Tcl Built-In Commands after(1T)
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 mil-
liseconds 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 com-
mand. 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 bgerror mechanism is used to report the error. 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
Tcl Last change: 7.5 1
Tcl Built-In Commands after(1T)
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 iden-
tifier that can be used to cancel the delayed command
using after cancel. If an error occurs while executing
the script then the bgerror mechanism is used to report
the error.
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 exist-
ing handler; id must have been the return value from
some previous call to after and it must not have trig-
gered 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)}]
}
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Tcl Built-In Commands after(1T)
This arranges for the command wakeup to be run in eight
hours (providing the event loop is active at that time):
after [expr {1000 * 60 * 60 * 8}] wakeup
The following command can be used to do long-running calcu-
lations (as represented here by ::mycalc::onestep, 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 pro-
cessing 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 {[::mycalc::onestep]} {
after idle [list after 0 doOneStep]
}
}
doOneStep
SEE ALSO
bgerror(1T), concat(1T), update(1T), vwait(1T)
KEYWORDS
cancel, delay, idle callback, sleep, time
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
Availability SUNWTcl
Interface Stability Uncommitted
NOTES
Source for Tcl is available on http:/opensolaris.org.
Tcl Last change: 7.5 3
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