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

page.title=Handling Data Layer Events

@jd:body

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

<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#Wait">Wait for the Status of Data Layer Calls</a></li>
  <li><a href="#Listen">Listen for Data Layer Events</a></li>
</ol>

</div>
</div>

<p>When you make calls to the Data Layer API, you can receive the status
of the call when it completes as well as listen for any changes that
the call ends up making with listeners.
</p>

<h2 id="Wait">Wait for the Status of Data Layer Calls</h2>

<p>You'll notice that calls to the Data Layer API sometimes return a
<a href="{@docRoot}reference/com/google/android/gms/common/api/PendingResult.html"><code>PendingResult</code></a>,
such as
<a href="{@docRoot}reference/com/google/android/gms/wearable/DataApi.html#putDataItem(com.google.android.gms.common.api.GoogleApiClient, com.google.android.gms.wearable.PutDataRequest)"><code>putDataItem()</code></a>.
As soon as the <a href="{@docRoot}reference/com/google/android/gms/common/api/PendingResult.html"><code>PendingResult</code></a> is created,
the operation is queued in the background. If you do nothing else after this, the operation
eventually completes silently. However, you'll usually want to do something with the result
after the operation completes, so the
<a href="{@docRoot}reference/com/google/android/gms/common/api/PendingResult.html"><code>PendingResult</code></a>
lets you wait for the result status, either synchronously or asynchronously.
</p>

<h3 id="async-waiting">Asynchronous calls</h3>
<p>If your code is running on the main UI thread, do not make blocking calls
to the Data Layer API. You can run the calls asynchronously by adding a callback method
to the <a href="{@docRoot}reference/com/google/android/gms/common/api/PendingResult.html"><code>PendingResult</code></a> object,
which fires when the operation is completed:</p>
<pre>
pendingResult.setResultCallback(new ResultCallback&lt;DataItemResult&gt;() {
    &#64;Override
    public void onResult(final DataItemResult result) {
        if(result.getStatus().isSuccess()) {
            Log.d(TAG, "Data item set: " + result.getDataItem().getUri());
        }
    }
});
</pre>

<h3 id="sync-waiting">Synchronous calls</h3>
<p>If your code is running on a separate handler thread in a background service (which is the case
in a <a href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html"><code>WearableListenerService</code></a>),
it's fine for the calls to block. In this case, you can call
<a href="{@docRoot}reference/com/google/android/gms/common/api/PendingResult.html#await()"><code>await()</code></a>
on the <a href="{@docRoot}reference/com/google/android/gms/common/api/PendingResult.html"><code>PendingResult</code></a>
object, which blocks until the request completes and returns a
<a href="{@docRoot}reference/com/google/android/gms/common/api/Result.html"><code>Result</code></a>
object:
</p>

<pre>
DataItemResult result = pendingResult.await();
if(result.getStatus().isSuccess()) {
    Log.d(TAG, "Data item set: " + result.getDataItem().getUri());
}
</pre>


<h2 id="Listen">Listen for Data Layer Events </h2>
<p>Because the data layer synchronizes and sends data across the handheld and
wearable, you normally want to listen for important events, such as when data items
are created, messages are received, or when the wearable and handset are connected.
</p>
<p>To listen for data layer events, you have two options:</p>

<ul>
  <li>Create a service that extends
  <a href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html"><code>WearableListenerService</code></a>.
  </li>
  <li>Create an activity that implements
  <a href="{@docRoot}reference/com/google/android/gms/wearable/DataApi.DataListener.html"><code>DataApi.DataListener</code></a>.
  </li>
</ul>

<p>With both these options, you override the data event callback methods for the events you
are interested in handling.</p>

<h3 id="listener-service">With a WearableListenerService</h3>

<p>
You typically create instances of this service in both your wearable and handheld apps. If you
are not interested in data events in one of these apps, then you don't need to implement this
service in that particular app.</p>

<p>For example, you can have a handheld app that sets and gets data item objects and a wearable app
that listens for these updates to update it's UI. The wearable never updates any of the data items,
so the handheld app doesn't listen for any data events from the wearable app.</p>

<p>You can listen for the following events with
<a href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html"><code>WearableListenerService</code></a>:</p>

<ul>
  <li><a href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html#onDataChanged(com.google.android.gms.wearable.DataEventBuffer)"><code>onDataChanged()</code></a>
- Called when data item objects are created, changed, or deleted. An event on one side of a connection
triggers this callback on both sides.</li>
  <li><a href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html#onMessageReceived(com.google.android.gms.wearable.MessageEvent)"><code>onMessageReceived()</code></a>
-  A message sent from one side of a connection triggers this callback on the other side of the connection.</li>
  <li><a href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html#onMessageReceived(com.google.android.gms.wearable.MessageEvent)"><code>onPeerConnected()</code></a>
  and <a href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html#onPeerDisconnected(com.google.android.gms.wearable.Node)"><code>onPeerDisconnected()</code></a> -
  Called when the connection with the handheld or wearable is connected or disconnected.
  Changes in connection state on one side of the connection trigger these callbacks on both sides
  of the connection.
  </li>
</ul>

