A layout defines the visual structure for a user interface, such as the UI for an  or . You can declare a layout in two ways:

• Declare UI elements in XML. Android provides a straightforward XML vocabulary that corresponds to the View classes and subclasses, such as those for widgets and layouts.
• Instantiate layout elements at runtime. Your application can create View and ViewGroup objects (and manipulate their properties) programmatically.

　　Each layout file must contain exactly one root element, which must be a View or ViewGroup object. Once you've defined the root element, you can add additional layout objects or widgets as child elements to gradually build a View hierarchy that defines your layout.

layout为用户接口定义一个可视化的结构，比如activity或者appt widget的用户接口。

每一个个layout文件必须含有一个(只能是一个)View 或者 ViewGroup对象的root element。(在疯狂android讲义里说过：每一个Layout都是ViewGroup对象)你可以增加额外的layout对象（这个layout对象也至少有一个root element)或者widgets作为child element。

layout的继承图:

from: