Android User Interface (UI) (Cont.)

The name of an XML element for a view is respective to the Android class it represents. So a TextView element creates a TextView widget in your UI, and a LinearLayout element creates a LinearLayout view group. For example, a simple vertical layout with a text view and a button looks like this: 

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
  xmlns:android           = "http://schemas.android.com/apk/res/android"
  android:layout_width    = "fill_parent" 
  android:layout_height   = "fill_parent"
  android:orientation     = "vertical" >
  <TextView
    android:id            = "@+id/text"
    android:layout_width  = "wrap_content"
    android:layout_height = "wrap_content"
    android:text          = "I am a TextView" />
  <Button
    android:id            = "@+id/button"
    android:layout_width  = "wrap_content"
    android:layout_height = "wrap_content"
    android:text          = "I am a Button" />
</LinearLayout>

When you load a layout resource in your app, Android initializes each node of the layout into a runtime object you can use to define additional behaviors, query the object state, or modify the layout. For a complete guide to creating a UI layout, see XML Layouts.

User Interface Components
You don’t have to build all of your UI using View and ViewGroup objects. Android provides several app components that offer a standard UI layout for which you simply need to define the content. These UI components each have a unique set of APIs that are described in their respective documents, such as Adding the App Bar, Dialogs, and Status Notifications.




      “The more I read, the more I acquire,    
      the more certain I am that I know nothing.”    
      ― Voltaire