Volume 6A: Motif Programming Manual

14 The Scale Widget

This chapter describes how to use the Scale widget to represent a range of values. The widget can be manipulated to

change the value.

The Scale widget displays a numeric value that falls within upper and lower bounds. The widget allows the user to

change that value interactively using a slider mechanism similar to that of a ScrollBar. This style of interface is useful

when it is inconvenient or inappropriate to have the user change a value using the keyboard. The widget is also

extremely intuitive to use; inexperienced users often understand how a Scale works when they first see one. the figure

shows how a Scale can be used with other widgets in an application.

A Scale widget in an application

A Scale can be oriented either horizontally or vertically. The values given to a Scale are stored as integers, but decimal

representation of values is possible through the use of a resource that allows you to place a decimal point in the value.

A Scale can be put in output−only mode, in which it is sometimes called a gauge. When a Scale is read−only, it

implies that the value is controlled by another widget or that it is being used to report status information specific to the

application. The standard way to create a read−only Scale is to specify that it is insensitive. Unfortunately, this

technique has the side−effect of graying out the widget. One workaround is to create a Scale widget that is sensitive,

but that has a null translation table.

14.1 Creating a Scale Widget

Applications that use the Scale widget must include the header file <Xm/Scale.h>. You can then create a Scale widget

as follows:

Widget scale;

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scale = XtVaCreateManagedWidget ("name",

xmScaleWidgetClass, parent,

resource−value−list,

NULL);

Even though the Scale widget functions as a primitive widget, it is actually subclassed from the Manager widget. All

the parts of a Scale are really other primitive widgets, but these subwidgets are not accessible through the Motif

toolkit. The fact that the Scale is a Manager widget means that you can create widgets that are children of a Scale. The

children are arranged so that they are evenly distributed along the vertical or horizontal axis parallel to the slider,

depending on the orientation of the Scale. This technique is used primarily to provide "tick marks" for the Scale, as

we'll describe later. In all other respects, a Scale can be treated just like other primitive widgets. the source code shows

a program that creates some Scale widgets. XtSetLanguageProc() is only available in X11R5; there is no

corresponding function in X11R4.

/* simple_scale.c −− demonstrate a few scale widgets. */

#include <Xm/Scale.h>

#include <Xm/RowColumn.h>

main(argc, argv)

int argc;

char *argv[];

{

Widget toplevel, rowcol, scale;

XtAppContext app;

void new_value(); /* callback for Scale widgets */

XtSetLanguageProc (NULL, NULL, NULL);

toplevel = XtVaAppInitialize (&app, "Demos", NULL, 0,

&argc, argv, NULL, NULL);

rowcol = XtVaCreateWidget ("rowcol",

xmRowColumnWidgetClass, toplevel,

XmNorientation, XmHORIZONTAL,

NULL);

scale = XtVaCreateManagedWidget ("Days",

xmScaleWidgetClass, rowcol,

XtVaTypedArg, XmNtitleString, XmRString, "Days", 5,

XmNmaximum, 7,

XmNminimum, 1,

XmNvalue, 1,

XmNshowValue, True,

NULL);

XtAddCallback (scale, XmNvalueChangedCallback, new_value, NULL);

scale = XtVaCreateManagedWidget ("Weeks",

xmScaleWidgetClass, rowcol,

XtVaTypedArg, XmNtitleString, XmRString, "Weeks", 6,

XmNmaximum, 52,

XmNminimum, 1,

XmNvalue, 1,

XmNshowValue, True,

NULL);

XtAddCallback (scale, XmNvalueChangedCallback, new_value, NULL);

scale = XtVaCreateManagedWidget ("Months",

xmScaleWidgetClass, rowcol,

14 The Scale Widget 14 The Scale Widget

368

XtVaTypedArg, XmNtitleString, XmRString, "Months", 7,

XmNmaximum, 12,

XmNminimum, 1,

XmNvalue, 1,

XmNshowValue, True,

NULL);

XtAddCallback (scale, XmNvalueChangedCallback, new_value, NULL);

scale = XtVaCreateManagedWidget ("Years",

xmScaleWidgetClass, rowcol,

XtVaTypedArg, XmNtitleString, XmRString, "Years", 6,

XmNmaximum, 20,

XmNminimum, 1,

XmNvalue, 1,

XmNshowValue, True,

NULL);

XtAddCallback (scale, XmNvalueChangedCallback, new_value, NULL);

XtManageChild (rowcol);

XtRealizeWidget (toplevel);

XtAppMainLoop (app);

}

void

new_value(scale_w, client_data, call_data)

Widget scale_w;

XtPointer client_data;

XtPointer call_data;

{

XmScaleCallbackStruct *cbs = (XmScaleCallbackStruct *) call_data;

printf("%s: %d0, XtName(scale_w), cbs−>value);

}

The output of this program is shown in the figure.

Output of simple_scale.c

The four Scales represent the number of days, weeks, months, and years, respectively. Each Scale displays a title that

is specified by the XmNtitleString resource. Just as with other Motif widgets that display strings, the

XmNtitleString must be set as a compound string, not a normal C string. The easiest way to make the conversion

is to use the XtVaTypedArg feature, as we've done in this example. The use of this conversion method is described

in detail in Chapter 19, Compound Strings.

14 The Scale Widget 14 The Scale Widget

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