android_frameworks_base/docs/html/training/wearables/data-layer/data-items.jd

page.title=Syncing Data Items

@jd:body

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

<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#SyncData">Sync Data with a Data Map</a></li>
  <li><a href="#ListenEvents">Listen for Data Item Events</a></li>
</ol>

</div>
</div>

<p>
A <a href="@{docRoot}reference/com/google/android/gms/wearable/DataItem.html"><code>DataItem</code></a>
defines the data interface that the system uses to synchronize data between handhelds
and wearables. A <a href="@{docRoot}reference/com/google/android/gms/wearable/DataItem.html"><code>DataItem</code></a> generally
consists of the following items:</p>
<ul>
  <li><b>Payload</b> - A byte array, which you can set with whatever data you wish, allowing you
  to do your own object serialization and deserialization. The size of the payload is limited
  to 100KB.</li>
  <li><b>Path</b> - A unique string that must start with a forward slash (for instance,
  <code>"/path/to/data"</code>)</li>
</ul>

<p>
You normally don't implement <a href="@{docRoot}reference/com/google/android/gms/wearable/DataItem.html"><code>DataItem</code></a>
directly. Instead, you:

<ol>
  <li>Create a <a href="{@docRoot}reference/com/google/android/gms/wearable/PutDataRequest.html"><code>PutDataRequest</code></a> object,
  specifying a string path to uniquely identify the item.
  </li>
  <li>Call <a href="{@docRoot}reference/com/google/android/gms/wearable/PutDataRequest.html#setData(byte[])">setData()</a> to set
  the payload.
  </li>
  <li>Call <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>DataApi.putDataItem()</code></a> to request the system to create the data item.
  </li>
  <li>When requesting data items, the system returns objects
  that properly implement the <a href="{@docRoot}reference/com/google/android/gms/wearable/DataItem.html"><code>DataItem</code></a> interface.
  </li>
</ol>

<p>
However, instead of working with raw bytes using <a href="{@docRoot}reference/com/google/android/gms/wearable/PutDataRequest.html#setData(byte[])">setData()</a>,
we recommend you <a href="#data-map">use a data map</a>, which exposes
a data item in an easy-to-use {@link android.os.Bundle}-like interface.
</p>

<h2 id="SyncData">Sync Data with a Data Map</h2>
<p>
When possible, use the <a href="{@docRoot}reference/com/google/android/gms/wearable/DataMap.html"><code>DataMap</code></a> class,
which lets you work with data items in the form of an Android {@link android.os.Bundle},
so object serialization and de-serialization is done for you, and you can manipulate data with key-value pairs.
</p>

<p>To use a data map:</p>

<ol>
  <li>Create a
<a href="{@docRoot}reference/com/google/android/gms/wearable/PutDataMapRequest.html"><code>PutDataMapRequest</code></a>
object, setting the path of the data item.
<p class="note"><b>Note:</b> The path string is a unique identifier for the
data item that allows you to access it from either side of the connection. The path must begin
with a forward slash. If you're using hierarchical data in your
app, you should create a path scheme that matches the structure of the data. 
</p>
</li>
<li>Call
<a href="{@docRoot}reference/com/google/android/gms/wearable/PutDataMapRequest.html#getDataMap()"><code>PutDataMapRequest.getDataMap()</code></a>
</a> to obtain a data map that you can set values on.</li>
  <li>Set any desired values for the data map using the <code>put...()</code> methods, such as
  <a href="{@docRoot}reference/com/google/android/gms/wearable/DataMap.html#putString(java.lang.String, java.lang.String)"><code>putString()</code></a>.
  </li>
  <li>Call <a href="{@docRoot}reference/com/google/android/gms/wearable/PutDataMapRequest.html#asPutDataRequest()"><code>PutDataMapRequest.asPutDataRequest()</code></a>
  to obtain a <a href="{@docRoot}reference/com/google/android/gms/wearable/PutDataRequest.html"><code>PutDataRequest</code></a> object.
   </li>
  <li>Call <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>DataApi.putDataItem()</code></a> to request the system to create the data item.
  <p class="note"><b>Note:</b>
  If the handset and wearable devices are disconnected,
  the data is buffered and and synced when the connection is re-established.
  </p>
  </li>
</ol>

<p>The following example shows how to create a data map, set data on it, and create it:</p>

<pre>
PutDataMapRequest dataMap = PutDataMapRequest.create("/count");
dataMap.getDataMap().putInt(COUNT_KEY, count++);
PutDataRequest request = dataMap.asPutDataRequest();
PendingResult&lt;DataApi.DataItemResult&gt; pendingResult = Wearable.DataApi
        .putDataItem(mGoogleApiClient, request);
</pre>

<h2 id="ListenEvents">Listen for Data Item Events</h2>
If one side of the data layer connection changes a data item, you probably want
to be notified of any changes on the other side of the connection.
You can do this by implementing a listener for data item events.

<p>For example, here's what a typical callback looks like to carry out certain actions
when data changes.</p>

<pre>
&#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>
<p>
This is just a snippet that requires more implementation details. Learn about
how to implement a full listener service or activity in
<a href="{@docRoot}training/wearables/data-layer/events.html">Listening for Data Layer Events</a>.
</p>