The resource system brings a number of different pieces together to form the final complete resource functionality. To help understand the overall system, here are some brief definitions of the core concepts and components you will encounter in using it:

Asset: A single blob of data associated with an application. This includes object files compiled from the Java source code, graphics (such as PNG images), XML files, etc. These files are organized in a directory hierarchy that, during final packaging of the application, is bundled together into a single ZIP file.

aapt: Android Asset Packaging Tool. The tool that generates the final ZIP file of application assets. In addition to collecting raw assets together, it also parses resource definitions into binary asset data.

Resource Table: A special asset that aapt generates for you, describing all of the resources contained in an application/package. This file is accessed for you by the Resources class; it is not touched directly by applications.

Resource: An entry in the Resource Table describing a single named value. Broadly, there are two types of resources: primitives and bags.

Resource Identifier: In the Resource Table all resources are identified by a unique integer number. In source code (resource descriptions, XML files, Java source code) you can use symbolic names that stand as constants for the actual resource identifier integer.

Primitive Resource: All primitive resources can be written as a simple string, using formatting to describe a variety of primitive types included in the resource system: integers, colors, strings, references to other resources, etc. Complex resources, such as bitmaps and XML describes, are stored as a primitive string resource whose value is the path of the underlying Asset holding its actual data.

Bag Resource: A special kind of resource entry that, instead of a simple string, holds an arbitrary list of name/value pairs. Each name is itself a resource identifier, and each value can hold the same kinds of string formatted data as a normal resource. Bags also support inheritance: a bag can inherit the values from another bag, selectively replacing or extending them to generate its own contents.

Kind: The resource kind is a way to organize resource identifiers for various purposes. For example, drawable resources are used to instantiate Drawable objects, so their data is a primitive resource containing either a color constant or string path to a bitmap or XML asset. Other common resource kinds are string (localized string primitives), color (color primitives), layout (a string path to an XML asset describing a view layout), and style (a bag resource describing user interface attributes). There is also a standard "attr" resource kind, which defines the resource identifiers to be used for naming bag items and XML attributes

Style: The name of the resource kind containing bags that are used to supply a set of user interface attributes. For example, a TextView class may be given a style resource that defines its text size, color, and alignment. In a layout XML file, you associate a style with a bag using the "style" attribute, whose value is the name of the style resource.

Style Class: Specifies a related set of attribute resources. This data is not placed in the resource table itself, but used to generate constants in the source code that make it easier for you to retrieve values out of a style resource and/or

XML tag's attributes. For example, the Android platform defines a "View" style class that contains all of the standard view attributes: padding, visibility, background, etc.; when View is inflated it uses this style class to retrieve those values from the XML file (at which point style and theme information is applied as approriate) and load them into its instance.

Configuration: For any particular resource identifier, there may be multiple different available values depending on the current configuration. The configuration includes the locale (language and country), screen orientation, screen density, etc. The current configuration is used to select which resource values are in effect when the resource table is loaded.

Theme: A standard style resource that supplies global attribute values for a particular context. For example, when writing an Activity the application developer can select a standard theme to use, such as the Theme.White or Theme.Black styles; this style supplies information such as the screen background image/color, default text color, button style, text editor style, text size, etc. When inflating a layout resource, most values for widgets (the text color, selector, background) if not explicitly set will come from the current theme; style and attribute values supplied in the layout can also assign their value from explicitly named values in the theme attributes if desired.

Overlay: A resource table that does not define a new set of resources, but instead replaces the values of resources that are in another resource table. Like a configuration, this is applied at load time to the resource data; it can add new configuration values (for example strings in a new locale), replace existing values (for example change the standard white background image to a "Hello Kitty" background image), and modify resource bags (for example change the font size of the Theme.White style to have an 18 pt font size). This is the facility that allows the user to select between different global appearances of their device, or download files with new appearances.

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