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ON KEY is an event-trapping statement that specifies a routine to branch to
when key n% is pressed.
 
ON KEY(n%) GOSUB {linenumber | linelabel}
    ■ linenumber or    A linenumber value of 0 disables event trapping and
      linelabel        does not specify line 0 as the start of the
                       routine.
 
Usage Notes
    ■ Keys are processed in the following order:
 
      1. The line printer's echo-toggle key is processed first. Defining
         this key as a user-defined key trap does not prevent characters
         from being echoed to the line printer when pressed.
      2. Function keys and the direction keys are examined next.
         Defining a function key or direction key as a user-defined key
         trap has no effect because these keys are predefined.
      3. Finally, the user-defined keys are examined.
 
    ■ The ON KEY statement can trap any key, including Break or system
      reset (Ctrl+Alt+Del). This makes it possible to prevent accidentally
      breaking out of a program or rebooting the machine.
    ■ The ON KEY statement specifies only the start of an event-trapping
      routine. The ◄KEY Statements► determine whether the routine is
      called and how events are handled when trapping is off:
 
        KEY ON      Enables event trapping. Event trapping occurs only
                    after a KEY ON statement is executed.
        KEY OFF     Disables event trapping. Even if an event takes place,
                    it is not remembered.
        KEY STOP    Suspends event trapping. Any events that occur are
                    remembered and are trapped as soon as a KEY ON statement
                    is executed.
 
    ■ If your program contains event-handling statements and you are
      compiling from the BC command line, use the BC /W or /V option.
      (The /W option checks for events at every label or line number; the
      /V option checks at every statement.) If you do not use these
      options and your program contains event traps, BASIC generates the
      error message, "ON event without /V or /W on command line."
    ■ The RETURN linenumber or RETURN linelabel forms of RETURN can be
      used to return to a specific line from the trapping routine. Use
      this type of return with care, however, because any GOSUB,
      WHILE, or FOR statements active at the time of the trap remain
      active. BASIC may generate error messages such as "NEXT without
      FOR." In addition, if an event occurs in a procedure, a RETURN
      linenumber or RETURN linelabel statement cannot get back into the
      procedure because the line number or label must be in the
      module-level code.
 
Important
    ■ After a key is trapped, the information on which key was trapped is
      no longer available. This means that you cannot subsequently use
      an INPUT statement or INKEY$ function to find out which key caused
      the trap. Because you do not know which key press caused the trap,
      you must create a different event-handling routine for each key you
      want to trap.