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All user interface elements in an Android app are built using Viewand ViewGroupobjects. A Viewis an object that draws something on the screen that the user can interact with. A ViewGroupis an object that holds other View(and ViewGroup) objects in order to define the layout of the interface.
Android provides a collection of both Viewand ViewGroupsubclasses that offer you common input controls (such as buttons and text fields) and various layout models (such as a linear or relative layout).
The user interface for each component of your app is defined using a hierarchy of Viewand ViewGroupobjects, as shown in figure 1. Each view group is an invisible container that organizes child views, while the child views may be input controls or other widgets that draw some part of the UI. This hierarchy tree can be as simple or complex as you need it to be (but simplicity is best for performance).
Figure 1. Illustration of a view hierarchy, which defines a UI layout.
To declare your layout, you can instantiate Viewobjects in code and start building a tree, but the easiest and most effective way to define your layout is with an XML file. XML offers a human-readable structure for the layout, similar to HTML.
The name of an XML element for a view is respective to the Android class it represents. So a element creates a TextViewwidget in your UI, and a element creates a LinearLayoutview group.
For example, a simple vertical layout with a text view and a button looks like this:
xml version="1.0" encoding="utf-8"?>
android:text="I am a TextView" />
android:text="I am a Button" />
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.
You don't have to build all of your UI using Viewand ViewGroupobjects. 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 Action Bar, Dialogs, and Status Notifications.