docs/html/training/wearables/watch-faces/service.jd

page.title=Building a Watch Face Service

@jd:body

<div id="tb-wrapper">
<div id="tb">
<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#CreateProject">Create and Configure Your Project</a></li>
  <li><a href="#CallbackMethods">Implement the Service Callback Methods</a></li>
  <li><a href="#RegisterService">Register the Service Implementation</a></li>
</ol>
<h2>Related Samples</h2>
<ul>
<li><a href="{@docRoot}samples/WatchFace/index.html">
WatchFace</a></li>
</ul>
<h2>You should also read</h2>
<ul>
  <li><a href="{@docRoot}design/wear/watchfaces.html">Watch Faces for Android Wear</a></li>
</ul>
</div>
</div>

<p>Watch faces in Android Wear are implemented as <a
href="{@docRoot}guide/components/services.html">services</a> and packaged inside a <a
href="{@docRoot}training/wearables/apps/index.html">wearable app</a>. When users install a
handheld app that contains a wearable app with watch faces, these watch faces become available
in the <a
href="https://play.google.com/store/apps/details?id=com.google.android.wearable.app&hl=en">Android
Wear companion app</a> on the handheld device and in the watch face picker on the wearable. When
users select one of the available watch faces, the wearable device shows the watch face and
invokes its service callback methods as required.</p>

<p>This lesson shows you how to configure an Android project to include watch faces and how
to implement the watch face service.</p>



<h2 id="CreateProject">Create and Configure Your Project</h2>

<p>To create an Android project for your watch face in Android Studio:</p>

<ol>
<li>Start Android Studio.</li>
<li>Create a new project:
  <ul>
  <li>If you don't have a project opened, in the <strong>Welcome</strong> screen, click
      <strong>New Project</strong>.</li>
  <li>If you have a project opened, from the <strong>File</strong> menu, select <strong>New
      Project</strong>.</li>
  </ul>
</li>
<li>Provide an application name and click <strong>Next</strong>.</li>
<li>Select the <strong>Phone and Tablet</strong> form factor.</li>
<li>Under <strong>Minimum SDK</strong>, choose API 18.</li>
<li>Select the <strong>Wear</strong> form factor.</li>
<li>Under <strong>Minimum SDK</strong>, choose API 21 and click <strong>Next</strong>.</li>
<li>Select <strong>Add No Activity</strong> and click <strong>Next</strong> in the two following
    screens.</li>
<li>Click <strong>Finish</strong>.</li>
<li>Click <strong>View</strong> > <strong>Tool Windows</strong> > <strong>Project</strong> in the
    IDE window.</li>
</ol>

<p>Android Studio creates a project with the <code>wear</code> and <code>mobile</code> modules.
For more information, see <a href="{@docRoot}sdk/installing/create-project.html">Creating a
Project</a>.</p>

<h3 id="Dependencies">Dependencies</h3>

<p>The
<a href="{@docRoot}reference/android/support/wearable/watchface/package-summary.html">Wearable Support Library</a>
provides the necessary classes that you extend to create watch
face implementations. The Google Play services client libraries (<code>play-services</code> and
<code>play-services-wearable</code>) are required to sync data items between the companion device
and the wearable with the <a href="{@docRoot}training/wearables/data-layer/index.html">Wearable
Data Layer API</a>.</p>

<p>Android Studio automatically adds the required entries in your <code>build.gradle</code>
files when you create the project in the instructions above.</p>

<h3 id="Reference">Wearable Support Library API Reference</h3>

<p>The reference documentation provides detailed information about the classes you use to
implement watch faces. Browse the
<a href="{@docRoot}reference/android/support/wearable/watchface/package-summary.html">API reference
documentation</a> for the Wearable Support Library.</p>

<h3 id="LibEclipse">Download the Wearable Support Library for Eclipse ADT</h3>

<p>If you are using the ADT plugin for Eclipse, download the
<a href="{@docRoot}shareables/training/wearable-support-lib.zip">Wearable Support Library</a> and
include it as a dependency in your project.</p>

<h3 id="Permissions">Declare Permissions</h3>

<p>Watch faces require the <code>PROVIDE_BACKGROUND</code> and <code>WAKE_LOCK</code> permissions.
Add the following permissions to the manifest files of both the wearable app and the mobile
app under the <code>manifest</code> element:</p>

<pre>
&lt;manifest ...>
    &lt;uses-permission
        android:name="com.google.android.permission.PROVIDE_BACKGROUND" />
    &lt;uses-permission
        android:name="android.permission.WAKE_LOCK" />
    ...
&lt;/manifest>
</pre>

<p class="caution"><strong>Caution:</strong> The handheld app must include all the permissions
declared in the wearable app.</p>



<h2 id="CallbackMethods">Implement the Service and Callback Methods</h2>

<p>Watch faces in Android Wear are implemented as
<a href="{@docRoot}guide/components/services.html">services</a>.
When a watch face is active, the system invokes the methods in its service when the time changes
or when an important event occurs (like switching to ambient mode or receiving a new
notification). The service implementation then draws the watch face on the screen using the
updated time and any other relevant data.</p>

<p>To implement a watch face, you extend the
<a href="{@docRoot}reference/android/support/wearable/watchface/CanvasWatchFaceService.html"><code>CanvasWatchFaceService</code></a>
and
<a href="{@docRoot}reference/android/support/wearable/watchface/CanvasWatchFaceService.Engine.html"><code>CanvasWatchFaceService.Engine</code></a>
classes, and then you override the callback methods in the
<a href="{@docRoot}reference/android/support/wearable/watchface/CanvasWatchFaceService.Engine.html"><code>CanvasWatchFaceService.Engine</code></a>
class. These classes are included in the
<a href="{@docRoot}reference/android/support/wearable/watchface/package-summary.html">Wearable Support Library</a>.
</p>

