1page.title=Layout Tricks: Using ViewStubs 2parent.title=Articles 3parent.link=../browser.html?tag=article 4@jd:body 5 6<p>Sharing and reusing UI components is very easy with Android, thanks to the <a 7href="layout-tricks-reuse.html"><include /></a> tag. Sometimes it's so 8easy to create complex UI constructs that your UI ends up with a large number of 9views, some of which are rarely used. Thankfully, Android offers a very special 10widget called {@link android.view.ViewStub}, which brings you all the benefits 11of the <code><include /></code> without polluting your user interface with 12rarely used views.</p> 13 14<p>A <code>ViewStub</code> is a dumb and lightweight view. It has no dimension, 15it does not draw anything and does not participate in the layout in any way. 16This means that a <code>ViewStub</code> is very cheap to inflate and very cheap 17to keep in a view hierarchy. A <code>ViewStub</code> can be best described as a 18<em>lazy include</em>. The layout referenced by a <code>ViewStub</code> is 19inflated and added to the user interface only when you decide so.</p> 20 21<p>The following screenshot comes from the <a 22href="http://code.google.com/p/shelves">Shelves</a> application. The main purpose of 23the activity shown in the screenshot is to present the user with a browsable 24list of books:</p> 25 26<img style="margin: 0px auto 10px; display: block; text-align: center;" src="images/viewstub1.png" alt="" id="BLOGGER_PHOTO_ID_5314039375336055346" border="0"> 27 28<p>The same activity is also used when the user adds or imports new books. 29During such an operation, Shelves shows extra bits of user interface. 30The screenshot below shows the progress bar and cancel button that 31appear at the bottom of the screen during an import:</p> 32 33<img style="margin: 0px auto 10px; display: block; text-align: center;" src="images/viewstub2.png" alt="" id="BLOGGER_PHOTO_ID_5314039800002559378" border="0"> 34 35<p>Because importing books is not a common operation, at least when compared to 36browsing the list of books, the import panel is originally represented 37by a <code>ViewStub</code>:</p> 38 39<img style="margin: 0px auto 10px; display: block; text-align: center;" src="images/viewstub3.png" alt="" id="BLOGGER_PHOTO_ID_5314040334008167378" border="0"> 40 41<p>When the user initiates the import process, the <code>ViewStub</code> is 42inflated and replaced by the content of the layout file it references:</p> 43 44<img style="margin: 0px auto 10px; display: block; text-align: center;" src="images/viewstub4.png" alt="" id="BLOGGER_PHOTO_ID_5314040638747834450" border="0"> 45 46<p>To use a <code>ViewStub</code>, all you need is to specify an 47<code>android:id</code> attribute, to later inflate the stub, and an 48<code>android:layout</code> attribute, to reference what layout file 49to include and inflate. A stub lets you use a third attribute, 50<code>android:inflatedId</code>, which can be used to override the 51<code>id</code> of the root of the included file. Finally, the layout 52parameters specified on the stub will be applied to the roof of the 53included layout. Here is an example:</p> 54 55<pre class="prettyprint"><ViewStub 56 android:id="@+id/stub_import" 57 android:inflatedId="@+id/panel_import" 58 59 android:layout="@layout/progress_overlay" 60 61 android:layout_width="fill_parent" 62 android:layout_height="wrap_content" 63 android:layout_gravity="bottom" /></pre> 64 65<p>When you are ready to inflate the stub, simply invoke the 66{@link android.view.ViewStub#inflate()} method. You can also simply change the 67visibility of the stub to {@link android.view.View#VISIBLE} or 68{@link android.view.View#INVISIBLE} and the stub will inflate. Note however that the 69<code>inflate()</code> method has the benefit of returning the root 70<code>View</code> of the inflate layout:</p> 71 72<pre class="prettyprint">((ViewStub) findViewById(R.id.stub_import)).setVisibility(View.VISIBLE); 73// or 74View importPanel = ((ViewStub) findViewById(R.id.stub_import)).inflate();</pre> 75 76<p>It is very important to remember that after the stub is inflated, the stub is 77<em>removed</em> from the view hierarchy. As such, it is unnecessary to keep a 78long-lived reference, for instance in an class instance field, to a 79<code>ViewStub</code>.</p> 80 81<p>A <code>ViewStub</code> is a great compromise between ease of programming and 82efficiency. Instead of inflating views manually and adding them at runtime to 83your view hierarchy, simply use a <code>ViewStub</code>. It's cheap and easy. 84The only drawback of <code>ViewStub</code> is that it currently does 85<em>not</em> support the <a href="layout-tricks-merge.html"><merge /> 86tag</a>.</p> 87
