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      1 page.title=Sending and Receiving Messages
      2 
      3 @jd:body
      4 
      5 <div id="tb-wrapper">
      6 <div id="tb">
      7 
      8 <h2>This lesson teaches you to</h2>
      9 <ol>
     10   <li><a href="#SendMessage">Send a Message</a></li>
     11   <li><a href="#ReceiveMessage">Receive a Message</a></li>
     12 </ol>
     13 </div>
     14 </div>
     15 
     16 <p>You send messages using the
     17 <a href="{@docRoot}reference/com/google/android/gms/wearable/MessageApi.html"><code>MessageApi</code></a>
     18 and attach the following items to the message:</p>
     19 
     20 <ul>
     21   <li>An arbitrary payload (optional)</li>
     22   <li>A path that uniquely identifies the message's action</li>
     23 </ul>
     24 <p>
     25 Unlike data items, there is no syncing between the handheld and wearable apps.
     26 Messages are a one-way communication mechanism that's good for remote procedure calls (RPC),
     27 such as sending a message to the wearable
     28 to start an activity. You can also use messages in request/response model
     29 where one side of the connection sends a message, does some work,
     30 sends back a response message.</p>
     31 
     32 <h2 id="SendMessage">Send a Message</h2>
     33 
     34 <p>The following example shows how to send a message that indicates to the other
     35 side of the connect to start an activity.
     36 This call is made synchronously, which blocks until the message
     37 is received or when the request times out:
     38 </p>
     39 
     40 <p class="note"><b>Note:</b> Read more about asynchronous and synchronous calls
     41 to Google Play services and when to use each in
     42 <a href="{@docRoot}google/auth/api-client.html#Communicating">Communicate with Google Play Services</a>.
     43 </p>
     44 
     45 <pre>
     46 Node node; // the connected device to send the message to
     47 GoogleApiClient mGoogleApiClient;
     48 public static final START_ACTIVITY_PATH = "/start/MainActivity";
     49 ...
     50 
     51     SendMessageResult result = Wearable.MessageApi.sendMessage(
     52             mGoogleApiClient, node, START_ACTIVITY_PATH, null).await();
     53     if (!result.getStatus().isSuccess()) {
     54         Log.e(TAG, "ERROR: failed to send Message: " + result.getStatus());
     55     }
     56 </pre>
     57 
     58 <p>
     59 Here's a simple way to get a list of connected nodes that you can potentially
     60 send messages to:</p>
     61 
     62 <pre>
     63 private Collection&lt;String&gt; getNodes() {
     64     HashSet &lt;String&gt;results= new HashSet&lt;String&gt;();
     65     NodeApi.GetConnectedNodesResult nodes =
     66             Wearable.NodeApi.getConnectedNodes(mGoogleApiClient).await();
     67     for (Node node : nodes.getNodes()) {
     68         results.add(node.getId());
     69     }
     70     return results;
     71 }
     72 </pre>
     73 
     74 <h2 id="ReceiveMessage">Receiving a Message</h2>
     75 
     76 <p>
     77 
     78 To be notified of received messages, you implement a listener for message events.
     79 This example shows how you might do this by checking the <code>START_ACTIVITY_PATH</code>
     80 that the previous example used to send the message. If this condition is <code>true</code>,
     81 a specific activity is started.
     82 </p>
     83 
     84 <pre>
     85 &#64;Override
     86 public void onMessageReceived(MessageEvent messageEvent) {
     87     if (messageEvent.getPath().equals(START_ACTIVITY_PATH)) {
     88         Intent startIntent = new Intent(this, MainActivity.class);
     89         startIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
     90         startActivity(startIntent);
     91     }
     92 }
     93 </pre>
     94 
     95 <p>
     96 This is just a snippet that requires more implementation details. Learn about
     97 how to implement a full listener service or activity in
     98 <a href="{@docRoot}training/wearables/data-layer/events.html">Listening for Data Layer Events</a>.
     99 </p>