<p>To create a <a href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html"><code>WearableListenerService</code></a>:</p>

<ol>
  <li>Create a class that extends
  <a href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html"><code>WearableListenerService</code></a>.
  </li>
  <li>Listen for the events that you're interested in, such as
  <a href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html#onDataChanged(com.google.android.gms.wearable.DataEventBuffer)"><code>onDataChanged()</code></a>.
  </li>
  <li>Declare an intent filter in your Android manifest to notify the system about your
  <a href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html"><code>WearableListenerService</code></a>.
  This allows the system to bind your service as needed.
  </li>
</ol>

  <p>The following example shows how to implement a simple
  <a href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html"><code>WearableListenerService</code></a>:
  </p>

<pre>
public class DataLayerListenerService extends WearableListenerService {

    private static final String TAG = "DataLayerSample";
    private static final String START_ACTIVITY_PATH = "/start-activity";
    private static final String DATA_ITEM_RECEIVED_PATH = "/data-item-received";

    &#64;Override
    public void onDataChanged(DataEventBuffer dataEvents) {
        if (Log.isLoggable(TAG, Log.DEBUG)) {
            Log.d(TAG, "onDataChanged: " + dataEvents);
        }
        final List<DataEvent> events = FreezableUtils
                .freezeIterable(dataEvents);

        GoogleApiClient googleApiClient = new GoogleApiClient.Builder(this)
                .addApi(Wearable.API)
                .build();

        ConnectionResult connectionResult =
                googleApiClient.blockingConnect(30, TimeUnit.SECONDS);

        if (!connectionResult.isSuccess()) {
            Log.e(TAG, "Failed to connect to GoogleApiClient.");
            return;
        }

        // Loop through the events and send a message
        // to the node that created the data item.
        for (DataEvent event : events) {
            Uri uri = event.getDataItem().getUri();

            // Get the node id from the host value of the URI
            String nodeId = uri.getHost();
            // Set the data of the message to be the bytes of the URI
            byte[] payload = uri.toString().getBytes();

            // Send the RPC
            Wearable.MessageApi.sendMessage(googleApiClient, nodeId,
                    DATA_ITEM_RECEIVED_PATH, payload);
        }
    }
}
</pre>

<p>Here's the corresponding intent filter in the Android manifest file:</p>

<pre>
&lt;service android:name=".DataLayerListenerService"&gt;
  &lt;intent-filter&gt;
      &lt;action android:name="com.google.android.gms.wearable.BIND_LISTENER" /&gt;
  &lt;/intent-filter&gt;
&lt;/service&gt;
</pre>


<h4>Permissions within Data Layer Callbacks</h4>

<p>
To deliver callbacks to your application for data layer events, Google Play services
binds to your <a href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html"><code>WearableListenerService</code></a>,
and calls your callbacks via IPC. This has the consequence
that your callbacks inherit the permissions of the calling process.</p>

<p>If you try to perform a privileged operation within a callback, the security check fails because your callback is
running with the identity of the calling process, instead of the identity of your app's
process.</p>

<p>To fix this, call {@link android.os.Binder#clearCallingIdentity} </a>,
to reset identity after crossing the IPC boundary, and then restore identity with
{@link android.os.Binder#restoreCallingIdentity restoreCallingIdentity()} when
you've completed the privileged operation:
</p>

<pre>
long token = Binder.clearCallingIdentity();
try {
    performOperationRequiringPermissions();
} finally {
    Binder.restoreCallingIdentity(token);
}
</pre>

<h3 id="Listen">With a Listener Activity</h3>

<p>
If your app only cares about data layer events when the user is interacting
with the app and does not need a long-running service to handle every data
change, you can listen for events in an activity by implementing one or more
of the following interfaces:

<ul>
  <li><a href="{@docRoot}reference/com/google/android/gms/wearable/DataApi.DataListener.html"><code>DataApi.DataListener</code></a></li>
  <li><a href="{@docRoot}reference/com/google/android/gms/wearable/MessageApi.MessageListener.html"><code>MessageApi.MessageListener</code></a></li>
  <li><a href="{@docRoot}reference/com/google/android/gms/wearable/NodeApi.NodeListener.html"><code>NodeApi.NodeListener</code></a></li>
</ul>
</p>

