page.title=Sending and Syncing Data
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    <!-- Required platform, tools, add-ons, devices, knowledge, etc. -->
    <h2>Dependencies and prerequisites</h2>
    <ul>
      <li>Android 4.3 (API Level 18) or higher on the handset device</li>
      <li>The latest version of <a href="{@docRoot}google/play-services/index.html">Google Play services</a></li>
      <li>An Android Wear device or Wear AVD</li>
    </ul>
  </div>
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<p>
The Wearable Data Layer API, which is part of Google Play services, provides a communication channel
for your handheld and wearable apps. The API consists of a set of data objects that the system can
send and synchronize over the wire and listeners that notify your apps of important events with
the data layer:</p>

<dl>
  <dt><b>Data Items</b></dt>
  <dd>A <a href="{@docRoot}reference/com/google/android/gms/wearable/DataItem.html"><code>DataItem</code></a>
  provides data storage with automatic syncing between the handheld and
  wearable.</dd>

  <dt><b>Messages</b></dt>
  <dd>The <a href="{@docRoot}reference/com/google/android/gms/wearable/MessageApi.html"><code>MessageApi</code></a> class
  can send messages and is good for remote procedure calls (RPC), such as controlling a handheld's
  media player from the wearable or starting an intent on the wearable from the handheld.
  Messages are also great for one-way requests or for a request/response communication model.
  If the handheld and wearable are connected, the system queues the message for delivery and
  returns a successful result code. If the devices are not connected, an error is returned. A
  successful result code does not indicate that the message was delivered successfully as the
  devices may disconnect after receiving the result code.
</p></dd>

  <dt><b>Asset</b></dt>
  <dd><a href="{@docRoot}reference/com/google/android/gms/wearable/Asset.html"><code>Asset</code></a> objects are for
  sending binary blobs of data, such as images. You attach assets to data items and the system
  automatically takes care of the transfer for you, conserving Bluetooth bandwidth by caching large assets
  to avoid re-transmission.</dd>

  <dt><b>WearableListenerService</b> (for services)</dt>
  <dd><p>Extending <a href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html"><code>WearableListenerService</code></a>
  lets you listen for important data layer events in a service. The system manages the lifecycle of
  the <a href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html"><code>WearableListenerService</code></a>,
  binding to the service when it needs to send data items or messages and unbinding the service when no work is needed.</p>
  </dd>

  <dt><b>DataListener</b> (for foreground activities)</dt>
  <dd>
  Implementing <a href="{@docRoot}reference/com/google/android/gms/wearable/DataApi.DataListener.html"><code>DataListener</code></a>
  in an activity lets you listen for important data layer events when an activity
  is in the foreground. Using this instead of the
  <a href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html"><code>WearableListenerService</code></a>
  lets you listen for changes only when the user is actively using your app.
  </dd>

  <dt><b>Channel</b></dt>
  <dd>
  You can use the
  <a href="{@docRoot}reference/com/google/android/gms/wearable/ChannelApi.html"><code>ChannelApi</code></a>
  class to transfer large data items, such as music and movie files, from a handheld to a wearable
  device. The Channel API for data transfer has the following benefits:
  <ul>
    <li>Transfer large data files between two or more connected devices, without
    the automatic synchronization provided when using
    <a href="{@docRoot}reference/com/google/android/gms/wearable/Asset.html"><code>Asset</code></a>
    objects attached to
    <a href="{@docRoot}reference/com/google/android/gms/wearable/DataItem.html"><code>DataItem</code></a>
    objects. The Channel API saves disk space unlike the
    <a href="{@docRoot}reference/com/google/android/gms/wearable/DataApi.html"><code>DataApi</code></a>
    class, which creates a copy of the assets on the local device before synchronizing with
    connected devices.</li>
    <li>Reliably send a file that is too large in size to send using the
    <a href="{@docRoot}reference/com/google/android/gms/wearable/MessageApi.html"><code>MessageApi</code></a>
    class.</li>
    <li>Transfer streamed data, such as music pulled from a network server or voice
    data from the microphone.</li>
  </ul>
  </dd>
</dl>

<p class="warning"><b>Warning:</b>
Because these APIs are designed for communication between handhelds and wearables,
these are the only APIs you should use to set up communication between these
devices. For instance, don't try to open low-level sockets to create a communication
channel.
</p>

<h2>Lessons</h2>
  <dl>
    <dt><a href="{@docRoot}training/wearables/data-layer/accessing.html">Accessing the Wearable Data Layer</a></dt>
    <dd>This lesson shows you how to create a client to access the Data Layer APIs.</dd>

    <dt><a href="{@docRoot}training/wearables/data-layer/data-items.html">Syncing Data Items</a></dt>
    <dd>Data items are objects that are stored in a replicated data store that is automatically
      synced from handhelds to wearables.</dd>

    <dt><a href="{@docRoot}training/wearables/data-layer/assets.html">Transferring Assets</a></dt>
      <dd>Assets are binary blobs of data that you typically use to transfer images or media.</dd>

    <dt><a href="{@docRoot}training/wearables/data-layer/messages.html">Sending and Receiving Messages</a></dt>
      <dd>Messages are designed for fire-and-forget messages that you can send back and forth
      between the wearable and handheld.</dd>

    <dt><a href="{@docRoot}training/wearables/data-layer/events.html">Handling Data Layer Events</a></dt>
      <dd>Be notified of changes and events to the data layer.</dd>
  </dl>