USER HOOKS

     The GetKey function provides a feature which allows the programmer to
     identify a procedure that will be continually called while the program
     is idle, i.e. waiting for a keypress. For example, whenever the
     program is waiting for keyboard input, a procedure could be called
     that updates the time on the screen. This type of hook is referred to
     as an Idle Hook.

     Similarly, it is possible to identify a procedure that will be called
     every time a key has been pressed. For example, the hooked procedure
     could trap for the key F1 and call a help system. This avoids the need
     to individually trap for the F1 key at every occurrence in the program
     where GetKey is called. This type of hook is referred to as a Pressed
     Hook.

Idle Hook
     The first task is to build a procedure in your program that you want
     to call every time the program is waiting for keyboard input. To use a
     procedure as an Idle Hook it must:

     1) Be a plain procedure with no passed parameters.

     2) Be declared as a far procedure, i.e. precede the procedure with a
     {$F+} compiler directive and succeed the procedure with the {$F-}
     compiler directive.

Pressed Hook

     As with the Idle Hook, the first task is to build the procedure that
     is to be called every time a key is pressed. To use a procedure as a
     Pressed Hook it must:

     1) Be declared with one passed parameter. This must be a variable
     parameter of type char, e.g.

          PROCEDURE KEY_OVERRIDE(VAR CHH : CHAR);

     2) Be declared as a far procedure, i.e. precede the procedure with a
     {$F+} compiler directive and succeed the procedure with the {$F-}
     compiler directive.

     The hooked procedure is passed the key pressed by the user. The
     procedure can update this character to effectively substitute the key
     pressed by the user with a different one.

Turbo 5 Example

     The example below illustrates the application of idle and pressed
     hooks in a Turbo Pascal 5.0 program.

     {$F+}
     PROCEDURE SHOW_CLOCK;
     BEGIN
         WRITECENTER(1,WHITE,BLACK,TIME)
     END;

     PROCEDURE KEY_OVERRIDE(VAR CHK : CHAR);
     BEGIN
         CASE CHK OF
         F1   : MAIN_HELP_PROCEDURE;
         F5   : DISPLAY_SECURITY_SCREEN;
         ALTX : EXIT_PROCEDURE;
         END; {CASE}
     END;
     {$F-}
     ASSIGN_IDLE_HOOK(SHOW_CLOCK);
     ASSIGN_PRESSED_HOOK(KEY_OVERRIDE);
     ....


Turbo 4 Example

     The example below illustrates the application of idle and pressed
     hooks in a Turbo Pascal 4.0 program.

     {$F+}
     PROCEDURE SHOW_CLOCK;
     BEGIN
         WRITECENTER(1,WHITE,BLACK,TIME)
     END;

     PROCEDURE KEY_OVERRIDE(VAR CHK : CHAR);
     BEGIN
         CASE CHK OF
         F1   : MAIN_HELP_PROCEDURE;
         F5   : DISPLAY_SECURITY_SCREEN;
         ALTX : EXIT_PROCEDURE;
         END; {CASE}
     END;
     {$F-}
     KTTT.IDLE_HOOK := @SHOW_CLOCK;
     KTTT.PRESSED_HOOK := @KEY_OVERRIDE;
     ....