1page.title=Loading Views On Demand
2parent.title=Improving Layout Performance
3parent.link=index.html
4
5trainingnavtop=true
6previous.title=Re-using Layouts with <include/>
7previous.link=reusing-layouts.html
8next.title=Making ListView Scrolling Smooth
9next.link=smooth-scrolling.html
10
11@jd:body
12
13
14<div id="tb-wrapper">
15<div id="tb">
16
17<!-- table of contents -->
18<h2>This lesson teaches you to</h2>
19<ol>
20  <li><a href="#ViewStub">Define a ViewStub</a></li>
21  <li><a href="#Load">Load the ViewStub Layout</a></li>
22</ol>
23
24<!-- other docs (NOT javadocs) -->
25<h2>You should also read</h2>
26<ul>
27  <li><a href="http://android-developers.blogspot.com/2009/03/android-layout-tricks-3-optimize-with.html"
28         >Optimize with stubs (blog post)</a></li>
29</ul>
30
31</div>
32</div>
33
34
35<p>Sometimes your layout might require complex views that are rarely used. Whether
36they are item details, progress indicators, or undo messages, you can reduce memory usage and speed
37up rendering by loading the views only when they are needed.</p>
38
39
40<h2 id="ViewStub">Define a ViewStub</h2>
41
42<p>{@link android.view.ViewStub} is a lightweight view with no dimension and doesn’t draw anything
43or participate in the layout. As such, it's cheap to inflate and cheap to leave in a view hierarchy.
44Each {@link android.view.ViewStub} simply needs to include the {@code android:layout} attribute to
45specify the layout to inflate.</p>
46
47<p>The following {@link android.view.ViewStub} is for a translucent progress bar overlay. It should
48be visible only when new items are being imported into the application.</p>
49
50<pre>
51&lt;ViewStub
52    android:id="@+id/stub_import"
53    android:inflatedId="@+id/panel_import"
54    android:layout="@layout/progress_overlay"
55    android:layout_width="fill_parent"
56    android:layout_height="wrap_content"
57    android:layout_gravity="bottom" /&gt;
58</pre>
59
60
61<h2 id="Load">Load the ViewStub Layout</h2>
62
63<p>When you want to load the layout specified by the {@link android.view.ViewStub}, either set it
64visible by calling {@link android.view.View#setVisibility setVisibility(View.VISIBLE)} or call
65{@link android.view.ViewStub#inflate()}.</p>
66
67<pre>
68((ViewStub) findViewById(R.id.stub_import)).setVisibility(View.VISIBLE);
69// or
70View importPanel = ((ViewStub) findViewById(R.id.stub_import)).inflate();
71</pre>
72
73<p class="note"><strong>Note:</strong> The {@link android.view.ViewStub#inflate()} method returns
74the inflated {@link android.view.View} once complete. so you don't need to call {@link
75android.app.Activity#findViewById findViewById()} if you need to interact with the layout.</p>
76
77<p>Once visible/inflated, the {@link android.view.ViewStub} element is no longer part of the view
78hierarchy. It is replaced by the inflated layout and the ID for the root view of that layout is
79the one specified by the {@code android:inflatedId} attribute of the ViewStub. (The ID {@code
80android:id} specified for the {@link android.view.ViewStub} is valid only until the {@link
81android.view.ViewStub} layout is visible/inflated.)</p>
82
83<p class="note"><strong>Note:</strong> One drawback of {@link android.view.ViewStub} is that it
84doesn’t currently support the {@code <merge>} tag in the layouts to be inflated.</p>
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