<p>The following snippet outlines the key methods you need to implement:</p>

<pre>
public class AnalogWatchFaceService extends CanvasWatchFaceService {

    &#64;Override
    public Engine onCreateEngine() {
        /* provide your watch face implementation */
        return new Engine();
    }

    /* implement service callback methods */
    private class Engine extends CanvasWatchFaceService.Engine {

        &#64;Override
        public void onCreate(SurfaceHolder holder) {
            super.onCreate(holder);
            /* initialize your watch face */
        }

        &#64;Override
        public void onPropertiesChanged(Bundle properties) {
            super.onPropertiesChanged(properties);
            /* get device features (burn-in, low-bit ambient) */
        }

        &#64;Override
        public void onTimeTick() {
            super.onTimeTick();
            /* the time changed */
        }

        &#64;Override
        public void onAmbientModeChanged(boolean inAmbientMode) {
            super.onAmbientModeChanged(inAmbientMode);
            /* the wearable switched between modes */
        }

        &#64;Override
        public void onDraw(Canvas canvas, Rect bounds) {
            /* draw your watch face */
        }

        &#64;Override
        public void onVisibilityChanged(boolean visible) {
            super.onVisibilityChanged(visible);
            /* the watch face became visible or invisible */
        }
    }
}
</pre>

<p>The
<a href="{@docRoot}reference/android/support/wearable/watchface/CanvasWatchFaceService.html"><code>CanvasWatchFaceService</code></a>
class provides an invalidate mechanism similar to
the {@link android.view.View#invalidate View.invalidate()} method. You can call the
<a href="{@docRoot}reference/android/support/wearable/watchface/CanvasWatchFaceService.Engine.html#invalidate()"><code>invalidate()</code></a>
method throughout your implementation when
you want the system to redraw the watch face. You can only use the
<a href="{@docRoot}reference/android/support/wearable/watchface/CanvasWatchFaceService.Engine.html#invalidate()"><code>invalidate()</code></a>
method in the main UI thread. To invalidate the canvas from another thread, call the
<a href="{@docRoot}reference/android/support/wearable/watchface/CanvasWatchFaceService.Engine.html#postInvalidate()"><code>postInvalidate()</code></a>
method.</p>

<p>For more information about implementing the methods in the
<a href="{@docRoot}reference/android/support/wearable/watchface/CanvasWatchFaceService.Engine.html"><code>CanvasWatchFaceService.Engine</code></a>
class, see <a
href="{@docRoot}training/wearables/watch-faces/drawing.html">Drawing Watch Faces</a>.</p>



<h2 id="RegisterService">Register the Watch Face Service</h2>

<p>After you implement the watch face service, you register the implementation in the manifest
file of the wearable app. When users install this app, the system uses the information about
the service to make the watch face available in the <a
href="https://play.google.com/store/apps/details?id=com.google.android.wearable.app&hl=en">Android
Wear companion app</a> and in the watch face picker on the wearable device.</p>

</p>The following snippet shows how to register a watch face implementation
under the <a href="{@docRoot}guide/topics/manifest/application-element.html">
<code>application</code></a> element:</p>

<pre>
&lt;service
    android:name=".AnalogWatchFaceService"
    android:label="&#64;string/analog_name"
    android:allowEmbedded="true"
    android:taskAffinity=""
    android:permission="android.permission.BIND_WALLPAPER" >
    &lt;meta-data
        android:name="android.service.wallpaper"
        android:resource="&#64;xml/watch_face" />
    &lt;meta-data
        android:name="com.google.android.wearable.watchface.preview"
        android:resource="&#64;drawable/preview_analog" />
    &lt;meta-data
        android:name="com.google.android.wearable.watchface.preview_circular"
        android:resource="&#64;drawable/preview_analog_circular" />
    &lt;intent-filter>
        &lt;action android:name="android.service.wallpaper.WallpaperService" />
        &lt;category
            android:name=
            "com.google.android.wearable.watchface.category.WATCH_FACE" />
    &lt;/intent-filter>
&lt;/service>
</pre>

<p>The <a
href="https://play.google.com/store/apps/details?id=com.google.android.wearable.app&hl=en">Android
Wear companion app</a> and the watch face picker on the wearable device use the preview image
defined by the <code>com.google.android.wearable.watchface.preview</code> metadata entry when
presenting users with all the watch faces installed on the device. To obtain this drawable,
run the watch face on your Android Wear device or in an emulator instance and <a
href="{@docRoot}sdk/installing/studio-debug.html#screenCap">take a screenshot</a>. On Android Wear
devices with hdpi screens, the preview image is typically 320x320 pixels in size.</p>

<p>Watch faces that look substantially different on round devices can provide both round and
square preview images. To specify a round preview image, use the
<code>com.google.android.wearable.watchface.preview_circular</code> metadata entry. If a watch
face includes both preview images, the companion app and the watch face picker on the wearable
show the appropriate one, depending on the shape of the watch. If a round preview image is not
included, the square preview image is used for both square and round devices. For round devices,
a square preview image is cropped using a circular shape.</p>

<p>The <code>android.service.wallpaper</code> metadata entry specifies the
<code>watch_face.xml</code> resource file, which contains a <code>wallpaper</code>
element:</p>

<pre>
&lt;?xml version="1.0" encoding="UTF-8"?>
&lt;wallpaper xmlns:android="http://schemas.android.com/apk/res/android" />
</pre>

<p>Your wearable app can contain more than one watch face. You must add a service entry to the
manifest file of the wearable app for each of your watch face implementations.</p>