1page.title=Optimizing Layout Hierarchies
2parent.title=Improving Layout Performance
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16<!-- table of contents -->
17<h2>This lesson teaches you to</h2>
18<ol>
19  <li><a href="#Inspect">Inspect Your Layout</a></li>
20  <li><a href="#Revise">Revise Your Layout</a></li>
21  <li><a href="#Lint">Use Lint</a></li>
22</ol>
23
24<!-- other docs (NOT javadocs) -->
25<h2>You should also read</h2>
26<ul>
27  <li><a href="{@docRoot}guide/topics/ui/declaring-layout.html">XML Layouts</a></li>
28  <li><a
29href="{@docRoot}guide/topics/resources/layout-resource.html#include- element">Layout
30Resource</a></li>
31</ul>
32
33</div>
34</div>
35
36
37<p>It is a common misconception that using the basic layout structures leads to the most efficient
38layouts. However, each widget and layout you add to your application requires initialization,
39layout, and drawing. For example, using nested instances of {@link android.widget.LinearLayout} can
40lead to an excessively deep view hierarchy. Furthermore, nesting several instances of {@link
41android.widget.LinearLayout} that use the {@code layout_weight} parameter can be especially
42expensive as each child needs to be measured twice. This is particularly important when the layout
43is inflated repeatedly, such as when used in a {@link android.widget.ListView} or {@link
44android.widget.GridView}.</p>
45
46<p>In this lesson you'll learn to use <a
47href="{@docRoot}tools/help/hierarchy-viewer.html">Hierarchy Viewer</a> and <a
48href="{@docRoot}tools/help/layoutopt.html">Layoutopt</a> to examine and optimize your
49layout.</p>
50
51
52
53<h2 id="Inspect">Inspect Your Layout</h2>
54
55<p>The Android SDK tools include a tool called <a
56href="{@docRoot}tools/help/hierarchy-viewer.html">Hierarchy Viewer</a> that allows
57you to analyze your layout while your application is running. Using this tool helps you discover
58bottlenecks in the layout performance.</p>
59
60<p>Hierarchy Viewer works by allowing you to select running processes on a connected device or
61emulator, then display the layout tree. The traffic lights on each block represent its Measure,
62Layout and Draw performance, helping you identify potential issues.</p>
63
64<p>For example, figure 1 shows a layout that's used as an item in a {@link
65android.widget.ListView}. This layout shows a small bitmap image on the left and two stacked items
66of text on the right. It is especially important that layouts that will be inflated multiple
67times&mdash;such as this one&mdash;are optimized as the performance
68benefits will be multiplied.</p>
69
70<img src="{@docRoot}images/training/layout-listitem.png" alt="" />
71<p class="img-caption"><strong>Figure 1.</strong> Conceptual layout for an item in a {@link
72android.widget.ListView}.</p>
73
74<p>The {@code hierarchyviewer} tool is available in  {@code &lt;sdk&gt;/tools/}. When opened,
75the Hierarchy Viewer shows a list of available devices and its running components. Click
76<strong>Load View Hierarchy</strong> to view the layout hierarchy of the selected component. For
77example, figure 2 shows the layout for the list item illustrated by figure 1.</p>
78
79<div style="float:left;width:455px">
80<img src="{@docRoot}images/training/hierarchy-linearlayout.png" alt="" />
81<p class="img-caption"><strong>Figure 2.</strong> Layout hierarchy for the layout in figure 1,
82using nested instances of {@link android.widget.LinearLayout}.</p>
83</div>
84
85<div style="float:left;width:155px;margin-left:2em">
86<img src="{@docRoot}images/training/hierarchy-layouttimes.png" alt="" />
87<p class="img-caption"><strong>Figure 3.</strong> Clicking a hierarchy node shows its
88performance times.</p>
89</div>
90
91<p style="clear:left">In figure 2, you can see there is a 3-level hierarchy with some problems
92laying out the text items. Clicking on the items shows the time taken for each stage of the process
93(figure 3). It becomes clear which items are taking the longest to measure, layout, and render, and
94where you should spend time optimizing.</p>
