Android Layout Trick #2: Include to Reuse

Android comes with a wide variety of widgets, small visual construction blocks you can glue together to present the users with complex and
useful interfaces. However applications often need higher level visual components. A component can be seen as a complex widget made of
several simple stock widgets. You could for instance reuse a panel containing a progress bar and a cancel button, a panel containing two buttons (positive and negative actions), a panel with an icon, a title and
a description, etc. Creating new components can be done easily by writing a custom

View

but
it can be done even more easily using only XML.

In Android XML layout files, each tag is mapped to an actual class instance (the class is always a subclass of View.)
The UI toolkit lets you also use three special tags that are not mapped to a

View

instance:

<requestFocus
/>

,

<merge />

and

<include
/>

. The latter,

<include />

, can be used to create pure XML visual components.
(Note: I will present the

<merge />

tag in the next installment of Android
Layout Tricks.)

The

<include />

does exactly what its name suggests; it includes another XML layout. Using this
tag is straightforward as shown in the following example, taken straight from the
source code of the Home application that currently ships with Android:

<com.android.launcher.Workspace
android:id="@+id/workspace"
android:layout_width="fill_parent"
android:layout_height="fill_parent"

launcher:defaultScreen="1">

<include android:id="@+id/cell1" layout="@layout/workspace_screen" />
<include android:id="@+id/cell2" layout="@layout/workspace_screen" />
<include android:id="@+id/cell3" layout="@layout/workspace_screen" />

</com.android.launcher.Workspace>

In the

<include
/>

only the

layout

attribute
is required. This attribute, without the

android

namespace
prefix, is a reference to the layout file you wish to include. In this example, the same layout is included three times in a row.
This tag also lets you override a few attributes of the included layout.
The above example shows that you can use

android:id

to
specify the id of the root view of the included layout; it will also override the id of the included layout if one is defined. Similarly, you can override all the layout parameters. This means that any

android:layout_*

attribute
can be used with the

<include />

tag.
Here is an example:

<include android:layout_width="fill_parent" layout="@layout/image_holder" />
<include android:layout_width="256dip" layout="@layout/image_holder" />

This tag is particularly useful when you need to customize only part of your UI depending on the device’s configuration.
For instance, the main layout of your activity can be placed in the

layout/

directory
and can include another layout which exists in two flavors, in

layout-land/

and

layout-port/

.
This allows you to share most of the UI in portrait and landscape.

Like I mentioned earlier, the next installment will explain the

<merge
/>

, which can be particularly powerful when combined with

<include />

.
原文URL: http://www.curious-creature.com/2009/02/25/android-layout-trick-2-include-to-reuse/