page.title=Optimizing Performance and Battery Life

@jd:body

<div id="tb-wrapper">
<div id="tb">
<h2>This lesson teaches you to</h2>
<ol>
  <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>
</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 provides some tips to speed up your animations and to measure and conserve
power on the device.</p>


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

<p>Many watch faces consist of a background image and other graphic assets that are transformed
and overlapped on top of the background image, such as clock hands and other elements of the design
that move over time. Typically these graphic elements are rotated (and sometimes scaled) inside the
<code>Engine.onDraw()</code> method every time the system redraws the watch face, as described in
<a href="{@docRoot}training/wearables/watch-faces/drawing.html#Drawing">Draw Your Watch
Face</a>.</p>

<p>The larger these graphic assets are, the more computationally expensive it is to transform them.
Transforming large graphic assets in the <code>Engine.onDraw()</code> method drastically reduces
the frame rate at which the system can run your animations.</p>

<div id="fig1" style="float:right;width:250px;margin-left:25px">
<img src="{@docRoot}training/wearables/watch-faces/images/ClockHandFull.png" alt=""
     width="180" height="180"/>
<img src="{@docRoot}training/wearables/watch-faces/images/ClockHandCropped.png" alt=""
     width="15" height="65" style="margin-left:25px"/>
<p class="img-caption">
<strong>Figure 1.</strong> Clock hands can be cropped to remove extra pixels.</p>
</div>

<p>To improve the performance of your watch face:</p>

<ul>
<li>Do not use graphic elements that are larger than you need.</li>
<li>Remove extra transparent pixels around the edges.</li>
</ul>

<p>The example clock hand on the left side of <a href="#fig1">Figure 1</a> can be reduced in size
by 97&#37;.</p>

<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
android.graphics.Canvas#drawBitmap(android.graphics.Bitmap, float, float, android.graphics.Paint)
Canvas.drawBitmap()} method, you can provide a {@link android.graphics.Paint} instance to configure
several options. To improve performance, disable anti-aliasing using the {@link
android.graphics.Paint#setAntiAlias setAntiAlias()} method, since this option does not have any
effect on bitmaps.</p>

<div id="fig2" style="float:right;width:250px;margin-left:40px;margin-top:12px">
<img src="{@docRoot}training/wearables/watch-faces/images/BitmapFilterDisabled.png" alt=""
     width="70" height="70" style="margin-left:20px"/>
<img src="{@docRoot}training/wearables/watch-faces/images/BitmapFilterEnabled.png" alt=""
     width="70" height="70" style="margin-left:20px"/>
<p class="img-caption"><strong>Figure 2.</strong> Example of bitmap filtering disabled (left) and
enabled (right).</p>
</div>

<h3 id="BitmapFiltering">Use bitmap filtering</h3>

<p>For bitmap assets that you draw on top of other elements, enable bitmap filtering on the same
{@link android.graphics.Paint} instance using the {@link android.graphics.Paint#setFilterBitmap
setFilterBitmap()} method. <a href="#fig2">Figure 2</a> shows a magnified view of a clock hand with
and without bitmap filtering.</p>


<h2 id="OutDrawing">Move Expensive Operations Outside the Drawing Method</h2>

<p>The system calls the <code>Engine.onDraw()</code> method every time it redraws your watch
face, so you should only include operations that are strictly required to update the watch
face inside this method to improve performance.<p>

<p>When possible, avoid performing these operations inside the <code>onDraw()</code> method:</p>

<ul>
<li>Loading images and other resources.</li>
<li>Resizing images.</li>
<li>Allocating objects.</li>
<li>Performing computations whose result does not change between frames.</li>
</ul>

<p>You can usually perform these operations in the <code>Engine.onCreate()</code> method instead.
You can resize images ahead of time in the {@link
android.service.wallpaper.WallpaperService.Engine#onSurfaceChanged(android.view.SurfaceHolder, int, int, int)
Engine.onSurfaceChanged()} method, which provides you with the size of the canvas.</p>

<p>To analyze the performance of your watch face, use the Android Device Monitor. In particular,
ensure that the execution time for your <code>Engine.onDraw()</code> implementation is short and
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>