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Esc.To.ExitOn the RevTech BBS a few months ago, one of the users asked the question, "how do I check to see if an <esc> key has been pressed without losing what was already in the input buffer ?". In other words, if I am in a lengthy processing loop from which the user ought to be able to escape by pressing <esc>, how do I allow this facility but still allow them to type ahead ?" As this query was never answered I have taken the liberty of using this query as an entry point to the discussion of ESC.TO.EXIT. Fortunately for the inquisitive developer, RevTech use meaningful names for most of their subroutines (with the possible execption of the Verbs, the RTPs and FAT.FINGER (see later)) so studying the VERBS file provided the answer, a routine called ESC.TO.EXIT. This is a function which, called from a processing loop, returns true if <esc> was pressed and false if it was not. However any keys awaiting processing whilst ESC.TO.EXIT was being called, would be left in the data buffer. To illustrate this try the following section of code. 0001 DECLARE FUNCTION ESC.TO.EXIT 0002 EQU TRUE$ TO 1 0003 EQU FALSE$ TO 0 0004 * 0005 * Set a time for the loop to repeat 0006 * 0007 END.LOOP = TIME() + 4 0008 USER.ESCAPED = FALSE$ 0009 LOOP 0010 * 0011 * Insert processing loop here 0012 * 0013 UNTIL TIME() > END.LOOP OR USER.ESCAPED 0014 IF ESC.TO.EXIT() THEN USER.ESCAPED = TRUE$ 0015 REPEAT 0016 IF USER.ESCAPED THEN 0017 CALL MSG("<Esc> key pressed", "", "", "") 0018 END ELSE 0019 * 0020 * Now build a string to show what has been buffered in the data 0021 * statement 0022 * 0023 NEW = "" 0024 LOOP 0025 INPUT X,-1 0026 WHILE X DO 0027 NEW := X 0028 REPEAT 0029 CALL MSG(NEW : " entered", "", "", "") 0030 END It is apparent from the above description that when using the ESC.TO.EXIT() function, the user is not restricted to the standard keyboard buffer (the data entered being appended to the DATA statement not stored in the keyboard buffer). Thus using this method the user could type ahead up to a maximum of 64K. This is either an advantage or a disadvantage depending upon your trusting your users to know what they are typing after 15 keystrokes ! (Volume 1, Issue 9, Page 9) |
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