## Message types

By default, a subscription finds all messages associated with the
app's [Google Cloud Console](https://console.cloud.google.com/) project. This
includes:

- Messages published by the same app on another device.
- Messages owned by that project, attached to beacons. See [Add Attachments to Beacons](https://developers.google.com/beacons/proximity/attachments).

Your app can use a [`MessageFilter`](https://developers.google.com/android/reference/com/google/android/gms/nearby/messages/MessageFilter.md.txt)
to subscribe for more types of nearby messages, including public beacon
attachments, and raw Bluetooth Low Energy (BLE) beacon IDs.

### Public beacon attachments

A developer can mark their beacon attachment namespace as `PUBLIC`. This allows
all apps to retrieve them, regardless of their Cloud Console Project. For
information on how to make attachment namespaces public, see
[Attachment visibility](https://developers.google.com/beacons/proximity/attachments#attachment_visibility).

Example:

    // Subscribe for two different public beacon attachment types.
    MessageFilter messageFilter = new MessageFilter.Builder()
      .includeNamespacedType(EXAMPLE_PUBLIC_NAMESPACE_A, EXAMPLE_PUBLIC_TYPE_A)
      .includeNamespacedType(EXAMPLE_PUBLIC_NAMESPACE_B, EXAMPLE_PUBLIC_TYPE_B)
      .build();
    SubscribeOptions options = new SubscribeOptions.Builder()
      .setStrategy(Strategy.BLE_ONLY)
      .setFilter(messageFilter)
      .build();

    MessageListener messageListener = new MessageListener() {
      @Override
      public void onFound(final Message message) {
        // We may want to handle the two types of message differently.
        if (EXAMPLE_PUBLIC_NAMESPACE_A.equals(message.getNamespace())
            && EXAMPLE_PUBLIC_TYPE_A.equals(message.getType())) {
          // Handle a "type A" message.
        } else if (EXAMPLE_PUBLIC_NAMESPACE_B.equals(message.getNamespace())
            && EXAMPLE_PUBLIC_TYPE_B.equals(message.getType())) {
          // Handle a "type B" message.
        }
      }
    };

    Nearby.getMessagesClient(this).subscribe(messageListener, options);

### BLE beacon IDs

You can use Google's beacon platform to attach arbitrary data in the cloud to
your beacons, abstracting away the actual beacon IDs that are advertised in BLE
packets. These attachments are discovered by default (see
[Message types](https://developers.google.com/nearby/messages/android/advanced.md.txt#message_types)).

However, if you do need to discover raw beacon IDs (to use your own beacon
registry, for example), you can. There are currently two supported formats:

- [Eddystone UIDs](https://github.com/google/eddystone/tree/master/eddystone-uid).
  - Find these with [`MessageFilter.Builder#includeEddystoneUids`](https://developers.google.com/android/reference/com/google/android/gms/nearby/messages/MessageFilter.Builder.md.txt#includeEddystoneUids(java.lang.String,%20java.lang.String)).
- iBeacon IDs.
  - Find these with [`MessageFilter.Builder#includeIBeaconIds`](https://developers.google.com/android/reference/com/google/android/gms/nearby/messages/MessageFilter.Builder.md.txt#includeIBeaconIds(java.util.UUID,%20java.lang.Short,%20java.lang.Short)).

Example:

    // Subscribe for all Eddystone UIDs whose first 10 bytes (the "namespace")
    // match MY_EDDYSTONE_UID_NAMESPACE.
    //
    // Note that the Eddystone UID namespace is separate from the namespace
    // field of a Nearby Message.
    MessageFilter messageFilter = new MessageFilter.Builder()
      .includeEddystoneUids(MY_EDDYSTONE_UID_NAMESPACE, null /* any instance */)
      .build();
    SubscribeOptions options = new SubscribeOptions.Builder()
      .setStrategy(Strategy.BLE_ONLY)
      .setFilter(messageFilter)
      .build();

    MessageListener messageListener = new MessageListener() {
      @Override
      public void onFound(final Message message) {
        // Note: Checking the type shown for completeness, but is unnecessary
        // if your message filter only includes a single type.
        if (Message.MESSAGE_NAMESPACE_RESERVED.equals(message.getNamespace())
            && Message.MESSAGE_TYPE_EDDYSTONE_UID.equals(message.getType())) {
          // Nearby provides the EddystoneUid class to parse Eddystone UIDs
          // that have been found nearby.
          EddystoneUid eddystoneUid = EddystoneUid.from(message);
          Log.i(TAG, "Found Eddystone UID: " + eddystoneUid);
        }
      }
    };

    Nearby.getMessagesClient(this).subscribe(messageListener, options);

## RSSI and distance callbacks

In addition to found and lost callbacks, a foreground subscription can update
your [MessageListener](https://developers.google.com/android/reference/com/google/android/gms/nearby/messages/MessageListener.md.txt)
when Nearby has new information about the BLE signal associated with a message.

> [!NOTE]
> **Note:**

- These extra callbacks are currently only delivered for BLE beacon messages (both attachments and [beacon IDs](https://developers.google.com/nearby/messages/android/advanced.md.txt#ble_beacon_ids)).
- These extra callbacks are not delivered to background (`PendingIntent`) subscriptions.

Example:

    MessageListener messageListener = new MessageListener() {
      /**
       * Called when a message is discovered nearby.
       */
      @Override
      public void onFound(final Message message) {
        Log.i(TAG, "Found message: " + message);
      }

      /**
       * Called when the Bluetooth Low Energy (BLE) signal associated with a message changes.
       *
       * This is currently only called for BLE beacon messages.
       *
       * For example, this is called when we see the first BLE advertisement
       * frame associated with a message; or when we see subsequent frames with
       * significantly different received signal strength indicator (RSSI)
       * readings.
       *
       * For more information, see the MessageListener Javadocs.
       */
      @Override
      public void onBleSignalChanged(final Message message, final BleSignal bleSignal) {
        Log.i(TAG, "Message: " + message + " has new BLE signal information: " + bleSignal);
      }

      /**
       * Called when Nearby's estimate of the distance to a message changes.
       *
       * This is currently only called for BLE beacon messages.
       *
       * For more information, see the MessageListener Javadocs.
       */
      @Override
      public void onDistanceChanged(final Message message, final Distance distance) {
        Log.i(TAG, "Distance changed, message: " + message + ", new distance: " + distance);
      }

      /**
       * Called when a message is no longer detectable nearby.
       */
      @Override
      public void onLost(final Message message) {
        Log.i(TAG, "Lost message: " + message);
      }
    };