docs/html/training/improving-layouts/optimizing-layout.jd

page.title=Optimizing Layout Hierarchies
parent.title=Improving Layout Performance
parent.link=index.html

trainingnavtop=true
next.title=Re-using Layouts with <include/>
next.link=reusing-layouts.html

@jd:body



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

<!-- table of contents -->
<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#Inspect">Inspect Your Layout</a></li>
  <li><a href="#Revise">Revise Your Layout</a></li>
  <li><a href="#Layoutopt">Use Layoutopt</a></li>
</ol>

<!-- other docs (NOT javadocs) -->
<h2>You should also read</h2>
<ul>
  <li><a href="{@docRoot}guide/topics/ui/declaring-layout.html">XML Layouts</a></li>
  <li><a
href="{@docRoot}guide/topics/resources/layout-resource.html#include- element">Layout
Resource</a></li>
</ul>

</div>
</div>


<p>It is a common misconception that using the basic layout structures leads to the most efficient
layouts. However, each widget and layout you add to your application requires initialization,
layout, and drawing. For example, using nested instances of {@link android.widget.LinearLayout} can
lead to an excessively deep view hierarchy. Furthermore, nesting several instances of {@link
android.widget.LinearLayout} that use the {@code layout_weight} parameter can be especially
expensive as each child needs to be measured twice. This is particularly important when the layout
is inflated repeatedly, such as when used in a {@link android.widget.ListView} or {@link
android.widget.GridView}.</p>

<p>In this lesson you'll learn to use <a
href="{@docRoot}guide/developing/tools/hierarchy-viewer.html">Heirachy Viewer</a> and <a
href="{@docRoot}guide/developing/tools/layoutopt.html">Layoutopt</a> to examine and optimize your
layout.</p>



<h2 id="Inspect">Inspect Your Layout</h2>

<p>The Android SDK tools include a tool called <a
href="{@docRoot}guide/developing/tools/hierarchy-viewer.html">Heirachy Viewer</a> that allows
you to analyze your layout while your application is running. Using this tool helps you discover
bottlenecks in the layout performance.</p>

<p>Hierarchy Viewer works by allowing you to select running processes on a connected device or
emulator, then display the layout tree. The traffic lights on each block represent its Measure,
Layout and Draw performance, helping you identify potential issues.</p>

<p>For example, figure 1 shows a layout that's used as an item in a {@link
android.widget.ListView}. This layout shows a small bitmap image on the left and two stacked items
of text on the right. It is especially important that layouts that will be inflated multiple
times&mdash;such as this one&mdash;are optimized as the performance
benefits will be multiplied.</p>

<img src="{@docRoot}images/training/layout-listitem.png" alt="" />
<p class="img-caption"><strong>Figure 1.</strong> Conceptual layout for an item in a {@link
android.widget.ListView}.</p>

<p>The {@code hierarchyviewer} tool is available in  {@code &lt;sdk&gt;/tools/}. When opened,
the Hierarchy Viewer shows a list of available devices and its running components. Click
<strong>Load View Hierarchy</strong> to view the layout hierarchy of the selected component. For
example, figure 2 shows the layout for the list item illustrated by figure 1.</p>

<div style="float:left;width:455px">
<img src="{@docRoot}images/training/hierarchy-linearlayout.png" alt="" />
<p class="img-caption"><strong>Figure 2.</strong> Layout hierarchy for the layout in figure 1,
using nested instances of {@link android.widget.LinearLayout}.</p>
</div>

<div style="float:left;width:155px;margin-left:2em">
<img src="{@docRoot}images/training/hierarchy-layouttimes.png" alt="" />
<p class="img-caption"><strong>Figure 3.</strong> Clicking a hierarchy node shows its
performance times.</p>
</div>

<p style="clear:left">In figure 2, you can see there is a 3-level hierarchy with some problems
laying out the text items. Clicking on the items shows the time taken for each stage of the process
(figure 3). It becomes clear which items are taking the longest to measure, layout, and render, and
where you should spend time optimizing.</p>

<p>The timings for rendering a complete list item using this layout are:</p>
<ul>
  <li>Measure: 0.977ms</li>
  <li>Layout: 0.167ms</li>
  <li>Draw: 2.717ms</li>
</ul>


<h2 id="Revise">Revise Your Layout</h2>

<p>Because the layout performance above slows down due to a nested {@link
android.widget.LinearLayout}, the performance might improve by flattening the layout&mdash;make
the layout shallow and wide, rather than narrow and deep. A {@link android.widget.RelativeLayout} as
the root node allows for such layouts. So, when this design is converted to use {@link
android.widget.RelativeLayout}, you can see that the layout becomes a 2-level hierarchy. Inspection
of the new layout looks like this:</p>

<img src="{@docRoot}images/training/hierarchy-relativelayout.png" alt="" />
<p class="img-caption"><strong>Figure 4.</strong> Layout hierarchy for the layout in figure 1,
using {@link android.widget.RelativeLayout}.</p>

<p>Now rendering a list item takes:</p>
<ul>
  <li>Measure: 0.598ms</li>
  <li>Layout: 0.110ms</li>
  <li>Draw: 2.146ms</li>
</ul>

<p>Might seem like a small improvement, but this time is multiplied several times because this
layout is used for every item in a list.</p>

<p>Most of this time difference is due to the use of {@code layout_weight} in the {@link
android.widget.LinearLayout} design, which can slow down the speed of measurement. It is just one
example of how each layout has appropriate uses and you should carefully consider whether using 
layout weight is necessary.</p>


<h2 id="Layoutopt">Use Layoutopt</h2>

<p>It is always good practice to also run the <a
href="{@docRoot}guide/developing/tools/layoutopt.html">layoutopt</a> tool on your final layout files
to search for places in your view hierarchy that may be optimized. Layoutopt is also in your SDK
{@code tools/} directory and takes a layout directory name or a space-separated list of layout files
that you'd like to inspect.</p>

<p>When you run {@code layoutopt} on a layout file, it prints a line number for each issue found, a
description of the issue, and for some types of issues it also suggests a resolution. For
example:</p>

<pre class="no-pretty-print classic">
$ layoutopt samples/
samples/compound.xml
   7:23 The root-level &lt;FrameLayout/&gt; can be replaced with &lt;merge/&gt;
   11:21 This LinearLayout layout or its FrameLayout parent is useless
samples/simple.xml
   7:7 The root-level &lt;FrameLayout/&gt; can be replaced with &lt;merge/&gt;
</pre>

<p>After you apply the suggested layout optimizations, run Hierarchy Viewer again to inspect the
performance changes.</p>