Volume 6A: Motif Programming Manual

XmNtopAttachment, XmATTACH_FORM,

XmNleftAttachment, XmATTACH_FORM,

NULL);

two = XtVaCreateManagedWidget ("Two",

xmPushButtonWidgetClass, parent,

XmNleftAttachment, XmATTACH_WIDGET,

XmNleftWidget, one,

/* attach top of widget to same y coordinate as top of "one" */

XmNtopAttachment, XmATTACH_OPPOSITE_WIDGET,

XmNtopWidget, one,

NULL);

three = XtVaCreateManagedWidget ("Three",

xmPushButtonWidgetClass, parent,

XmNtopAttachment, XmATTACH_WIDGET,

XmNtopWidget, one,

/* attach left of widget to same x coordinate as left side of "one" */

XmNleftAttachment, XmATTACH_OPPOSITE_WIDGET,

XmNleftWidget, one,

NULL);

XtRealizeWidget (toplevel);

XtAppMainLoop (app);

}

The example uses three PushButton gadgets inside of a Form widget. The output of the program is shown in the

figure.

Output of attach.c

You should notice that the widgets are packed together quite tightly, which might not be how you expected them to

appear. In order to space the widgets more reasonably, we need to specify some distance between them using

attachment offsets.

9.4.2 Attachment Offsets

Attachment offsets control the spacing between widgets and the objects to which they are attached. The following

resources represent the attachment offsets for the four sides of a widget:

XmNleftOffset

XmNrightOffset

XmNtopOffset

XmNbottomOffset

9 Manager Widgets 9.4.2 Attachment Offsets

213

the figure shows the graphic representation of attachment offsets.

By default, offsets are set to 0 (zero), which means that there is no offset, as shown in the output for the source code

To make the output more reasonable, we need only to set the left offset between widgets One and Two and the top

offset to between widgets One and Three. The resources values can be hard−coded in the application or set in a

resource file, using the following specification:

*form.One.leftOffset: 10

*form.One.topOffset: 10

*form.Two.leftOffset: 10

*form.Three.topOffset: 10

Attachment offsets

Our choice of the value 10 was arbitrary. The widgets are now spaced more appropriately, as shown in the figure.

Output of attach.c with offset resources set to 10

While the layout of the widgets can be improved by setting offset resources, it is also possible to disrupt the layout.

Consider the following resource specifications:

9 Manager Widgets 9.4.2 Attachment Offsets

214

*form*leftOffset: 10

*form*topOffset: 10

While it might seem that these resource values are simply a terser way to specify the offsets shown earlier, the figure

makes it clear that these specifications do not produce the desired effect.

Output of attach.c with inappropriate offset resources

An application should hard−code whatever resources may be necessary to prevent the user from setting values that

would make the application non−functional or aesthetically unappealing. Offset resource values can be tricky because

they apply individually to each side of each widget in a Form. The problem with the resource specifications used to

produce the figure is that the offsets are being applied to each side of every widget, when some of the alignments need

to be precise. In order to prevent this problem, we need to hard−code the offsets for particular attachments, as shown

in the following code fragment:

two = XtVaCreateManagedWidget ("Two",

xmPushButtonWidgetClass, parent,

XmNleftAttachment, XmATTACH_WIDGET,

XmNleftWidget, one,

XmNtopAttachment, XmATTACH_OPPOSITE_WIDGET,

XmNtopWidget, one,

XmNtopOffset, 0,

NULL);

three = XtVaCreateManagedWidget ("Three",

xmPushButtonWidgetClass, parent,

XmNtopAttachment, XmATTACH_WIDGET,

XmNtopWidget, one,

XmNleftAttachment, XmATTACH_OPPOSITE_WIDGET,

XmNleftWidget, one,

XmNleftOffset, 0,

NULL);

The use of zero−length offsets guarantees that the widgets they are associated with are aligned exactly with the

widgets to which they are attached, regardless of any resource specifications made by the user. A general rule of

thumb is that whenever you use XmATTACH_OPPOSITE_WIDGET, you should also set the appropriate offset to zero

so that the alignment remains consistent.

In some situations it is necessary to use negative offsets to properly arrange widgets in a Form. The most common

example of this situation occurs when using the XmATTACH_OPPOSITE_FORM attachment. Unless you use a

negative offset, as shown in the figure, the widgets are placed off the edge of the Form and are not visible.

9 Manager Widgets 9.4.2 Attachment Offsets

215