1page.title=Sending and Syncing Data
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4<div id="tb-wrapper">
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7    <!-- Required platform, tools, add-ons, devices, knowledge, etc. -->
8    <h2>Dependencies and prerequisites</h2>
9    <ul>
10      <li>Android 4.3 (API Level 18) or higher on the handset device</li>
11      <li>The latest version of <a href="{@docRoot}google/play-services/index.html">Google Play services</a></li>
12      <li>An Android Wear device or Wear AVD</li>
13    </ul>
14  </div>
15</div>
16
17<p>
18The Wearable Data Layer API, which is part of Google Play services, provides a communication channel
19for your handheld and wearable apps. The API consists of a set of data objects that the system can
20send and synchronize over the wire and listeners that notify your apps of important events with
21the data layer:</p>
22
23<dl>
24  <dt><b>Data Items</b></dt>
25  <dd>A <a href="{@docRoot}reference/com/google/android/gms/wearable/DataItem.html"><code>DataItem</code></a>
26  provides data storage with automatic syncing between the handheld and
27  wearable.</dd>
28
29  <dt><b>Messages</b></dt>
30  <dd>The <a href="{@docRoot}reference/com/google/android/gms/wearable/MessageApi.html"><code>MessageApi</code></a> class
31  can send messages and is good for remote procedure calls (RPC), such as controlling a handheld's
32  media player from the wearable or starting an intent on the wearable from the handheld.
33  Messages are also great for one-way requests or for a request/response communication model.
34  If the handheld and wearable are connected, the system queues the message for delivery and
35  returns a successful result code. If the devices are not connected, an error is returned. A
36  successful result code does not indicate that the message was delivered successfully as the
37  devices may disconnect after receiving the result code.
38</p></dd>
39
40  <dt><b>Asset</b></dt>
41  <dd><a href="{@docRoot}reference/com/google/android/gms/wearable/Asset.html"><code>Asset</code></a> objects are for
42  sending binary blobs of data, such as images. You attach assets to data items and the system
43  automatically takes care of the transfer for you, conserving Bluetooth bandwidth by caching large assets
44  to avoid re-transmission.</dd>
45
46  <dt><b>WearableListenerService</b> (for services)</dt>
47  <dd><p>Extending <a href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html"><code>WearableListenerService</code></a>
48  lets you listen for important data layer events in a service. The system manages the lifecycle of
49  the <a href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html"><code>WearableListenerService</code></a>,
50  binding to the service when it needs to send data items or messages and unbinding the service when no work is needed.</p>
51  </dd>
52
53  <dt><b>DataListener</b> (for foreground activities)</dt>
54  <dd>
55  Implementing <a href="{@docRoot}reference/com/google/android/gms/wearable/DataApi.DataListener.html"><code>DataListener</code></a>
56  in an activity lets you listen for important data layer events when an activity
57  is in the foreground. Using this instead of the
58  <a href="{@docRoot}reference/com/google/android/gms/wearable/WearableListenerService.html"><code>WearableListenerService</code></a>
59  lets you listen for changes only when the user is actively using your app.
60  </dd>
61
62  <dt><b>Channel</b></dt>
63  <dd>
64  You can use the
65  <a href="{@docRoot}reference/com/google/android/gms/wearable/ChannelApi.html"><code>ChannelApi</code></a>
66  class to transfer large data items, such as music and movie files, from a handheld to a wearable
67  device. The Channel API for data transfer has the following benefits:
68  <ul>
69    <li>Transfer large data files between two or more connected devices, without
70    the automatic synchronization provided when using
71    <a href="{@docRoot}reference/com/google/android/gms/wearable/Asset.html"><code>Asset</code></a>
72    objects attached to
73    <a href="{@docRoot}reference/com/google/android/gms/wearable/DataItem.html"><code>DataItem</code></a>
74    objects. The Channel API saves disk space unlike the
75    <a href="{@docRoot}reference/com/google/android/gms/wearable/DataApi.html"><code>DataApi</code></a>
76    class, which creates a copy of the assets on the local device before synchronizing with
77    connected devices.</li>
78    <li>Reliably send a file that is too large in size to send using the
79    <a href="{@docRoot}reference/com/google/android/gms/wearable/MessageApi.html"><code>MessageApi</code></a>
80    class.</li>
81    <li>Transfer streamed data, such as music pulled from a network server or voice
82    data from the microphone.</li>
83  </ul>
84  </dd>
85</dl>
86
87<p class="warning"><b>Warning:</b>
88Because these APIs are designed for communication between handhelds and wearables,
89these are the only APIs you should use to set up communication between these
90devices. For instance, don't try to open low-level sockets to create a communication
91channel.
92</p>
93
94<p>Android Wear supports multiple wearables connected to a handheld device. For example, when the
95user saves a note on a handheld, it automatically appears on both of the user's Wear devices. To
96synchronize data between devices, Google’s servers host a cloud node in the network of devices. The
97system synchronizes data to directly connected devices, the cloud node, and to wearable devices
98connected to the cloud node via Wi-Fi.</p>
99
100<img src="{@docRoot}wear/images/wear_cloud_node.png" alt="" width="330" height="375"/>
101<p class="img-caption"><strong>Figure 1.</strong> A sample network of nodes with
102handheld and wearable devices.</p>
103
104
105<h2>Lessons</h2>
106  <dl>
107    <dt><a href="{@docRoot}training/wearables/data-layer/accessing.html">Accessing the Wearable Data Layer</a></dt>
108    <dd>This lesson shows you how to create a client to access the Data Layer APIs.</dd>
109
110    <dt><a href="{@docRoot}training/wearables/data-layer/data-items.html">Syncing Data Items</a></dt>
111    <dd>Data items are objects that are stored in a replicated data store that is automatically
112      synced from handhelds to wearables.</dd>
113
114    <dt><a href="{@docRoot}training/wearables/data-layer/assets.html">Transferring Assets</a></dt>
115      <dd>Assets are binary blobs of data that you typically use to transfer images or media.</dd>
116
117    <dt><a href="{@docRoot}training/wearables/data-layer/messages.html">Sending and Receiving Messages</a></dt>
118      <dd>Messages are designed for fire-and-forget messages that you can send back and forth
119      between the wearable and handheld.</dd>
120
121    <dt><a href="{@docRoot}training/wearables/data-layer/events.html">Handling Data Layer Events</a></dt>
122      <dd>Be notified of changes and events to the data layer.</dd>
123  </dl>
124