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From: bzs@BU-CS.BU.EDU (Barry Shein)
Newsgroups: comp.windows.x
Subject: A Call for Callforwards...
Keywords: Xt Toolkit
Message-ID: <8906260010.AA16100@bu-cs.BU.EDU>
Date: 26 Jun 89 00:10:53 GMT
Sender: daemon@bloom-beacon.MIT.EDU
Organization: The Internet
Lines: 79


This all relates to the Xt Toolkit though might be applicable to other
toolkits.

The Xt Toolkit supports "Callback" functions: Functions defined in a
user application which are called by a widget typically (though not
necessarily) as a result of some XEvent. An example is a callback
which is invoked when a commandwidget is pressed so the application
can perform whatever corresponds to that user action.

In implementing Xt widgets I have run across a need for something
similar which I shall refer to as "Callforward" functions: A function
called by the widget for the sole purpose of getting information from
the application that the widget can not (or should not) generate for
itself.

An Illustrative Example:

Consider a scrollbar widget with at least the functions incrementUnit
and decrementUnit. In a text application this would correspond to
moving the scroll region one line of text up or down. Currently there
would be three ways to specify how much motion is needed to scroll one
line of text (this is needed to move the slider an appropriate
amount):

	1. A default value.
	2. A value set by the application (eg. XtincrementValue)
	3. By relieving the scrollwidget entirely of this and setting
	the slider position on every callback.

The first two work fine so long as all lines are the same height (ie.
a constant value.) The last works for anything but is not the way we
expect to use scrollbar widgets, more like hand-to-hand combat.

A Callforward is simply an application-supplied function which the
widget calls (if it exists) and expects a return value from.

In this case the scrollbar widget calls the callforward passing the
widget, a value indicating how much to scroll, typically one unit in
this example, (this can be genericized to provide a uniform interface)
and, possibly, user data (a closure.)

The definition is almost identical to that of a Callback but the
return type is changed from void to some generic type (eg. caddr_t).

Thus, the logic for scrolling (forward) becomes:

	IF CALL_FORWARD_EXISTS THEN
		MoveSlider(CallCallforward(w,name,amount,data))
	ELSE
		MoveSlider(incrementValue)

The major differences between a callback and a callforward are that
the latter returns a value and I think the ability to have a list of
them (as with callbacks) is a bad idea (I don't think this community
is ready for multiple-value-returns in C.)

Other exemplary uses of callforwards:

	1. Querying the font to use at a particular x/y position
	2. Boolean requests to check legality of a particular
	action, such as whether or not to allow a grip to be moved
	any further.
	3. Advice on setting dimensions within a resize request
	(eg. an application might want to round down or up to
	contain an entire object to avoid, eg, chopping a spread sheet
	cell.)
	4. Advice within GeometryRequests.

I am sure there are many other uses.

Comments?

	-Barry Shein

Software Tool & Die, Purveyors to the Trade
1330 Beacon Street, Brookline, MA 02146, (617) 739-0202
Internet: bzs@skuld.std.com
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