<p>To create an activity that listens for data events:</p>
<ol>
<li>Implement the desired interfaces.</li>
<li>In {@link android.app.Activity#onCreate}, create an instance of
<a href="{@docRoot}reference/com/google/android/gms/common/api/GoogleApiClient.html"><code>GoogleApiClient</code></a>
to work with the Data Layer API.
<li>
In {@link android.app.Activity#onStart onStart()}, call <a href="{@docRoot}reference/com/google/android/gms/common/api/GoogleApiClient.html#connect()"><code>connect()</code></a> to connect the client to Google Play services.
</li>
<li>When the connection to Google Play services is established, the system calls
<a href="{@docRoot}reference/com/google/android/gms/common/api/GoogleApiClient.ConnectionCallbacks.html#onConnected(android.os.Bundle)"><code>onConnected()</code></a>. This is where you call
<a href="{@docRoot}reference/com/google/android/gms/wearable/DataApi.html#addListener(com.google.android.gms.common.api.GoogleApiClient, com.google.android.gms.wearable.DataApi.DataListener)"><code>DataApi.addListener()</code></a>,
  <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>,
  or <a href="{@docRoot}reference/com/google/android/gms/wearable/NodeApi.html#addListener(com.google.android.gms.common.api.GoogleApiClient, com.google.android.gms.wearable.NodeApi.NodeListener)"><code>NodeApi.addListener()</code></a>
  to notify Google Play services that your activity is interested in listening for data layer events.
</li>
<li>In {@link android.app.Activity#onStop onStop()}, unregister any listeners with
<a href="{@docRoot}reference/com/google/android/gms/wearable/DataApi.html#removeListener(com.google.android.gms.common.api.GoogleApiClient, com.google.android.gms.wearable.DataApi.DataListener)"><code>DataApi.removeListener()</code></a>,
<a href="{@docRoot}reference/com/google/android/gms/wearable/MessageApi.html#removeListener(com.google.android.gms.common.api.GoogleApiClient, com.google.android.gms.wearable.MessageApi.MessageListener)"><code>MessageApi.removeListener()</code></a>,
or <a href="{@docRoot}reference/com/google/android/gms/wearable/NodeApi.html#removeListener(com.google.android.gms.common.api.GoogleApiClient, com.google.android.gms.wearable.NodeApi.NodeListener)"><code>NodeApi.removeListener()</code></a>.
</li>
<li>Implement <a href="{@docRoot}reference/com/google/android/gms/wearable/DataApi.DataListener.html#onDataChanged(com.google.android.gms.wearable.DataEventBuffer)"><code>onDataChanged()</code>,
  <a href="{@docRoot}reference/com/google/android/gms/wearable/NodeApi.NodeListener.html#onPeerConnected(com.google.android.gms.wearable.Node)"><code>onMessageReceived()</code></a>,
    <a href="{@docRoot}reference/com/google/android/gms/wearable/NodeApi.NodeListener.html#onPeerConnected(com.google.android.gms.wearable.Node)"><code>onPeerConnected()</code></a>, and
  <a href="{@docRoot}reference/com/google/android/gms/wearable/NodeApi.NodeListener.html#onPeerDisconnected(com.google.android.gms.wearable.Node)"><code>onPeerDisconnected()</code></a>, depending on the interfaces that you implemented.
</li>
</ol>

<p>Here's an example that implements
<a href="{@docRoot}reference/com/google/android/gms/wearable/DataApi.DataListener.html"><code>DataApi.DataListener</code></a>:</p>

<pre>
public class MainActivity extends Activity implements
        DataApi.DataListener, ConnectionCallbacks, OnConnectionFailedListener {

    private GoogleApiClient mGoogleApiClient;

    &#64;Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.main);
        mGoogleApiClient = new GoogleApiClient.Builder(this)
                .addApi(Wearable.API)
                .addConnectionCallbacks(this)
                .addOnConnectionFailedListener(this)
                .build();
    }

    &#64;Override
    protected void onStart() {
        super.onStart();
        if (!mResolvingError) {
            mGoogleApiClient.connect();
        }
    }

    &#64;Override
    public void onConnected(Bundle connectionHint) {
        if (Log.isLoggable(TAG, Log.DEBUG)) {
            Log.d(TAG, "Connected to Google Api Service");
        }
        Wearable.DataApi.addListener(mGoogleApiClient, this);
    }

    &#64;Override
    protected void onStop() {
        if (null != mGoogleApiClient && mGoogleApiClient.isConnected()) {
            Wearable.DataApi.removeListener(mGoogleApiClient, this);
            mGoogleApiClient.disconnect();
        }
        super.onStop();
    }

    &#64;Override
    public void onDataChanged(DataEventBuffer dataEvents) {
        for (DataEvent event : dataEvents) {
            if (event.getType() == DataEvent.TYPE_DELETED) {
                Log.d(TAG, "DataItem deleted: " + event.getDataItem().getUri());
            } else if (event.getType() == DataEvent.TYPE_CHANGED) {
                Log.d(TAG, "DataItem changed: " + event.getDataItem().getUri());
            }
        }
    }
}
</pre>