Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!silver!sl161022
From: sl161022@silver.bacs.indiana.edu
Newsgroups: comp.sys.mac.programmer
Subject: Re: VBL Tasks
Message-ID: <99000007@silver>
Date: 13 Feb 89 15:36:00 GMT
References: <4398@hubcap.UUCP>
Organization: Indiana University CSCI, Bloomington
Lines: 71
Nf-ID: #R:hubcap.UUCP:-439800:silver:99000007:000:3154
Nf-From: silver.bacs.indiana.edu!sl161022    Feb 13 10:36:00 1989


"VInstall and i understand about setting up a VBLTask record.  However, what
"if the procedure I install needs to be called only when a certain key combo
"is hit? Does it have to scan the event queue or something?

"I want it running all the time in the background.

Actually, another way to do this (and the way I use to do a similar thing)
is to install a patch for _GetOSEvent, which is called quite frequently.
In fact, whenever the Finder, the running application, or any DA is ready
to respond to an event, it will call either _GetNextEvent (which goes
through the Toolbox Event Manager) or _GetOSEvent (which goes directly
through the Operating System.)  Since _GetNextEvent calls _GetOSEvent,
you can patch _GetOSEvent and you'll be essentially guaranteed access
to any keypress, and you can then do whatever you want.

Of course, I do program in assembly language, where one has direct control
of the registers.  (_GetOSEvent passes and returns parameters in A0 and D0.)
I am not certain how a high level language gets access to the same
parameters.  Anyway, in assembly language, it looks something like. . .

; the initial code to install your background routine:

         push.l    A4             ;we'll be using this, so save it

         push.l    #'blah'        ;fetch the background routine
         push.w    #0             ;(push = move ?,-(SP))
         _GetResource             ;resource handle still on stack

         move.l    (SP),A4        ;let A4 point to our routine, and
         move.l    (A4),A4        ; detach resource, just to keep it safe
         _DetachResource

         move.w    #$A031,D0      ;$A031 is the trap code of _GetOSEvent
         _GetTrapAddress          ;A0 <- address of "old" GetOSEvent routine
         move.l    A0,OldGetOS(?) ;save this address, we'll need it later
         move.w    #$A031,D0      ;and now we'll redirect _GetOSEvent to
         move.l    A4,A0          ; my replacement routine
         _SetTrapAddress          ;ta da

         pop.l     A4             ;restore old value of A4

         . . .

; the replacement routine, which is a resource ('blah' #0) with its
; resSysHeap and resLocked attributes set (important!!)

Beginning
         push.l    A0             ;we'll need pointer to event record later
         move.l    OldGetOS(?),A1 ;call original routine (it's as if we're
         jsr       (A1)           ; not even here)
         pop.l     A0             ;we now have access to event
         push.l    D0             ;better save D0, caller is expecting it

         . . .                    ;do whatever you want based on the event

         pop.l     D0             ;restore D0
         rts


The chore of translating this into your language of choice is up to you,
but I think this will accomplish what you're after.
__________________________________________________________________

"May the forces of evil become confused on the way to your house."

Sincerely,                                        -- George Carlin
           Phaedrus
           (aka Colin Klipsch)
           sl161022@silver.bacs.indiana.edu
           Indiana University at Bloomington