Abrupt Branch to ON Event GOSUB Handler from Separate Handler (64430)

MORE INFORMATION

Within an ON <event> GOSUB handler, event trapping is suspended within the handler for only the trapped event. Any other active event traps (set with other ON <event> GOSUB statements) triggered within an event handler will cause control to immediately branch to the other ON <event> handler during execution of the initial handler. Upon terminating the second handler, control resumes where it left off in the original handler. This abrupt transfer of control may be problematic for those who want all actions surrounding a particular event, a key press for example, to be processed in full before subsequent key presses or events are handled.

The code example below demonstrates a case of control branching from one ON KEY handler to another when a second key is pressed within the first handler. The example contains the code (remarked out) necessary to temporarily disable all ON KEY and other event trapping within the initial ON KEY handler. If you unremark (uncomment) the code shown, the program will complete each handler without interruption.

Code Example

'DECLARE SUB EventsStop ()  ' These are the DECLARE statements for the
'DECLARE SUB EventsOn ()    ' SUBs that disable and enable key and
                            ' event trapping. (DECLARE statements are
                            ' not needed or supported for QuickBasic
                            ' 3.00 or earlier versions.)
CLS

ON KEY(1) GOSUB F1KeyHandler   ' Trapping for F1 function key.
ON KEY(2) GOSUB F2KeyHandler   ' Trapping for F2 function key.

KEY(1) ON
KEY(2) ON

PRINT "Please, press the F1 key"
SLEEP
PRINT "Exiting program"
END

' To temporarily disable all key and event trapping within an
' event handler, the CALL to SUB EventsStop must be the first
' statement and the CALL to SUB EventsOn must be the last statement
' of the handler before the RETURN.

F1KeyHandler:
        'CALL EventsStop       ' Include this statement to temporarily
                               ' disable all or selected ON KEY and
                               ' event statements

        PRINT "In F1 key handler"
        PRINT "Press F2 to jump to F2 key handler"
        SLEEP
        PRINT "Exiting F1 key handler"

        'CALL EventsOn         ' Include this statement so that any
                               ' keys pressed or events occurring
                               ' during the execution of this handler
                               ' will be processed at this point.
        RETURN

F2KeyHandler:
        'CALL EventsStop       ' Include this statement to temporarily
                               ' disable all or selected key and event
                               ' trapping within this handler.
        PRINT "In F2 key handler"
        PRINT "Exiting F2 key handler"

        'CALL EventsOn         ' Include this statement so that
                               ' any trappable keys or events triggered
                               ' during the execution of this handler
                               ' will be processed at this point.
        RETURN

SUB EventsOn               'All keys and events to enable go in here
        KEY(1) ON
        KEY(2) ON

        'TIMER ON          'Examples of events that may be enabled
        'COM(1) ON
END SUB

SUB EventsStop             'All keys and events to temporarily disable
        KEY(1) STOP        'go in here
        KEY(2) STOP

        'TIMER STOP        'Examples of events that may be temporarily
        'COM(1) STOP       'disabled
END SUB