Scroll to navigation

after(3tcl) Tcl Built-In Commands 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:



  

  
Ms must be an integer giving a time in milliseconds. A negative
      number is treated as 0. The command sleeps for ms milliseconds and
      then returns. While the command is sleeping the application does not
      respond to events.

  

  
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. A ms value of 0 (or
      negative) queues the event immediately with priority over other event
      types (if not installed with an event proc, which will wait for next round
      of events).

  

  
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.

  

  
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 canceled and will never be executed; if no such
      command is currently pending then the after cancel command has no
      effect.

  

  
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.

  

  
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
      canceled. 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







  

    7.5
    Tcl