Q69046: Predefined Keys Trap All SHIFT Combinations; PEEK SHIFT Status

Article: Q69046
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S910117-124 B_BASICCOM | mspl13_basic
Last Modified: 6-FEB-1991

The predefined key traps KEY(0) through KEY(14), KEY(30), and KEY(31)
are active no matter what combinations of the SHIFT, CTRL, ALT, CAPS
LOCK, and NUM LOCK keys are active. If you want to distinguish and
trap both the SHIFTed and unSHIFTed status for a given key, you must
either set up two user-defined key traps, or check the contents of the
SHIFT register while the predefined key is being trapped.

This information applies to Microsoft QuickBASIC versions 4.00, 4.00b,
and 4.50 for MS-DOS, to Microsoft BASIC Compiler 6.00 and 6.00b for
MS-DOS, and to Microsoft BASIC Professional Development System (PDS)
versions 7.00 and 7.10 for MS-DOS.

The following program demonstrates that even though a user-defined key
trap, ON KEY(15), is activated for the SHIFT+DOWN ARROW key
combination, the trap is never executed because the predefined DOWN
ARROW key trap, ON KEY(14), takes precedence:

Code Example 1
--------------

KEY 15, CHR$(3) + "P"
ON KEY(15) GOSUB ShiftArrow
KEY(15) ON
ON KEY(14) GOSUB DownArrow
KEY(14) ON
CLS
DO
   a$ = INKEY$
LOOP UNTIL a$ = "q"

KEY(15) OFF
KEY(14) OFF
END
ShiftArrow:
   PRINT "Shift-Down arrow..."
   RETURN
DownArrow:
   PRINT "Down arrow..."
   RETURN

The next program demonstrates how to use the PEEK function to
determine if the SHIFT key is currently being pressed when the key
trap occurs:

Code Example 2
--------------

ON KEY(14) GOSUB ArrowKey
KEY(14) ON
CLS
DO
    a$ = INKEY$
LOOP UNTIL a$ = "q"
KEY(14) OFF
END
ArrowKey:
    DEF SEG = 0
    IF (PEEK(1047) AND 3) = 0 THEN
         PRINT "Down arrow..."
    ELSE
         PRINT "Shift-Down arrow..."
    END IF
    DEF SEG
    RETURN

The disadvantage of the method in example 2 is that if anything should
happen to delay the time between the key press and the time that the
trap routine is called (for example, if an INPUT statement is being
executed when the key is pressed), then the user may release the SHIFT
key before it can be detected.

The last option is to invoke a user-defined key trap for both the
SHIFTed and unSHIFTed keys; and not to use the predefined key trap.

Reference:

For more information on this topic, query on the following words:

   key and trap and CAPS and NUM and LOCK and user and defined

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