Widget Basics

  

In the last Chapter we introduced some basic Motif programming concepts and introduced one specific widget, the PushButton, used in the example program developed. There are many other classes of widgets defined in Motif. Motif widgets are provided to perform a wide variety of common GUI tasks. This Chapter overviews the classes of Motif widgets. Following Chapters go on to study individual widgets in detail.

Widget Classes

The organisation of a large system of GUI components in Motif can be quite complex. In order to aid the design and understanding of Motif various widget classes have been constructed. Each class can be categorised by a broad functionality, at a variety of levels. Motif defines a hierarchy of widget classes (Fig 6.1) and a widget will inherit properties from a higher class in the widget hierarchy (Section 6.5). Some levels of the hierarchy have strong relationships with Xt Intrinsics since widgets are actually created in this toolkit.

 

Fig. 6.1 Widget Hierarchy The levels of the hierarchy and a broad functionality of each level are:

Core

-- The top of the hierarchy comes from Xt intrinsics. This superclass defines background, size and position properties that are common to all widgets.

XmPrimitive

-- The superclass of all primitive widgets. Primitive widgets are the basic building blocks of any Motif GUI (See Section 6.1.3 below).

Composite

-- The superclass of containers for widgets. Two sub-classes of the Composite class are defined:

Shell

widgets control the interfacing of Motif with the window manager.

Constraint

widgets are concerned with the organisation (positioning, alignment, etc.) of widgets contained within them. XmManager is a subclass of the constraint widget class. Manager widgets are discussed further in Section 6.1.3 below.

Shell Widgets

   

All widgets are contained in a shell widget. This is usually the top level widget. The primary function of a shell widget is as an interface to the window manager.

The application shell is normally the top level for an application and is created by XtVaAppInitialze()  or related functions.

There are two other shells:

• The Override shell  is used for pop-up menus that must be at the top-level. To achieve this, the window manager must usually be bypassed. Consequently, the override shell is not often used by Motif programs.
• The Transient shell is used for dialogs. However Motif repackages this shell so that dialogs become a subclass widget.

Constraint Widgets

   

Constraint widgets are concerned with the positioning and alignment of widgets contained within them. XmManager is a (Motif) subclass of the constraint widget specifying general manager facilities concerned with,for example, callbacks and highlight colour.

Construction widgets

 

Motif defines two basic classes of widgets that provide the basic building blocks of any GUI: primitive   and manager  . The majority of the remainder of this text is devoted to these widget classes.

Within each of these basic classes, several different sub-classes of widget are defined.

The broad function of each class is as follows:

Primitive

widgets   are designed to work as a single entity. They provide the building blocks with which we assemble our GUI. The PushButton is an example of a primitive widget class.

Manager

widgets   are designed to be containers and may have primitive and manager widgets placed under their control. The main function of manager widgets is to help control the design of a GUI. Manager widgets control how we organise a GUI by prescribing standard, or uniform, layouts (such as the MainWindow widget  , Chapter 9) or providing widgets that let us place widgets in an ordered fashion (e.g. RowColumn   or Form   widgets, Chapter 8).

Following Chapters will give details and examples of all types of widgets. For the remainder of this chapter we will give a brief introduction to these widgets.

Primitive Widgets

  

The following primitive widgets are defined in Motif:

ArrowButton

   -- A button with an orientatable arrow (Fig. 6.2). This button is defined and used in a similar fashion to the PushButton widget. An example of the ArrowButton can be seen in the arrows.c program in Chapter 8.

  
Figure 6.2: The Four Orientations of the ArrowButton Widget

Label

   -- A widget which has text or an image (Pixmap)  associated with it (Fig 6.3). The basic Label widget, as its name implies, does not do anything interactively. Its sole purpose is to help facilitate visual aids within the GUI. The Label widget does, however, have four (interactive) sub-classes of button:

  
Figure 6.3: A Label Widget

PushButton

   -- A button that can be labelled with a String (Fig 5.1). We have already met this widget in our first program, push.c (Chapter 5).

DrawnButton

   -- A button with which an icon (Pixmap) can be associated (Fig. 6.4).

  
Figure 6.4: A DrawnButton Widget

CascadeButton

   -- A button usually associated with a PullDown menu (Fig. 9.2). This widget is described in association with PullDown menu widgets in Chapter 9.

  
Figure 6.5: A CascadeButton Widget and Associated PullDown Menu

ToggleButton

   -- A button that displays text or graphics together with a graphic indicator of the state of the ToggleButton. The ToggleButton has two states: either on or off. Toggle buttons may be grouped together to provide a variety of configurations. RadioBox   are groups of ToggleButtons, where only one button can be selected at a time. A CheckBox  , on the other hand, allows any number of buttons to be selected at a given time (Fig. 6.6). Examples of both configurations of ToggleButton are given in Chapter 15.

