC website: http://www.nearfieldcommunication.org.
We can use the Android NFC API to do several things, including reading or writing to passive tags and communicating with other devices.
Like any other hardware feature we want to use, we have to request permission to do so. There is the usual NFC permission that we need to request as well as an additional feature that we have to specify. The feature is used by the package manager to let the store know that we may require a device that supports NFC. The feature can be set to required or optional.
Before we can interact via NFC, we have to obtain the NfcAdapter instance from the current context. Once we have the adapter, we start listening for NFC tags or devices using the EnableForegroundDispatch() method. We provide the ActionTagDiscovered action filter to this method and a PendingIntent instance that holds an Intent instance to return to our activity. We could specify any activity here.
Once the listener is started, we need to make sure we stop it as soon as the app leaves the foreground; otherwise, an exception will be thrown. We can cancel the NFC listener by invoking the DisableForegroundDispatch() method.
Because we specified that the listener should return to our activity, the OnNewIntent() method is invoked, along with the Intent instance we specified in the PendingIntent instance, as soon as a tag is detected.
The Intent instance will contain the NFC extras that we can use to read the tags and message. If we request the ExtraTag extra from the intent, we will obtain the Tag instance that initiated the event. We can use this Tag instance to read various details out as well as to write to the tag.
We can also read any of the messages stored in the tag using the ExtraNdefMessages extra. This extra is an array of NdefMessage types, each of which contains one or more NdefRecord types. On each record, we can get type information, such as the MIME type, from the GetTypeInfo() method. We can also get the actual payload using the GetPayload() method.
Some tags support being written to, and we can do this by creating NdefMessage and NdefRecord instances. We can also use these messages to send data to another device.