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="https://developers.google.com/android/reference/com/google/android/gms/common/api/PendingResult.html"><code>PendingResult</code></a>,
such as
<a href="https://developers.google.com/android/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="https://developers.google.com/android/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="https://developers.google.com/android/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="https://developers.google.com/android/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="https://developers.google.com/android/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="https://developers.google.com/android/reference/com/google/android/gms/common/api/PendingResult.html#await()"><code>await()</code></a>
on the <a href="https://developers.google.com/android/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="https://developers.google.com/android/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, it is usually necessary to listen for important events. Examples of
such events include creation of data items and receipt of messages.
</p>
<p>
To listen for data layer events, you have two options:
</p>
<ul>
   <li>Create a service that extends <a href
="https://developers.google.com/android/reference/com/google/android/gms/wearable/WearableListenerService.html">
{@code WearableListenerService}</a>.</li>
   <li>Create an activity that implements <a
href="https://developer.android.com/reference/com/google/android/gms/wearable/DataApi.DataListener.html">
{@code DataApi.DataListener}</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>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 its 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>
Some of the events you can listen for using <a
href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html">
{@code WearableListenerService}</a> are as follows:
</p>
<ul>
  <li><a
href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html#onDataChanged(com.google.android.gms.wearable.DataEventBuffer)">
{@code onDataChanged()}</a>:
Whenever a data item object is created, deleted, or changed, the system triggers
this callback on all connected nodes.
</li>
<li><a
href="http://developer.android.com/reference/com/google/android/gms/wearable/WearableListenerService.html#onMessageReceived(com.google.android.gms.wearable.MessageEvent)">
{@code onMessageReceived()}</a>:  A message sent from a node triggers
this callback on the target node.</li>
<li><a
href="https://developers.google.com/android/reference/com/google/android/gms/wearable/WearableListenerService.html#onCapabilityChanged(com.google.android.gms.wearable.CapabilityInfo)">
{@code onCapabilityChanged()}</a>:
When a capability that an instance of your app advertises becomes available
on the network, that event triggers this callback. If you're looking for a
nearby node you can query the
<a
href="https://developers.google.com/android/reference/com/google/android/gms/wearable/Node.html#isNearby()">
{@code isNearby()}</a> method of the nodes provided in the callback.</li>

<p>
In addition to those on this list, you can listen for events from
<a href="https://developers.google.com/android/reference/com/google/android/gms/wearable/ChannelApi.ChannelListener">
{@code ChannelApi.ChannelListener}</a>, such as
<a href="https://developers.google.com/android/reference/com/google/android/gms/wearable/ChannelApi.ChannelListener.html#onChannelOpened(com.google.android.gms.wearable.Channel)">
{@code onChannelOpened()}</a>.
</p>
</ul>
<p>To create a <a
href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html"><code>WearableListenerService</code></a>, follow these steps:</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 declaration 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 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>
The next section explains how to use an intent filter with this listener.
</p>

<h3>Using filters with WearableListenerService</h3>

<p>
An intent filter for the
<a href="https://developers.google.com/android/reference/com/google/android/gms/wearable/WearableListenerService.html">
{@code WearableListenerService}</a> example shown in the previous section might look like this:

<pre>
&lt;service android:name=".DataLayerListenerService"&gt;
  &lt;intent-filter>
      &lt;action android:name="com.google.android.gms.wearable.DATA_CHANGED" /&gt;
      &lt;data android:scheme="wear" android:host="*"
               android:path="/start-activity" /&gt;
  &lt;/intent-filter&gt;
&lt;/service&gt;
</pre>

<p>
In this filter, the {@code DATA_CHANGED} action replaces the
previously recommended {@code BIND_LISTENER} action so that only specific
events wake or launch your application. This change improves system efficiency
and reduces battery consumption and other overhead associated with your
application. In this example, the watch listens for the
{@code /start-activity} data item, and the
phone listens for the {@code /data-item-received} message response.
</p>
<p>
Standard Android filter matching rules apply. You can specify multiple services
per manifest, multiple intent filters per service, multiple actions per filter,
and multiple data stanzas per filter. Filters can match on a wildcard host or on
a specific one. To match on a wildcard host, use {@code host="*"}. To match
on a specific host, specify {@code host=&lt;node_id&gt;}.
</p>

<p>
You can also match a literal path or path prefix. If you are matching by path
or path prefix, you must specify a wildcard or specific host.
If you do not do so, the system ignores the path you specified.
</p>

<p>
For more information on the filter types that Wear supports, see the
API reference documentation for <a
href="https://developers.google.com/android/reference/com/google/android/gms/wearable/WearableListenerService">
{@code WearableListenerService}</a>.
</p>

<p>
For more information on data filters and matching rules, see the API reference
documentation for the <a
href="{@docRoot}guide/topics/manifest/data-element.html">{@code data}</a>
manifest element.
</p>