  
Figure 6.6: A RadioBox and CheckBox ToggleButton Widget Configuration

Scrollbar

   -- A widget that allows the contents of a window to be displayed in a reduced area where the user can scroll the window to view hidden parts of the window. This widget can be defined and used explicitly in Motif programs (Chapter 14). Frequently, scrolling control will be defined when the ScrollBar (Fig. 6.7) is compounded with another widget. Chapters 11 and  17 discuss examples of the Scrollbar used in conjunction with Text (Fig. 6.10) and DrawingArea widgets respectively.

  
Figure 6.7: A Scrollbar Widget

Separator

   -- A widget used to separate items in a GUI to aid visual display. Fig. 6.8 illustrates the use of the Separator widget to delineate items (Load and Quit) in a single pulldown menu.

  
Figure 6.8: A Separator Widget Between Two Menu Items

List

   -- A widget that allows selection from a list of text items (Fig. 6.9). This widget is described in Chapter 12.

  
Figure 6.9: A List Widget

Text

   -- A complete text editor widget (Fig. 6.10). This widget provides default callback resources for text selection, editing, cutting and pasting of text as well the editing style of the widget. Additional callback resources can easily be attached to extend and customise the text widget for many text editing applications. Chapter 11 addresses all such issues.

  
Figure 6.10: A Text Widget

TextField

   -- A single-line text editor. This is basically a Text widget which is limited to a single-line of text entry. The TextField widget has many uses in GUI design where simple input is required. For example, when a file name needs to specified as input in a GUI or a key word is required for some search.

  
Figure 6.11: A TextField Widget

Motif 2.0 also prescribes another form of text widget, CSText  . This widget provides the same facilities a the Text widget but uses an alternative (compound) text string representation, XmString (Section 6.6), which is capable of supporting multiple fonts .

Gadgets

 

There may be occasions in Motif programming when we want to have the properties of a primitive widget but do not wish to worry about the management of the window properties of the widget. Motif has to manage each created widget's window and associated resources. If we have several widgets within our interface this could cause complications for our application.

To attempt to alleviate many of these problems Motif provides Gadgets . Gadgets are basically windowless widgets and, therefore, require less resources than a widget. Control of the gadget is the responsibility of the parent of the gadget.

Not all widgets have corresponding gadgets. The following gadgets are available: ArrowButton, Label and Separator. They behave in a similar manner to their corresponding widgets.

We will not consider gadgets further in this book since our programs are relatively simple and the relevance of their use cannot be effectively illustrated. The primary use of gadgets is when there is a real need to save memory on the X server or within the application. Gadgets may actually increase computer processor load since X finds some events more difficult to track within them. Gadgets are basically an artefact from earlier versions of Motif that were developed when window construction and management was more critical. Later versions of X have optimised the X server and coupled with the fact that computer power has increased and memory is less expensive, the use of gadgets is not as important as it once used to be.

Manager Widgets

  

Manager widgets are the basic Motif widgets for constructing and organising our interfaces in Motif. The following manager widget classes are available:

Frame

   -- A widget that provides an embossed effect to the child widget it contains (Fig 6.4). This is the simplest manager widget. An example of this widget is given in frame.c (see Exercise 6.1). 

  
Figure 6.12: A Frame Widget

ScrolledWindow

   -- A widget that allows scrolling of its child widget. Fig. 6.10 illustrates a common example of this: a ScrolledText widget that has a Text widget capable of being scrolled in a horizontal and vertical direction.

One common subclass of the ScrolledWindow widget is the MainWindow widget:

MainWindow

   -- A typical top level container widget for an application. This widget provides a common, uniform look and feel for any GUI (similar to MS Windows look and feel (Section 3.1)). The MainWindow widget has well defined mechanisms for the provision of Menubars, Scrollbars and command and message windows (Fig. 6.13). Chapter 9 describes the major aspects of MainWindow programming.

  
Figure 6.13: A MainWindow Widget

DrawingArea

   -- A widget where graphics can be displayed (Fig. 6.14). Note: Motif does not provide any graphics functions, Xlib provides all the graphic drawing and manipulation routines. Graphic programming and the interface with Xlib is one of the more difficult aspects of Motif to understand. Chapters 16--18 discuss these more advanced issues of programming and show how the DrawingArea widget is used in practice.

  
Figure 6.14: A DrawingArea Widget

PanedWindow

   -- Allows vertical tiling of child widgets. The use of this widget is not as common as other widgets and we will not address its use on this particular journey through Motif.

