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
Free and Open Source Software