docs/html/training/improving-layouts/loading-ondemand.jd

page.title=Loading Views On Demand
parent.title=Improving Layout Performance
parent.link=index.html

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previous.title=Re-using Layouts with <include/>
previous.link=reusing-layouts.html
next.title=Making ListView Scrolling Smooth
next.link=smooth-scrolling.html

@jd:body


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<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#ViewStub">Define a ViewStub</a></li>
  <li><a href="#Load">Load the ViewStub Layout</a></li>
</ol>

<!-- other docs (NOT javadocs) -->
<h2>You should also read</h2>
<ul>
  <li><a href="http://android-developers.blogspot.com/2009/03/android-layout-tricks-3-optimize-with.html"
         >Optimize with stubs (blog post)</a></li>
</ul>

</div>
</div>


<p>Sometimes your layout might require complex views that are rarely used. Whether
they are item details, progress indicators, or undo messages, you can reduce memory usage and speed
up rendering by loading the views only when they are needed.</p>


<h2 id="ViewStub">Define a ViewStub</h2>

<p>{@link android.view.ViewStub} is a lightweight view with no dimension and doesn’t draw anything
or participate in the layout. As such, it's cheap to inflate and cheap to leave in a view hierarchy.
Each {@link android.view.ViewStub} simply needs to include the {@code android:layout} attribute to
specify the layout to inflate.</p>

<p>The following {@link android.view.ViewStub} is for a translucent progress bar overlay. It should
be visible only when new items are being imported into the application.</p>

<pre>
&lt;ViewStub
    android:id="@+id/stub_import"
    android:inflatedId="@+id/panel_import"
    android:layout="@layout/progress_overlay"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:layout_gravity="bottom" /&gt;
</pre>


<h2 id="Load">Load the ViewStub Layout</h2>

<p>When you want to load the layout specified by the {@link android.view.ViewStub}, either set it
visible by calling {@link android.view.View#setVisibility setVisibility(View.VISIBLE)} or call
{@link android.view.ViewStub#inflate()}.</p>

<pre>
((ViewStub) findViewById(R.id.stub_import)).setVisibility(View.VISIBLE);
// or
View importPanel = ((ViewStub) findViewById(R.id.stub_import)).inflate();
</pre>

<p class="note"><strong>Note:</strong> The {@link android.view.ViewStub#inflate()} method returns
the inflated {@link android.view.View} once complete. so you don't need to call {@link
android.app.Activity#findViewById findViewById()} if you need to interact with the layout.</p>

<p>Once visible/inflated, the {@link android.view.ViewStub} element is no longer part of the view
hierarchy. It is replaced by the inflated layout and the ID for the root view of that layout is
the one specified by the {@code android:inflatedId} attribute of the ViewStub. (The ID {@code
android:id} specified for the {@link android.view.ViewStub} is valid only until the {@link
android.view.ViewStub} layout is visible/inflated.)</p>

<p class="note"><strong>Note:</strong> One drawback of {@link android.view.ViewStub} is that it
doesn’t currently support the {@code &lt;merge/&gt;} tag in the layouts to be inflated.</p>