The XView Programmer’s Model
XView is intended to simplify application development under the X Window System by pro-
viding the programmer with a set of predefined user interface components. These compo-
nents implement the “look and feel” of the OPEN LOOK Graphical User Interface, developed
by Sun Microsystems, Inc. and AT&T.
This chapter presents a model of XView for the programmer. It is important to understand
this model before you begin writing XView applications. However, you might wish to skim
the concepts presented in this chapter and proceed to Chapter 3, Creating XView Applica-
tions, to examine sample programs.
2.1 Object-oriented Programming
To the programmer, XView is an object-oriented toolkit. XView objects can be considered
building blocks from which the user interface of the application is assembled. Each piece
can be considered an object from a particular package. Each package provides a list of pro-
perties from which you can choose to configure the object. By selecting objects from the
available packages, you can build the user interface for an application.
XView is based on several of the fundamental principles of object-oriented programming:
Objects are represented in a class hierarchy.
Objects are opaque data types.
Objects have attributes which can be set via message passing functions.
Objects may have callback procedures that are triggered by events.
We will look at how these concepts are implemented in XView in the sections that follow.
XView Programmer’s
The XView Programmer’s Model 17
2.1.1 Object Class Hierarchy
XView defines classes of objects in a tree hierarchy. For example, frame is a subclass of the
more general class window, which in turn is a subclass of drawable. Drawable, like user
interface object classes, is a subclass of the Generic Object class. Figure 2-1 shows the
XView class hierarchy and the relationships between the classes. Each class has identifying
features that make it unique from other classes or packages. In XView, a class is often called
a package, meaning a set of related functional elements. However, there are XView pack-
ages that are not members of the object class hierarchy, such as the Notifier package.
Drop Site
Panel Item
Panel Item
Figure 2-1. XView class hierarchy
Some objects are visual and others are not. Examples of visual objects include windows,
scrollbars, frames, panels, and panel items. Nonvisual objects are objects which have no
appearance, per se, but they have information which aids in the display of visual objects.
Examples of nonvisual objects include the server, screen, and font objects. The screen, for
example, provides information such as the type of color it can display or the default fore-
ground and background colors that objects might inherit. The display can provide informa-
tion about what fonts are available for objects that display text.
18 XView Programming Manual
All objects, both visual and nonvisual, are a part of this object classing system. The system is
extensible, so you can create new classes that might or might not be based on existing
XView uses static subclassing and chained inheritance as part of its object-oriented model.
All objects of a particular class inherit the properties of the parent class (also known as a
superclass). The Generic Object XV_OBJECT contains certain basic properties that all objects
share. For example, the same object can appear in many places on the screen to optimize
storage. To keep a record of this, the Generic Object maintains a reference count of its
instances. Since all objects have an owner, the parent of the object is stored in a field of the
generic part of the object. As the needs of an object get more specific to a particular look or
functionality, lower-level classes define properties to implement it.
Each class contains properties that are shared among all instances of that object. For
example, panels are a part of the PANEL package, which has properties that describe, among
other things, its layout (horizontal or vertical) or the spacing between items (buttons) in the
panel. All panels share these properties, even though the state of the properties might differ
for each instance of the object.
As mentioned earlier, XView uses subclassing so that each package can inherit the properties
of its superclass. The PANEL package is subclassed from the WINDOW package, which has pro-
perties specific to all windows, such as window dimensions, location on the screen, border
thickness, depth, visual, and colormap information. The WINDOW package is subclassed from
the root object XV_OBJECT, as are all objects, and the panel can access generic information
such as the size and position of itself.
2.1.2 Object Handles
When you create an object, the XView function returns a handle for the object. Later, when
you wish to manipulate the object or inquire about its state, you pass its handle to the appro-
priate function. This reliance on object handles is a way of information-hiding. The handles
are opaque in the sense that you cannot see through them to the actual data structure which
represents the object.
Each object type has a corresponding type of handle. Since C does not have an opaque
type, all the opaque data types mentioned above are typedef’d to the XView type
Xv_opaque or Xv_object.
