Android Layout Tricks #4: Optimize, Part 2

Sharing and reusing layouts is very easy with Android thanks to the <include
/> tag, sometimes even too easy and you might end up with user interfaces that contain a large number of views, some of which are rarely used. Thankfully, Android offers a very special widget called ViewStub,
which brings you all the benefits of the 

<include />

 without polluting your user interface with
rarely used views.

A 

ViewStub

 is a dumb and lightweight view. It has
no dimension, it does not draw anything and does not participate in the layout in any way. This means a 

ViewStub

 is
very cheap to inflate and very cheap to keep in a view hierarchy. A 

ViewStub

 can be best described
as a lazy include. The layout referenced by a 

ViewStub

 is
inflated and added to the user interface only when you decide so.

The following screenshot comes from the Shelves application. The
main purpose of the activity shown in the screenshot is to present the user with a browsable list of books:



The same activity is also used when the user adds or imports new books. During such an operation, Shelves shows extra bits of
user interface. The screenshot below shows the progress bar and cancel button that appear at the bottom of the screen during an import:



Because
importing books is not a common operation, at least when compared to browsing the list of books, the import panel is originally represented by a 

ViewStub

:



When
the user initiates the import process, the 

ViewStub

 is
inflated and replaced by the content of the layout file it references:



To
use a 

ViewStub

 all
you need is to specify an 

android:id

 attribute,
to later inflate the stub, and an 

android:layout

 attribute,
to reference what layout file to include and inflate. A stub lets you use a third attribute, 

android:inflatedId

,
which can be used to override the id of the root of the included file. Finally,
the layout parameters specified on the stub will be applied to the roof of the included layout.
Here is an example:

<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" />
When you are ready to inflate the stub, simply invoke the inflate() method.
You can also simply change the visibility of the stub to VISIBLE or INVISIBLE and
the stub will inflate. Note however that the 

inflate()

 method
has the benefit of returning the root 

View

 of
the inflate layout:

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

It is very important to remember that after the stub is inflated, the stub is removed from the view hierarchy. As such, it is unnecessary
to keep a long-lived reference, for instance in an class instance field, to a 

ViewStub

.

A 

ViewStub

 is a great compromise between ease of programming and efficiency. Instead of inflating
views manually and adding them at runtime to your view hierarchy, simply use a 

ViewStub

. It’s
cheap and easy. The only drawback of 

ViewStub

 is that it currently does not support
the <merge />
tag.

Happy coding!

原文URL:http://www.curious-creature.com/2009/03/16/android-layout-tricks-4-optimize-part-2/