after(3tcl)


NAME

   after - Execute a command after a time delay

SYNOPSIS

   after ms

   after ms ?script script script ...?

   after cancel id

   after cancel script script script ...

   after idle ?script script script ...?

   after info ?id?
______________________________________________________________________________

DESCRIPTION

   This  command is used to delay execution of the program or to execute a
   command in background sometime in the future.  It  has  several  forms,
   depending on the first argument to the command:

   after ms
          Ms  must  be  an  integer  giving  a  time in milliseconds.  The
          command sleeps for ms milliseconds and then returns.  While  the
          command is sleeping the application does not respond to events.

   after ms ?script script script ...?
          In  this  form  the command returns immediately, but it arranges
          for a Tcl command to be executed ms  milliseconds  later  as  an
          event  handler.   The  command will be executed exactly once, at
          the given time.  The delayed command is formed by  concatenating
          all  the  script  arguments  in  the  same fashion as the concat
          command.  The command will be executed at global level  (outside
          the  context  of  any  Tcl procedure).  If an error occurs while
          executing the delayed command then the background error will  be
          reported  by  the  command registered with  interp bgerror.  The
          after command returns an identifier that can be used  to  cancel
          the delayed command using after cancel.

   after cancel id
          Cancels  the  execution of a delayed command that was previously
          scheduled.  Id indicates which command should be  canceled;   it
          must  have  been the return value from a previous after command.
          If the command given by id has already been  executed  then  the
          after cancel command has no effect.

   after cancel script script ...
          This  command  also  cancels the execution of a delayed command.
          The  script  arguments  are  concatenated  together  with  space
          separators  (just  as  in  the  concat  command).  If there is a
          pending command that matches the string,  it  is  cancelled  and
          will never be executed;  if no such command is currently pending
          then the after cancel command has no effect.

   after idle script ?script script ...?
          Concatenates the script arguments together with space separators
          (just  as in the concat command), and arranges for the resulting
          script to be evaluated later as an idle  callback.   The  script
          will  be  run  exactly  once,  the  next  time the event loop is
          entered and there are no events to process.  The command returns
          an  identifier  that  can  be used to cancel the delayed command
          using after cancel.  If an  error  occurs  while  executing  the
          script then the background error will be reported by the command
          registered with  interp bgerror.

   after info ?id?
          This command returns information about existing event  handlers.
          If no id argument is supplied, the command returns a list of the
          identifiers for all existing event handlers created by the after
          command  for  this interpreter.  If id is supplied, it specifies
          an existing handler; id must have been  the  return  value  from
          some  previous  call to after and it must not have triggered yet
          or been cancelled.  In this case the command returns a list with
          two  elements.   The  first  element  of  the list is the script
          associated with id, and the second element  is  either  idle  or
          timer to indicate what kind of event handler it is.

   The  after  ms  and  after  idle  forms  of the command assume that the
   application is event driven:  the delayed commands will not be executed
   unless the application enters the event loop.  In applications that are
   not normally event-driven, such as tclsh, the event loop can be entered
   with the vwait and update commands.

EXAMPLES

   This defines a command to make Tcl do nothing at all for N seconds:
          proc sleep {N} {
             after [expr {int($N * 1000)}]
          }

   This  arranges  for  the  command  wake_up  to  be  run  in eight hours
   (providing the event loop is active at that time):
          after [expr {1000 * 60 * 60 * 8}] wake_up

   The following command can be used to do long-running  calculations  (as
   represented  here  by ::my_calc::one_step, which is assumed to return a
   boolean indicating whether another step should be performed) in a step-
   by-step  fashion, though the calculation itself needs to be arranged so
   it can work step-wise.  This technique is extra careful to ensure  that
   the  event  loop is not starved by the rescheduling of processing steps
   (arranging for the next step to  be  done  using  an  already-triggered
   timer  event  only when the event queue has been drained) and is useful
   when you want to ensure that a Tk GUI remains responsive during a  slow
   task.
          proc doOneStep {} {
             if {[::my_calc::one_step]} {
                after idle [list after 0 doOneStep]
             }
          }
          doOneStep

SEE ALSO

   concat(3tcl), interp(3tcl), update(3tcl), vwait(3tcl)

KEYWORDS

   cancel, delay, idle callback, sleep, time





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