Scale

   -- This widget provides a slider object that can be used for user input (Fig. 6.4). Chapter 13 describes the Scale widget.

  
Figure 6.15: A Scale Widget

RowColumn

   -- A widely used widget that can lay out widgets in an orderly 2D fashion. Chapter 8 describes this widget.

BulletinBoard

   -- There are two sub-classes of this widget:

Form

   -- A widget similar in use to RowColumn but allows greater control of the placement and sizing of widgets. Chapter 8 compares and contrasts the Form and RowColumn widgets.

Dialog

   -- There are two forms of dialog:

• a MessageBox   which simply gives information to the user (Fig. 6.16) and
• a SelectionBox   which allows interaction with the user. Motif provides two sub-classes of the SelectionBox widget: a Command widget   for command line type input and a FileSelectionBox   for directory/file selection (Fig. 6.17).

  
Figure 6.16: A MessageBox (ErrorDialog) Widget

  
Figure 6.17: A FileSelectionBox Widget

Dialog widgets, as the name implies, provide the direct line of communication between the user and the application. In the case of the MessageBox widgets this could simply be the program informing the user of some event or action. There are several prescribed classes of MessageBox widgets that are associated with a special event. For example there are WarningDialog, InformationDialog and ErrorDialog widgets (Fig. 6.16). Also some form of prescribed user interaction is provided by the Command and FileSelectionBox widgets. Motif also provides base MessageBox and SelectionBox widgets so that the programmer can assemble customised Dialogs. However, Motif programming style (Chapter 20) suggests that wherever appropriate the prescribed Dialog widgets should be used to provide uniformity across applications. Chapter 10 deals with many aspects of Dialog widget programming and usage.

Motif 2.0 Widgets

Motif 2.0 defines a few new Manager widgets:

ComboBox

   -- A widget that combines the capabilities of a single line TextField and an XList.

IconGadget

   -- A widget that can be used to display Icons.

Container

   -- A widget that manages IconGadget children.

Notebook

   -- A widget that organizes children into pages, tabs, status area and page scroller.

Scale (thermometer)

   -- A modified version of the Scale widget with new resources added for thermometer behavior (see Chapter 13).

SpinBox

   -- A widget that manages multiple traversable children.

The use of many of these new widgets is fairly advanced and, except where indicated, these widgets are not dealt with further in this introductory text. Chapter 21 gives some further details on Motif 2.0.

Widget Resources

 

Each widget has a number of resources. These control many features of the widget such as the foreground and background colours, size etc.. A particular widget will have specialised resources such as callback resources which define how the widget responds to an event etc..

Every widget is documented in the Motif Reference Manual ([Hel94b]) which gives a complete list of the resources that a particular widget employs. When discussing individual widgets we will only consider the important resources that define the main characteristics of the widget concerned. The following Chapter addresses how widget resources can be set and altered for a given application.

There is a hierarchy of widgets (Fig 6.1) and a widget will inherit resources from a higher resource class in the widget hierarchy.

The levels of the hierarchy and related widget resources are:

Core

-- This superclass gives background, size and position resources that are common to all widgets.

XmPrimitive

-- The superclass of all primitive widgets defines resources related areas such as foreground and highlight colour.

Composite

-- The superclass of containers for widgets has two sub-classes:

Shell

widgets have resources related to interfacing with the window manager.

Constraint

widgets have resources that are concerned with the positioning and alignment of widgets contained within.

XmManager is a subclass of constraint and specifies general manager widget resources such as callbacks and highlight colour.

Widget level

-- Resources particular to a specific widget.

Strings in Motif

    

Motif programs (in C/C++) will typically need to use both types of string available:

• A data type String -- A ``normal'' C string (an array of characters). However, for convenience Motif has defined String as a standard Motif Data Type. Clearly, as Motif programs are usually written in C, this data type will be the main means of communication between a program and the standard input and output mechanisms.
• A data type XmString  -- Motif internal string data structure. When Motif needs to draw strings on the display, to achieve this more information is needed than simply the array of characters. Consequently, Motif defines an XmString with a more complicated structure. Most widgets can have a label resource associated with them, which is usually an XmString data type. It is rarely that the Motif programmer will need to know the internal structure of an XmString.

Motif provides a number of functions to convert a String into an XmString or vice versa:

XmStringCreateLocalized(),XmStringCreateLtoR(), XmStringGetLtoR(), XmStringCompare(), XmStringConcat(), XmStringCopy(), etc. are common examples. Many behave in a similar manner to their C standard library string handling function counterparts.

Their use is fairly straightforward -- the reference manuals should be consulted for more details.

Exercises

NEED SOME MORE