In addition to the opaque data types, there are several typedefs that refer not to pointers
but to structures: Event, Rect, and Rectlist. Generally pointers to these structures are
passed to XView functions, so they are declared as Event *, Rect *, etc. The reason that
the asterisk (*) is not included in the typedef is that the structures are publicly available.
XView Programmer’s
The XView Programmer’s Model 19
Table 2-1 lists each XView object, its owner, the package that defines it, and its data type.
Table 2-1. XView Objects, Owners, Packages, and Data Types
Name Owner Package Data Type
canvas frame CANVAS Canvas
canvas view window or screen CANVAS_VIEW Canvas_view
cms window CMS Cms
cursor window or screen CURSOR Xv_Cursor
drag drop window DRAGDROP Dnd
drop site item window DROP_SITE_ITEM Drop_site_item
font root window FONT Xv_Font
frame frame or root window FRAME Frame
base frame frame or root window FRAME_BASE
command frame frame or root window FRAME_CMD
property frame frame or root window FRAME_PROPS
fullscreen root window FULLSCREEN Fullscreen
icon window or screen ICON Icon
menu server MENU Menu
command menu null MENU_COMMAND_MENU Menu
choice menu null MENU_CHOICE_MENU Menu
pullright menu menu item MENU Menu
toggle menu null MENU_TOGGLE_MENU Menu
menu item menu MENUITEM Menu_item
openwin frame OPENWIN Openwin
notice window NOTICE Xv_Notice
panel frame PANEL Panel
panel button panel PANEL_BUTTON Panel_button_item
panel choice panel PANEL_CHOICE Panel_choice_item
panel drop site panel PANEL_DROP_SITE Panel_drop_site_item
panel item panel PANEL_ITEM Panel_item
panel list panel PANEL_LIST Panel_list_item
panel message panel PANEL_MESSAGE Panel_message_item
panel multi-line text panel PANEL_MULTILINE_TEXT Panel_multiline_
panel numeric text panel PANEL_NUMERIC_TEXT Panel_numeric_
panel slider panel PANEL_SLIDER Panel_slider_item
panel text panel PANEL_TEXT Panel_text_item
screen null SCREEN Screen
scrollbar panel or canvas SCROLLBAR Scrollbar
selection window SELECTION Selection
selection owner window SELECTION Selection_owner
selection requestor window SELECTION Selection_requestor
selection item selection owner SELECTION_ITEM Selection_item
server null SERVER Server
server image screen SERVER_IMAGE Server_image
20 XView Programming Manual
Table 2-1. XView Objects, Owners, Packages, and Data Types (continued)
Name Owner Package Data Type
text subwindow frame TEXTSW Textsw
tty frame TTY Tty
window frame WINDOW Xv_Window
2.2 Attribute-based Functions
A model such as that used by XView, which is based on complex and flexible objects, pres-
ents the problem of how the client is to manipulate the objects. The basic idea behind the
XView interface is to provide a small number of functions, which take as arguments a large
set of attributes. For a given call to create or modify an object, only a subset of all applicable
attributes will be of interest.
2.2.1 Creating and Manipulating Objects
There is a common set of functions that allows the programmer to manipulate any object by
referencing the object handle. The functions are listed in Table 2-2.
Table 2-2. Generic Functions
Function Role
Establishes the connection to the server, initializes the Notifier and the
Defaults/Resource-Manager database, loads the Server Resource
Manager database, and parses any generic toolkit command line
xv_create() Creates an object.
xv_destroy() Destroys an object.
xv_find() Finds an object that meets certain criteria; or if the object doesn’t exist,
creates it.
xv_get() Gets the value of an attribute.
xv_set() Sets the value of an attribute.
Using these six routines, objects can be created and manipulated from all packages available
in XView. When the programmer wants to create an instance of an object from a certain
package, the routine xv_create() is used. For example:
Panel panel;
panel = xv_create(panel_parent, PANEL, NULL);
Here, an instance of a panel has been created from the PANEL package. All its attributes are
set to the panel’s default properties because no object-specific attributes have been specified.
XView Programmer’s
The XView Programmer’s Model 21

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