Sending and Receiving Messages

You send messages using the MessageApi and attach the following items to the message:

• An arbitrary payload (optional)
• A path that uniquely identifies the message's action

Unlike with data items, there is no syncing between the handheld and wearable apps. Messages are a one-way communication mechanism that's good for remote procedure calls (RPC), such as sending a message to the wearable to start an activity.

Multiple wearable devices can be connected to a user’s handheld device. Each connected device in the network is considered a node. With multiple connected devices, you must consider which nodes receive the messages. For example, in a voice transcription app that receives voice data on the wearable device, you should send the message to a node with the processing power and battery capacity to handle the request, such as a handheld device.

Note: With versions of Google Play services prior to 7.3.0, only one wearable device could be connected to a handheld device at a time. You may need to update your existing code to take the multiple connected nodes feature into consideration. If you don’t implement the changes, your messages may not get delivered to intended devices.

Send a Message

A wearable app can provide functionality for users such as voice transcription. Users can speak into their wearable device's microphone, and have a transcription saved to a note. Since a wearable device typically does not have the processing power and battery capacity required to handle the voice transcription activity, the app should offload this work to a more capable, connected device.

The following sections show you how to advertise device nodes that can process activity requests, discover the nodes capable of fulfilling a requested need, and send messages to those nodes.

Advertise capabilities

To launch an activity on a handheld device from a wearable device, use the MessageApi class to send the request. Since multiple wearables can be connected to the handheld device, the wearable app needs to determine that a connected node is capable of launching the activity. In your handheld app, advertise that the node it runs on provides specific capabilities.

To advertise the capabilities of your handheld app:

1. Create an XML configuration file in the res/values/ directory of your project and name it wear.xml.
2. Add a resource named android_wear_capabilities to wear.xml.
3. Define capabilities that the device provides.

Note: Capabilities are custom strings that you define and must be unique within your app.

The following example shows how to add a capability named voice_transcription to wear.xml:

<resources>
    <string-array name="android_wear_capabilities">
        <item>voice_transcription</item>
    </string-array>
</resources>

Retrieve the nodes with the required capabilities

Initially, you can detect the capable nodes by calling the CapabilityApi.getCapability() method. The following example shows how to manually retrieve the results of reachable nodes with the voice_transcription capability:

private static final String
        VOICE_TRANSCRIPTION_CAPABILITY_NAME = "voice_transcription";

private GoogleApiClient mGoogleApiClient;

...

private void setupVoiceTranscription() {
    CapabilityApi.GetCapabilityResult result =
            Wearable.CapabilityApi.getCapability(
                    mGoogleApiClient, VOICE_TRANSCRIPTION_CAPABILITY_NAME,
                    CapabilityApi.FILTER_REACHABLE).await();

    updateTranscriptionCapability(result.getCapability());
}

To detect capable nodes as they connect to the wearable device, register a CapabilityApi.CapabilityListener() instance to your GoogleApiClient. The following example shows how to register the listener and retrieve the results of reachable nodes with the voice_transcription capability:

private void setupVoiceTranscription() {
    ...

    CapabilityApi.CapabilityListener capabilityListener =
            new CapabilityApi.CapabilityListener() {
                @Override
                public void onCapabilityChanged(CapabilityInfo capabilityInfo) {
                    updateTranscriptionCapability(capabilityInfo);
                }
            };

    Wearable.CapabilityApi.addCapabilityListener(
            mGoogleApiClient,
            capabilityListener,
            VOICE_TRANSCRIPTION_CAPABILITY_NAME);
}

Note: If you create a service that extends WearableListenerService to detect capability changes, you may want to override the onConnectedNodes() method to listen to finer-grained connectivity details, such as when a wearable device switches from Wi-Fi to a Bluetooth connection to the handset. For an example implementation, see the DisconnectListenerService class in the FindMyPhone sample. For more information on how to listen for important events, see Listen for Data Layer Events.

After detecting the capable nodes, determine where to send the message. You should pick a node that is in close proximity to your wearable device to minimize message routing through multiple nodes. A nearby node is defined as one that is directly connected to the device. To determine if a node is nearby, call the Node.isNearby() method.

The following example shows how you might determine the best node to use:

private String transcriptionNodeId = null;

private void updateTranscriptionCapability(CapabilityInfo capabilityInfo) {
    Set<Node> connectedNodes = capabilityInfo.getNodes();

    transcriptionNodeId = pickBestNodeId(connectedNodes);
}

private String pickBestNodeId(Set<Node> nodes) {
    String bestNodeId = null;
    // Find a nearby node or pick one arbitrarily
    for (Node node : nodes) {
        if (node.isNearby()) {
            return node.getId();
         }
         bestNodeId = node.getId();
    }
    return bestNodeId;
}

Deliver the message

Once you’ve identified the best node to use, send the message using the MessageApi class.

The following example shows how to send a message to the transcription-capable node from a wearable device. Verify that the node is available before you attempt to send the message. This call is synchronous and blocks processing until the system queues the message for delivery.

Note: A successful result code does not guarantee delivery of the message. If your app requires data reliability, use DataItem objects or the ChannelApi class to send data between devices.

public static final String VOICE_TRANSCRIPTION_MESSAGE_PATH = "/voice_transcription";

private void requestTranscription(byte[] voiceData) {
    if (transcriptionNodeId != null) {
        Wearable.MessageApi.sendMessage(googleApiClient, transcriptionNodeId,
            VOICE_TRANSCRIPTION_MESSAGE_PATH, voiceData).setResultCallback(
                  new ResultCallback() {
                      @Override
                      public void onResult(SendMessageResult sendMessageResult) {
                          if (!sendMessageResult.getStatus().isSuccess()) {
                              // Failed to send message
                          }
                      }
                  }
            );
    } else {
        // Unable to retrieve node with transcription capability
    }
}

Note: To learn more about asynchronous and synchronous calls to Google Play services and when to use each, see Communicate with Google Play Services.

You can also broadcast messages to all connected nodes. To retrieve all of the connected nodes that you can send messages to, implement the following code:

private Collection<String> getNodes() {
    HashSet <String>results = new HashSet<String>();
    NodeApi.GetConnectedNodesResult nodes =
            Wearable.NodeApi.getConnectedNodes(mGoogleApiClient).await();
    for (Node node : nodes.getNodes()) {
        results.add(node.getId());
    }
    return results;
}

Receive a Message

To be notified of received messages, implement the MessageListener interface to provide a listener for message events. Then, register the listener with the MessageApi.addListener() method. This example shows how you might implement the listener to check the VOICE_TRANSCRIPTION_MESSAGE_PATH. If this condition is true, start an activity to process the voice data.

@Override
public void onMessageReceived(MessageEvent messageEvent) {
    if (messageEvent.getPath().equals(VOICE_TRANSCRIPTION_MESSAGE_PATH)) {
        Intent startIntent = new Intent(this, MainActivity.class);
        startIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        startIntent.putExtra("VOICE_DATA", messageEvent.getData());
        startActivity(startIntent);
    }
}

This is just a snippet that requires more implementation details. Learn about how to implement a full listener service or activity in Listening for Data Layer Events.