Merge \"Docs: Watch face performance update\" into mnc-io-docs

<div id="tb">

<h2>This lesson teaches you to</h2>

<ol>

  <li><a href="#Basic">Basic Optimization</a></li>

  <li><a href="#Animations">Best Practices for Animations</a></li>

  <li><a href="#ReduceSize">Reduce the Size of Your Bitmap Assets</a></li>

  <li><a href="#CombineBitmaps">Combine Bitmap Assets</a></li>

  <li><a href="#AntiAlias">Disable Anti-Aliasing when Drawing Scaled Bitmaps</a></li>

  <li><a href="#OutDrawing">Move Expensive Operations Outside the Drawing Method</a></li>

  <li><a href="#SavePower">Follow Best Practices to Save Power</a></li>

</ol>

<h2>You should also read</h2>

<ul>

  <li><a href="{@docRoot}design/wear/watchfaces.html">Watch Faces for Android Wear</a></li>

  <li><a href="http://android-developers.blogspot.com/2014/12/making-performant-watch-face.html">

Making a performant watch face</a></li>

  <li><a href="http://android-developers.blogspot.com/2016/04/deprecation-of-bindlistener.html">

Deprecation of BIND_LISTENER with Android Wear APIs</a></li>

</ul>

</div>

</div>

<p>In addition to accommodating notification cards and system indicators, you need to ensure that

the animations in your watch face run smoothly and that your service does not perform unnecessary

computations. Watch faces in Android Wear run continuously on the device, so it is critical

that your watch face uses power efficiently.</p>

<p>This lesson has tips for conserving power and improving performance.

A watch face runs continuously, so it must use power

efficiently. </p>

<p>This lesson provides some tips to speed up your animations and to measure and conserve

power on the device.</p>

<p>Services must not perform unnecessary computations.

Watch faces with animations must run smoothly while accommodating

notification cards and system indicators.</p>

<h2 id="Basic">Basic Optimization</h2>

<p>This section contains best practices for improving efficiency during

periods when a watch face is inactive.</p>

<h3>Use callbacks in WatchFaceService.Engine</h3>

<p>Ensure that your watch face performs

computations only when active; use callbacks

in <a href="{@docRoot}reference/android/support/wearable/watchface/

WatchFaceService.Engine.html">{@code WatchFaceService.Engine}</a>.

Preferably, use the following methods of that class to determine if

the watch face is visible:</p>

<ul>

  <li>{@code onVisibilityChanged(boolean)}</li>

  <li>{@code isVisible()}</li>

</ul>

<p>Alternatively, use the following methods of the same class

(<a href="{@docRoot}reference/android/support/wearable/watchface/

WatchFaceService.Engine.html">{@code WatchFaceService.Engine}</a>):</p>

<ul>

  <li>{@code onCreate()}</li>

  <li>{@code onDestroy()}</li>

</ul>

<h3>Use listeners registered with the DataApi interface</h3>

<p>To listen for events, use live listeners that are registered

with <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}</a>.

For an example, see <a href="{@docRoot}training/wearables/data-layer/

data-items.html#ListenEvents">Syncing Data Items</a>.</p>

<p>Do not use <a href="https://developers.google.com/

android/reference/com/google/android/gms/wearable/

WearableListenerService">{@code WearableListenerService}</a> to listen for

events, because it is

called whether or not a watch face is active. For more information, see

<a href="http://android-developers.blogspot.com/2016/04/

deprecation-of-bindlistener.html">Deprecation of BIND_LISTENER

with Android Wear APIs</a>.</p>

<p>Do not register a broadcast receiver in the Android manifest file

to get system events such as time zone changes, battery events, etc., because

the <a href="{@docRoot}reference/android/content/BroadcastReceiver.html">{@code BroadcastReceiver}</a>

is called whether or not a watch face is active. However, you can use the

<a href="{@docRoot}reference/android/content/Context.html#registerReceiver(android.

content.BroadcastReceiver, android.content.IntentFilter)">{@code registerReceiver}</a> method

of the {@code Context} class to register a receiver.</p>

<h3>Monitor power consumption</h3>

<p>The <a href="https://play.google.com/store/apps/details?id=com.google.android.wearable.app&hl=en">

Android Wear companion app</a> enables developers and users to see how much battery

is consumed by different processes

on the wearable device (under <strong>Settings</strong> > <strong>Watch

battery</strong>).</p>

<p>For information about features introduced in Android 5.0 that help you improve battery life,

see <a href="{@docRoot}about/versions/android-5.0.html#Power">Project Volta</a>.</p>

<h2 id="Animations">Best Practices for Animations</h2>

<p>The best practices in this section help to reduce the power consumption of animations.</p>

<h3>Reduce the frame rate of animations</h3>

<p>Animations are often computationally expensive and consume a significant amount of power. Most

animations look fluid at 30 frames per second, so you should avoid running your animations

at a higher frame rate.</p>

<h3>Let the CPU sleep between animations</h3>

<p>Animations and small changes to the contents of the watch face wake up the CPU. Your watch

face should let the CPU sleep in between animations. For example, you can use short bursts of

animation every second in interactive mode and then let the CPU sleep until the next second.

Letting the CPU sleep often, even briefly, can significantly reduce power consumption.</p>

<p>To maximize battery life, use animations sparingly. Even a blinking colon wakes up the CPU with

every blink and hurts battery life.</p>

<h2 id="ReduceSize">Reduce the Size of Your Bitmap Assets</h2>

<p>Reducing the size of your bitmap assets as described in this section not only improves

the performance of your animations, but it also saves power.</p>

<h2 id="CombineBitmaps">Combine Bitmap Assets</h2>

<p>If you have bitmaps that are often drawn together, consider combining them into the same

graphic asset. You can often combine the background image in interactive mode with the tick

marks to avoid drawing two full-screen bitmaps every time the system redraws the watch face.</p>

<h2 id="AntiAlias">Disable Anti-Aliasing when Drawing Scaled Bitmaps</h2>

<p>When you draw a scaled bitmap on the {@link android.graphics.Canvas} object using the {@link

consistent across invocations. For more information, see

<a href="{@docRoot}tools/debugging/ddms.html">Using DDMS</a>.</p>

<h2 id="SavePower">Follow Best Practices to Save Power</h2>

<p>In addition to the techniques described in the previous sections, follow the best

practices in this section to reduce the power consumption of your watch face.</p>

<h3>Reduce the frame rate of animations</h3>

<p>Animations are often computationally expensive and consume a significant amount of power. Most

animations look fluid at 30 frames per second, so you should avoid running your animations

at a higher frame rate.</p>

<h3>Let the CPU sleep</h3>

<p>Animations and small changes to the contents of the watch face wake up the CPU. Your watch

face should let the CPU sleep in between animations. For example, you can use short bursts of

animation every second in interactive mode and then let the CPU sleep until the next second.

Letting the CPU sleep often, even briefly, can significantly reduce power consumption.</p>

<p>To maximize battery life, use animations sparingly. Even a blinking colon wakes up the CPU with

every blink and hurts battery life.</p>

<h3>Monitor power consumption</h3>

<p>The <a href="https://play.google.com/store/apps/details?id=com.google.android.wearable.app&hl=en">

Android Wear companion app</a> lets developers and users see how much battery different processes

on the wearable device are consuming under <strong>Settings</strong> > <strong>Watch

battery</strong>.</p>

<p>For more information about new features in Android 5.0 that help you improve battery life,

see <a href="{@docRoot}about/versions/android-5.0.html#Power">Project Volta</a>.</p>