Send and receive messages on Wear

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

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

Unlike in the case of data items, no syncing occurs between the handheld and wearable apps. Messages are a one-way communication mechanism useful 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, send the message to a node with the processing power and battery capacity to handle the request, such as a handheld device.

Note: When you specify the details of your message, consider the possibility of multiple connected nodes. Make sure the message is delivered to the intended devices or nodes.

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 transcript saved to a note. Since a wearable device typically doesn't have the processing power and battery capacity required to handle the voice transcription activity, the app needs to 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 MessageClient 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, do the following:

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 xmlns:tools="http://schemas.android.com/tools"
           tools:keep="@array/android_wear_capabilities">
    <string-array name="android_wear_capabilities">
        <item>voice_transcription</item>
    </string-array>
</resources>

Retrieve nodes with the required capabilities

Initially, you can detect the capable nodes by calling the getCapability method of the CapabilityClient class. To use this method, your Wear OS app and phone app must have the same application ID. The following example shows how to manually retrieve the results of reachable nodes with the voice_transcription capability:

private const val VOICE_TRANSCRIPTION_CAPABILITY_NAME = "voice_transcription"
...
private fun setupVoiceTranscription() {
    val capabilityInfo: CapabilityInfo = Tasks.await(
            Wearable.getCapabilityClient(context)
                    .getCapability(
                            VOICE_TRANSCRIPTION_CAPABILITY_NAME,
                            CapabilityClient.FILTER_REACHABLE
                    )
    )
    // capabilityInfo has the reachable nodes with the transcription capability
    updateTranscriptionCapability(capabilityInfo)
}

private static final String
    VOICE_TRANSCRIPTION_CAPABILITY_NAME = "voice_transcription";
    ...
private void setupVoiceTranscription() {
    CapabilityInfo capabilityInfo = Tasks.await(
        Wearable.getCapabilityClient(context).getCapability(
            VOICE_TRANSCRIPTION_CAPABILITY_NAME, CapabilityClient.FILTER_REACHABLE));
    // capabilityInfo has the reachable nodes with the transcription capability
    updateTranscriptionCapability(capabilityInfo);
}

To detect capable nodes as they connect to the wearable device, register an instance of a listener, specifically an OnCapabilityChangedListener of a CapabilityClient object. The following example shows how to register the listener and retrieve a result with reachable nodes that have the voice_transcription capability:

private fun setupVoiceTranscription() {
    updateTranscriptionCapability(capabilityInfo).also { capabilityListener ->
        Wearable.getCapabilityClient(context).addListener(
                capabilityListener,
                VOICE_TRANSCRIPTION_CAPABILITY_NAME
        )
    }
}

private void setupVoiceTranscription() {
    ...
    // This example uses a Java 8 Lambda. You can use named or anonymous classes.
    CapabilityClient.OnCapabilityChangedListener capabilityListener =
        capabilityInfo -> { updateTranscriptionCapability(capabilityInfo); };
    Wearable.getCapabilityClient(context).addListener(
        capabilityListener,
        VOICE_TRANSCRIPTION_CAPABILITY_NAME);
}

After detecting the capable nodes, determine where to send the message. Choose 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 whether a node is nearby such as connected via Bluetooth, call the Node.isNearby() method. If more than one node is nearby, choose one arbitrarily; similarly, if no capable node is nearby, then choose a capable node arbitrarily.

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

private var transcriptionNodeId: String? = null

private fun updateTranscriptionCapability(capabilityInfo: CapabilityInfo) {
    transcriptionNodeId = pickBestNodeId(capabilityInfo.nodes)
}

private fun pickBestNodeId(nodes: Set<Node>): String? {
    // Find a nearby node or pick one arbitrarily.
    return nodes.firstOrNull { it.isNearby }?.id ?: nodes.firstOrNull()?.id
}

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 a node to use, send the message using the MessageClient class.

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

Note: A successful result code doesn't guarantee delivery of the message. If your app requires data reliability, consider using DataItem objects or the ChannelClient class to send data between devices.

const val VOICE_TRANSCRIPTION_MESSAGE_PATH = "/voice_transcription"
...
private fun requestTranscription(voiceData: ByteArray) {
    transcriptionNodeId?.also { nodeId ->
        val sendTask: Task<*> = Wearable.getMessageClient(context).sendMessage(
                nodeId,
                VOICE_TRANSCRIPTION_MESSAGE_PATH,
                voiceData
        ).apply {
            addOnSuccessListener { ... }
            addOnFailureListener { ... }
        }
    }
}

public static final String VOICE_TRANSCRIPTION_MESSAGE_PATH = "/voice_transcription";
private void requestTranscription(byte[] voiceData) {
    if (transcriptionNodeId != null) {
        Task<Integer> sendTask =
            Wearable.getMessageClient(context).sendMessage(
              transcriptionNodeId, VOICE_TRANSCRIPTION_MESSAGE_PATH, voiceData);
         // You can add success and/or failure listeners,
         // Or you can call Tasks.await() and catch ExecutionException
         sendTask.addOnSuccessListener(...);
         sendTask.addOnFailureListener(...);
    } 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 the Tasks API.

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

private fun getNodes(): Collection<String> {
    return Tasks.await(Wearable.getNodeClient(context).connectedNodes).map { it.id }
}

private Collection<String> getNodes() {
    HashSet <String>results = new HashSet<String>();
    List<Node> nodes =
        Tasks.await(Wearable.getNodeClient(context).getConnectedNodes());
    for (Node node : nodes.getNodes()) {
        results.add(node.getId());
    }
    return results;
}

Receive a message

To be notified of received messages, implement the MessageClient.OnMessageReceivedListener interface to provide a listener for message events. Then, register the listener with the addListener method. The following 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.

fun onMessageReceived(messageEvent: MessageEvent) {
    if (messageEvent.path == VOICE_TRANSCRIPTION_MESSAGE_PATH) {
        val startIntent = Intent(this, MainActivity::class.java).apply {
            addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
            putExtra("VOICE_DATA", messageEvent.data)
        }
        startActivity(this, startIntent)
    }
}

@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(this, startIntent);
    }
}

This code requires more implementation details. Learn about how to implement a full listener service or activity in Listen for data layer events.