95
96<p>The timings for rendering a complete list item using this layout are:</p>
97<ul>
98  <li>Measure: 0.977ms</li>
99  <li>Layout: 0.167ms</li>
100  <li>Draw: 2.717ms</li>
101</ul>
102
103
104<h2 id="Revise">Revise Your Layout</h2>
105
106<p>Because the layout performance above slows down due to a nested {@link
107android.widget.LinearLayout}, the performance might improve by flattening the layout&mdash;make
108the layout shallow and wide, rather than narrow and deep. A {@link android.widget.RelativeLayout} as
109the root node allows for such layouts. So, when this design is converted to use {@link
110android.widget.RelativeLayout}, you can see that the layout becomes a 2-level hierarchy. Inspection
111of the new layout looks like this:</p>
112
113<img src="{@docRoot}images/training/hierarchy-relativelayout.png" alt="" />
114<p class="img-caption"><strong>Figure 4.</strong> Layout hierarchy for the layout in figure 1,
115using {@link android.widget.RelativeLayout}.</p>
116
117<p>Now rendering a list item takes:</p>
118<ul>
119  <li>Measure: 0.598ms</li>
120  <li>Layout: 0.110ms</li>
121  <li>Draw: 2.146ms</li>
122</ul>
123
124<p>Might seem like a small improvement, but this time is multiplied several times because this
125layout is used for every item in a list.</p>
126
127<p>Most of this time difference is due to the use of {@code layout_weight} in the {@link
128android.widget.LinearLayout} design, which can slow down the speed of measurement. It is just one
129example of how each layout has appropriate uses and you should carefully consider whether using
130layout weight is necessary.</p>
131
132
133<h2 id="Lint">Use Lint</h2>
134
135<p>It is always good practice to run the <a href="{@docRoot}tools/help/lint.html">lint</a>
136tool on your layout files to search for possible view hierarchy optimizations. Lint has replaced
137the Layoutopt tool and has much greater functionality. Some examples of lint <a
138href="http://tools.android.com/tips/lint-checks">rules</a> are:</p>
139
140<ul>
141<li>Use compound drawables - A {@link android.widget.LinearLayout} which contains an {@link android.widget.ImageView} and a {@link android.widget.TextView} can be more efficiently handled as a compound drawable.</li>
142<li>Merge root frame - If a {@link android.widget.FrameLayout} is the root of a layout and does not provide background or padding etc, it can be replaced with a merge tag which is slightly more efficient.</li>
143<li>Useless leaf - A layout that has no children or no background can often be removed (since it is invisible) for a flatter and more efficient layout hierarchy.</li>
144<li>Useless parent - A layout with children that has no siblings, is not a {@link android.widget.ScrollView} or a root layout, and does not have a background, can be removed and have its children moved directly into the parent for a flatter and more efficient layout hierarchy.</li>
145<li>Deep layouts - Layouts with too much nesting are bad for performance. Consider using flatter layouts such as {@link android.widget.RelativeLayout} or {@link android.widget.GridLayout} to improve performance. The default maximum depth is 10.</li>
146</ul>
147
148<p>Another benefit of Lint is that it is integrated into Android Studio. Lint automatically runs
149whenever you compile your program. With Android Studio, you can also run lint inspections for a
150specific build variant, or for all build variants. </p>
151
152<p>You can also manage inspection profiles and configure inspections within Android Studio with the
153<strong>File&gt;Settings&gt;Project Settings</strong> option. The Inspection Configuration page
154appears with the supported inspections.</p>
155<p><img src="{@docRoot}images/tools/studio-inspections-config.png" alt="" /> </p>
156<p class="img-caption"><strong>Figure 5.</strong> Inspection Configuration</p>
157
158<p>Lint has the ability to automatically fix some issues, provide suggestions for others and jump
159directly to the offending code for review.</p>
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