<p>When matching intent filters, there are two important rules to remember:</p>
<ul>
    <li>If a scheme is not specified for the intent filter, the system ignores
    all the other URI attributes.</li>
    <li>If no host is specified for the filter, the system ignores the
    port attribute and all the path attributes.</li>
</ul>

<h3>With a listener activity</h3>
<p>
If your app only cares about data-layer events when the user is interacting
with the app, it may not need a long-running service to handle every data
change. In such a case, you can listen for events in an activity by
implementing one or more of the following interfaces:
</p>
<ul>
  <li><a href="https://developers.google.com/android/reference/com/google/android/gms/wearable/DataApi.DataListener.html"><code>
  DataApi.DataListener</code></a></li>

  <li><a href="https://developers.google.com/android/reference/com/google/android/gms/wearable/MessageApi.MessageListener.html">
  <code>MessageApi.MessageListener</code></a></li>

  <li><a href="https://developers.google.com/android/reference/com/google/android/gms/wearable/CapabilityApi.CapabilityListener.html">{@code CapabilityApi.CapabilityListener}</a></li>
</ul>

<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 onCreate()}, create an instance of
<a href="https://developers.google.com/android/reference/com/google/android/gms/common/api/GoogleApiClient.html"><code>GoogleApiClient</code>
</a>to work with the Data Layer API.</li>

<li>
In {@link android.app.Activity#onStart onStart()}, call <a href="https://developers.google.com/android/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="https://developers.google.com/android/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="https://developers.google.com/android/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="https://developers.google.com/android/reference/com/google/android/gms/wearable/MessageApi.html#addListener(com.google.android.gms.common.api.GoogleApiClient, com.google.android.gms.wearable.MessageApi.MessageListener, android.net.Uri, int)">
<code>MessageApi.addListener()</code></a>, or

<a href="https://developers.google.com/android/reference/com/google/android/gms/wearable/CapabilityApi.html#addListener(com.google.android.gms.common.api.GoogleApiClient,%20com.google.android.gms.wearable.CapabilityApi.CapabilityListener,%20android.net.Uri,%20int)">
{@code CapabilityApi.addListener()}</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="https://developers.google.com/android/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="https://developers.google.com/android/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="https://developers.google.com/android/reference/com/google/android/gms/wearable/CapabilityApi.html#removeListener(com.google.android.gms.common.api.GoogleApiClient, com.google.android.gms.wearable.CapabilityApi.CapabilityListener)">
{@code CapabilityApi.removeListener()}</a>.</li>


<p>An alternative to adding listeners in
<a href="https://developers.google.com/android/reference/com/google/android/gms/common/api/GoogleApiClient.ConnectionCallbacks.html#onConnected(android.os.Bundle)"><code>onConnected()</code></a>
and removing them in
{@link android.app.Activity#onStop onStop()} is to add a filtered listener in an activity’s {@link android.app.Activity#onResume onResume()} and
remove it in {@link android.app.Activity#onPause onPause()}, so as to only receive data that is relevant to the
current application state.</p>


<li>Implement
<a href="https://developers.google.com/android/reference/com/google/android/gms/wearable/DataApi.DataListener.html#onDataChanged(com.google.android.gms.wearable.DataEventBuffer)">
<code>onDataChanged()</code></a>,
  <a href="https://developers.google.com/android/reference/com/google/android/gms/wearable/MessageApi.MessageListener.html#onMessageReceived(com.google.android.gms.wearable.MessageEvent)">
  <code>onMessageReceived()</code></a>,
  <a href="https://developers.google.com/android/reference/com/google/android/gms/wearable/WearableListenerService.html#onCapabilityChanged(com.google.android.gms.wearable.CapabilityInfo)">
{@code onCapabilityChanged()}</a>,
or methods from <a href="https://developers.google.com/android/reference/com/google/android/gms/wearable/ChannelApi.ChannelListener.html">
Channel API listener methods</a>, depending on the interfaces that you implemented.</li>
</ol>

<p>Here's an example that implements
<a href="https://developers.google.com/android/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>
<h3>Using Filters with Listener Activities</h3>
<p>
Just as you can specify intent filters for manifest-based
<a href="https://developers.google.com/android/reference/com/google/android/gms/wearable/WearableListenerService.html">
<code>WearableListenerService</code></a> objects, you can also use intent filters when registering a
listener through the Wearable API. The same rules are applicable to both
API-based listeners manifest-based listeners.
</p>

<p>
A common pattern is to register a listener with a specific path or path prefix
in an activity’s {@link android.app.Activity#onResume onResume()} method, and to
remove the listener in the activity’s {@link android.app.Activity#onPause onPause()} method.
Implementing listeners in this fashion allows your application to more selectively receive events,
improving its design and efficiency.
</p>