Android Layout Tricks #3: Optimize with stubs（Android 布局技巧3：使用stub优化）

[title2]30 March 2009

[/title2]

Android Layout Tricks #3: Optimize with stubs

http://android-developers.blogspot.com/2009/03/android-layout-tricks-3-optimize-with.html

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!

Posted byRomain Guyat2:00
PM
Labels:How-to,Optimization,User
Interface