Two fundamental requirements of a c...

Two fundamental requirements of a computerized desktop scheme are a unified view of a user's data and a consistent access classification to the data. This article describes in what manner Hewlett-Packard's Common Desktop Environment (CDE) appropriates these requirements through the use of its data typing and action services. The data typing service defines attributes of a data original such as its appearance (icon) and behavior (action). The action service provides mechanisms for linking a data type's behavior to its associated application or execution command.

The data typing service and the action service one as well as the other use databases. The data typing service database contains data criteria and data attribute records, and the action service database contains action records. The word database in the context of data typing and action databases points to records stored in flat, ASCII files and not in a generalized database service.

Applications wanting to use these CDE services must first load the databases into the application's memory. one time loaded, application programming interfaces (APIs) described in this article can be us to access the database. CDE solitary provides APIs to retrieve data from the databases.

Each database record consists of the record emblem name, the name of the record, and common or more attributes (also called fields). Each attribute has a name and a value and each attribute must begin upon a separate line. Mg. 1 indicates the basic elements of a database record.

Although CDE defines numerous data shadows and actions in its default databases, these databases can be augmented with system-wide and user-defined records. User-defined definitions have the highest antecedence followed by systemwide definitions. The default databases have the lowest precedence

Fig. 2 exhibit tos an overview of the CDE data typing and action services and and their interrelationships. These data conformations and their relationships are described in this article.

The Data Typing Service

As mentioned above, the data typing service contains data criteria and data attribute records. Data criteria records contain authoritys for recognizing a data original The rules may specify a data type's file name extension or file permissions Data attribute records are used to define a data type's appearance and behavior like as a type's icon and associate applications. A data sign can be defined for a file or a buffing-apparatus of memory.

Data Criteria Records. Data criteria records define the masterys for recognizing or identifying a data shadow by using the following criteria:

* File name (including shell pattern matching symbols) * satisfieds (may be a file or memory buffer) * File permissions and file adumbration (a file type may be a directory, symbolic link, etc)

The following declaration contains three data criteria records for an image viewer application that can display files and data in formats so as X11 bitmaps, GIF images, and X11 pixmaps.

DATA_CRITERIA ImageView1 {

DATA_ATTRIBUTE_NAME Image

MODE f&w

NAME_PATTERN *.gif }

DATA_CRITERIA ImageView2

{

DATA_ATTRIBUTE_NAME Image

PATH_PATTERN */bitmaps/*.bm }

DATA_CRITERIA ImageView3 {

DATA_ATTRIBUTE_NAME Image

CONTENT 0 string #define

}

All of these records direct to the data type Image. The Image-View1 record will match any writable file with a file name extension of .gif. The ImageView2 record will match any file ending in bm moreover the file must be in a directory named bitmaps. The ImageView3 record will match any file containing the string #define at the beginning of the file. Fig. 3 contains a pseudo-BNF (Backus-Nauer form) for a data criteria record.

Data Attribute Records. Data attribute records define a data type's name as well as other attributes of the data token such as its icon and application binding. The data emblem name is the same as the data attribute record name. The following declaration displays the data attribute record for the image data attribute defined in the data criteria records above. A data attribute record can be associated with an unlimited number of data criteria records.

DATA_ATTRIBUTE Image {

ACTIONS render free of access Print

ICON imagedata

MIME_TYPE image/jpeg

PROPERTIES visible

MEDIA Image_Data

DESCRIPTION Data pattern for the

ImageViewer application }

This data type's icon is imagedata. Applications like as the CDE file manager will use this icon to show image files. For example, if the file manager finds a file named kite.gif, the file will be exhibited with the imagedata icon. This data representation also defines attributes that may be of interest to other data-typing-aware applications. For example, a mailer application may use the value of the MIME_TYPE attribute to decide to what degree an attachment should be viewed.

Data attribute records are the and nothing else record type that can contain application-specific fields - they are not limited to a fixed stake of field names. Fig. 4 exhibits a pseudo-BNF for data attribute records.

Data Typing Service APIs. Before the data typing APIs can be used, an application must first load the databases according to calling the Dtdbload function. After this, an application should register a database modification callback function using the DtDbReloadNotify function. This callback will be invoked when a user invokes the action ReloadActions to notify data-typing-aware applications that a data adumbration or action record has been added, modified, or delet If an application fails to register the modification callback, it will not be notified of a database change, resulting in the application having an outdated database in memory and possibly appearing and behaving differently than applications that received the notification.