AIX Version 4.3 Differences Guide

hierarchical relationship between these objects or any particular implementation
of the trait.
For example, consider the I am a navigator trait. This trait characterizes the fact that
an object can set a value between a minimum and a maximum limit, regardless of
the particular method used to set the value or the particular look and feel. A
navigator can be implemented by a scroll bar, a thermometer-like control, or even
a choice in the list of all possible values. The navigator provides the abstraction
that guarantees that any object carrying this trait can be used to navigate
arbitrarily between two values.
The I am a navigator trait implements methods that make it possible for clients of
the object carrying the trait to set and get the min/max/current value
independently of the implementation.
In Motif 2.1, both the XmScrollBar (an XmPrimitive subclass) widget and the
XmSpinBox (an XmManager subclass) hold the I am a navigator trait, and they
can both inherit, or specialize, the methods of that trait.
Therefore, the traits mechanism makes it possible to implement a form of multiple
inheritance on top of the Xt object oriented framework, which only supports single
inheritance. In the Motif 2.1 model, traits are seen as light abstract classes,
sometimes called mix-ins.
Traits also make it possible to implement a form of polymorphism on the various
widgets. For example, if a trait has a setvalue method, it can be used to set
values of resources that actually have a different name. For example, a
trait::setvalue() replaces XtSetValues() on multiple XmNresource names.
A side effect of traits is the potential reduction of code size in applications. In
many cases, a Motif Manager widget would check the class of its children. Doing
so requires access to the class pointer and at link time. Linking the class pointer
global variable usually links chained modules in the application. Because those
tests are replaced in the manager code with checking the children’s traits, the
spaghetti effect is eliminated.
Traits also augment code re-use inside Motif. A widget writer can simply decide to
inherit a trait from another widget. It then inherits the class methods implemented
by the OSF engineers at no cost.
8.3.3.2 Uniform Transfer Model (UTM)
OSF/Motif 1.2 provides the application programmer and widget writer with a
number of different mechanisms to interchange data between applications. There
are four forms of data transfer supported within Motif:
• Primary transfer. When the user clicks on pointer button BTransfer, the data
currently selected (possibly in another application) is immediately transferred
at the pointer location.
• The clipboard. This is probably the most widely-known mechanism. When the
user activates the cut key (or the cut command into the Edit menu) the
selected data is cut into the clipboard. Activating the paste key inserts the data
at the current location of the destination cursor.
• Drag and drop. The visual metaphor for data exchange. The user can select
data, drag it over the screen and drop it onto a recipient.
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