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From: thomas@utah-gr.UUCP (Spencer W. Thomas)
Newsgroups: comp.sys.mac
Subject: Re: Scroll bars in Modal Dialogs
Message-ID: <1845@utah-gr.UUCP>
Date: Fri, 14-Nov-86 03:39:33 EST
Article-I.D.: utah-gr.1845
Posted: Fri Nov 14 03:39:33 1986
Date-Received: Fri, 14-Nov-86 07:54:47 EST
References: <636@bunny.UUCP>
Reply-To: thomas@utah-gr.UUCP (Spencer W. Thomas)
Distribution: na
Organization: University of Utah CS Dept
Lines: 82

[Note: I am cross posting because this message came in here on
net.micro.mac, so I want to make sure the original poster sees the
response.]

I think you are hitting a well-known bug in the handling of scroll bar
controls.  It is even documented in IM (if you know what you are looking
for).  The actionProc will be called with two arguments (the control
handle and the part code), *except* when moving the thumb.  In assembly
language, you can do a hack to try and determine which it is (since you
want to just return for thumb actions, anyway), by looking at the
supposed "theControl" argument and seeing if it is the handle to your
control.  This is not certain, since the garbage on the stack could
possibly have the same value you are looking for. 

In LSC, there seems to be nothing you can do about this. The problem is
that the actionProc is a "pascal" procedure, so it will unstack the
arguments on return.  If it was called with NO arguments, this can be a
disaster.

The problem comes up worst when using controls inside a dialog, since
you don't have the option of whether you pass an actionProc based on
which part of the control is hit.  If the control is in an ordinary
window, you can pass a NIL actionProc when the thumb is selected.

Hope this explanation makes sense, it feels a lit disjointed to me.

As an example, here is an actionProc I wrote using Sumacc.

/* 
 * XL_ScrollAdj - Scroll an adjuster control.
 * Since this is called from inside a dialogue, we have no control
 * over what part of the scroll bar the user may have chosen.  If the
 * thumb was picked, this routine will be called with no arguments.
 * Use a kludge to detect this.  If args were passed, pick them with
 * getpargs into the struct below.
 */
struct TCargs {         /* args passed from Pascal TrackControl */
        short   theCode;
        ControlHandle whichControl;
};

XL_ScrollAdj(args)
struct TCargs args;
{
	register int i;
        struct TCargs a;

	/* Oh what a kludge.  If called because user is moving thumb
	 * directly, no args are passed.  Hopefully, this will detect
	 * that.  We don't want to unstack the args if there aren't any!
	 * 
	 * Note also dependency on a single global adjuster window.
	 */
	for ( i = 0; i < 7; i++ )
	    if ( args.whichControl == XL_Adjust_wind.u.adj.adjCtl[i] )
		break;
	if ( i >= 7 )
	    return;			/* nope, no match, so no args */

	/* Getpargs pops the args off the stack */
        getpargs(&a, sizeof a);

	switch( a.theCode )
	{
	    case inUpButton:
		SetCtlValue(a.whichControl,GetCtlValue(a.whichControl)-1);
		break;
	    case inDownButton:
		SetCtlValue(a.whichControl,GetCtlValue(a.whichControl)+1);
		break;
	    case inPageUp:
		SetCtlValue(a.whichControl,GetCtlValue(a.whichControl)-10);
		break;
	    case inPageDown:
		SetCtlValue(a.whichControl,GetCtlValue(a.whichControl)+10);
		break;
	}
	XL_adjustItem = i + 1;		/* end-run around DialogSelect */
}

-- 
=Spencer   ({ihnp4,decvax}!utah-cs!thomas, thomas@utah-cs.ARPA)