User Interface

[sukishan]

In an Android application, the user interface is built using View and ViewGroup objects. There are many types of views and view groups, each of which is a descendant of the View class.

View objects are the basic units of user interface expression on the Android platform. The View class serves as the base for subclasses called "widgets," which offer fully implemented UI objects, like text fields and buttons. The ViewGroup class serves as the base for subclasses called "layouts," which offer different kinds of layout architecture, like linear, tabular and relative.

A View object is a data structure whose properties store the layout parameters and content for a specific rectangular area of the screen. A View object handles its own measurement, layout, drawing, focus change, scrolling, and key/gesture interactions for the rectangular area of the screen in which it resides. As an object in the user interface, a View is also a point of interaction for the user and the receiver of the interaction events.

Layout
The most common way to define your layout and express the view hierarchy is with an XML layout file. XML offers a human-readable structure for the layout, much like HTML. Each element in XML is either a View or ViewGroup object (or descendent thereof). View objects are leaves in the tree, ViewGroup objects are branches in the tree (see the View Hierarchy figure above).

The name of an XML element is respective to the Java class that it represents. So a <TextView> element creates a TextView in your UI, and a <LinearLayout> element creates a LinearLayout view group. When you load a layout resource, the Android system initializes these run-time objects, corresponding to the elements in your layout.

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="Hello, I am a TextView" />
    <Button android:id="@+id/button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Hello, I am a Button" />
</LinearLayout>

Notice that the LinearLayout element contains both the TextView and the Button. You can nest another LinearLayout (or other type of view group) inside here, to lengthen the view hierarchy and create a more complex layout.

For more on building a UI layout, read Declaring Layout.

Tip: You can also draw View and ViewGroups objects in Java code, using the addView(View) methods to dynamically insert new View and ViewGroup objects.
There are a variety of ways in which you can layout your views. Using more and different kinds of view groups, you can structure child views and view groups in an infinite number of ways. Some pre-defined view groups offered by Android (called layouts) include LinearLayout, RelativeLayout, TableLayout, GridLayout and others. Each offers a unique set of layout parameters that are used to define the positions of child views and layout structure.

[alenfort]

User interface is one of the most important part of any plan, because it determines you can easily make the program what you want. A powerful program with a poorly designed user interface of little value.

[jeckdenis]

User Interface is the graphic design and appearance of a website, its function as seen and used by the person on the user end, at the website in a browser. The UI of a website is ultimately how it lets users know what it has to offer them. The importance of user interface design of your website is the first step in achieving high volume online visitors.