docs/html/training/wearables/data-layer/messages.jd

page.title=Sending and Receiving Messages

@jd:body

<div id="tb-wrapper">
<div id="tb">

<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#SendMessage">Send a Message</a></li>
  <li><a href="#ReceiveMessage">Receive a Message</a></li>
</ol>
</div>
</div>

<p>You send messages using the
<a href="{@docRoot}reference/com/google/android/gms/wearable/MessageApi.html"><code>MessageApi</code></a>
and attach the following items to the message:</p>

<ul>
  <li>An arbitrary payload (optional)</li>
  <li>A path that uniquely identifies the message's action</li>
</ul>
<p>
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.</p>

<h2 id="SendMessage">Send a Message</h2>

<p>The following example shows how to send a message that indicates to the other
side of the connection to start an activity.
This call is synchronous and blocks processing until the message is received or until the request
times out:</p>

<p class="note"><b>Note:</b> Read more about asynchronous and synchronous calls
to Google Play services and when to use each in
<a href="{@docRoot}google/auth/api-client.html#Communicating">Communicate with Google Play Services</a>.
</p>

<pre>
GoogleApiClient mGoogleApiClient;
public static final String START_ACTIVITY_PATH = "/start/MainActivity";
...

private void sendStartActivityMessage(String nodeId) {
    Wearable.MessageApi.sendMessage(
      mGoogleApiClient, nodeId, START_ACTIVITY_PATH, new byte[0]).setResultCallback(
          new ResultCallback&lt;SendMessageResult>() {
              &#64;Override
              public void onResult(SendMessageResult sendMessageResult) {
                  if (!sendMessageResult.getStatus().isSuccess()) {
                      Log.e(TAG, "Failed to send message with status code: "
                              + sendMessageResult.getStatus().getStatusCode());
                  }
              }
          }
      );
}
</pre>

<p>
Here's a simple way to get a list of connected nodes that you can potentially
send messages to:</p>

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

<h2 id="ReceiveMessage">Receive a Message</h2>

<p>
To be notified of received messages, you implement the
<a href="{@docRoot}reference/com/google/android/gms/wearable/MessageApi.MessageListener.html">
<code>MessageListener</code></a> interface to provide a listener for message events. Then you register your
listener with the
<a href="{@docRoot}reference/com/google/android/gms/wearable/MessageApi.html#addListener(com.google.android.gms.common.api.GoogleApiClient, com.google.android.gms.wearable.MessageApi.MessageListener)">
<code>MessageApi.addListener()</code></a> method. This example shows how you might implement the listener
to check the <code>START_ACTIVITY_PATH</code> that the previous example used to send the message.
If this condition is <code>true</code>, a specific activity is started.
</p>

<pre>
&#64;Override
public void onMessageReceived(MessageEvent messageEvent) {
    if (messageEvent.getPath().equals(START_ACTIVITY_PATH)) {
        Intent startIntent = new Intent(this, MainActivity.class);
        startIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        startActivity(startIntent);
    }
}
</pre>

<p>
This is just a snippet that requires more implementation details. Learn about
how to implement a full listener service or activity in
<a href="{@docRoot}training/wearables/data-layer/events.html#Listen">Listening for Data Layer
Events</a>.
</p>