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after(n) Tcl Built-In Commands after(n)
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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 back-
ground 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 exe-
cuted exactly once, at the given time. The delayed command is formed by concate-
nating 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 proce-
dure). 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 indi-
cates 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 previ-
ous 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(n), interp(n), update(n), vwait(n)
KEYWORDS
cancel, delay, idle callback, sleep, time
Tcl 7.5 after(n)
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On Apache
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