after(n) Tcl Built-In Commands after(n) ________________________________________________________________________________
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?
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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. 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.
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. A ms value of 0 (or negative) queues the event
immediately with priority over other event types (if not installed
withn an event proc, which will wait for next round of events).
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 canceled 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
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(n), interp(n), update(n), vwait(n)
KEYWORDS
cancel, delay, idle callback, sleep, time
Tcl 7.5 after(n)
tcl 8.6.13 - Generated Sun Jan 8 09:02:48 CST 2023