Layout resources let you decouple your presentation layer by designing user interface layouts in XML rather than constructing them in code.
The most common use of a layout is for defining the user interface for an Activity. Once defined in XML, the layout is "inflated" within an Activity using setContentView, usually within the onCreate method. You can also reference layouts from within other layout resources, such as layouts for each row in a List View. More detailed information on using and creating layouts in Activities can be found in Chapter 4.
Using layouts to create your screens is best-practice UI design in Android. The decoupling of the layout from the code lets you create optimized layouts for different hardware configurations, such as varying screen sizes, orientation, or the presence of keyboards and touchscreens.
Each layout definition is stored in a separate file, each containing a single layout, in the res/layout folder. The file name then becomes the resource identifier.
A thorough explanation of layout containers and View elements is included in the next chapter, but as an example Listing 3-2 shows the layout created by the New Project Wizard. It uses a Linear Layout (described in more detail in Chapter 4) as a layout container for a Text View that displays the "Hello World'' greeting.
^^ LISTING 3-2: Hello World layout Available for download on <?xml version="1.0" encoding="utf-8"?>
Wrax.com <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:orientation="vertical" android:layout_width="fill_parent" android:layout_height="fill_parent"> <TextView android:layout_width="fill_parent" android:layout_height="wrap_content" android:text="Hello World!"
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