Topic       : The GFA-Basic Compendium
Author      : GFA Systemtechnik GmbH
Version     : GFABasic.HYP v2.98 (12/31/2023)
Subject     : Documentation/Programming
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View Ref-FileTRON proc
TRACE$

proc: procedure name

With help of the instruction 'TRON proc', a procedure can be specified which is
called before the execution of each individual command. The variable TRACE$
then contains the command which is to be processed next. The command TRON proc
makes for very efficient error tracing when used in conjunction with TRACE$. In
addition, as well the next command to be processed being displayed, specified
variables can be output to screen or printer, allowing changes in the variables
to be followed during the course of the program run.

It is important that the TRON procedure should not affect the program itself
while running, so while in use, no PRINT commands should be made to the screen
(TEXT, ATEXT, ...) and the use of VDI routines should be avoided because of
GDOS's use of DEFTEXT, Line-A, etc.

Example:

    TRON tr_proc
    GRAPHMODE 3
    DO UNTIL MOUSEK
      x1%=100+RAND(200)
      y1%=100+RAND(100)
      x2%=200+RAND(200)
      y2%=200+RAND(100)
      PBOX xl%,y1%,x2%,y2%
    LOOP
    '
    PROCEDURE tr_proc
      IF BIOS(11,-1) AND &X100 ! Control key
        adr%=XBIOS(2)
        BMOVE adr%+1280,adr%,4*1280
        PRINT AT(1,5);SPACE$(80);
        PRINT AT(1,5);LEFT$(TRACE$,79);
        PAUSE 20
      ENDIF
    RETURN

--> This program draws randomly-distributed rectangles on the screen. Pressing
    the Control key causes the command just processed to appear on the screen.
    The program is terminated by the pressing of a mouse button.

Memo: TRON is not usable in compiled applications.
      The editor does not report a missing procedure with TRON proc.