path: root/doc/after.n

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.TH after n 7.5 Tcl "Tcl Built-In Commands"
.so man.macros
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
after \- Execute a command after a time delay
.SH SYNOPSIS
\fBafter \fIms\fR
.sp
\fBafter \fIms \fR?\fIscript script script ...\fR?
.sp
\fBafter cancel \fIid\fR
.sp
\fBafter cancel \fIscript script script ...\fR
.sp
\fBafter idle \fR?\fIscript script script ...\fR?
.sp
\fBafter info \fR?\fIid\fR?
.BE
.SH DESCRIPTION
.PP
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:
.TP
\fBafter \fIms\fR
.
\fIMs\fR must be an integer giving a time in milliseconds.
A negative number is treated as 0.
The command sleeps for \fIms\fR milliseconds and then returns.
While the command is sleeping the application does not respond to
events.
.TP
\fBafter \fIms \fR?\fIscript script script ...\fR?
.
In this form the command returns immediately, but it arranges
for a Tcl command to be executed \fIms\fR 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 \fIscript\fR
arguments in the same fashion as the \fBconcat\fR 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 \fBinterp bgerror\fR.
The \fBafter\fR command returns an identifier that can be used
to cancel the delayed command using \fBafter cancel\fR.
A \fIms\fR 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).
.TP
\fBafter cancel \fIid\fR
.
Cancels the execution of a delayed command that
was previously scheduled.
\fIId\fR indicates which command should be canceled;  it must have
been the return value from a previous \fBafter\fR command.
If the command given by \fIid\fR has already been executed then
the \fBafter cancel\fR command has no effect.
.TP
\fBafter cancel \fIscript script ...\fR
.
This command also cancels the execution of a delayed command.
The \fIscript\fR arguments are concatenated together with space
separators (just as in the \fBconcat\fR 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 \fBafter cancel\fR command has no effect.
.TP
\fBafter idle \fIscript \fR?\fIscript script ...\fR?
.
Concatenates the \fIscript\fR arguments together with space
separators (just as in the \fBconcat\fR 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 \fBafter cancel\fR.
If an error occurs while executing the script then the
background error will be reported by the command
registered with \fBinterp bgerror\fR.
.TP
\fBafter info \fR?\fIid\fR?
.
This command returns information about existing event handlers.
If no \fIid\fR argument is supplied, the command returns
a list of the identifiers for all existing
event handlers created by the \fBafter\fR command for this
interpreter.
If \fIid\fR is supplied, it specifies an existing handler;
\fIid\fR must have been the return value from some previous call
to \fBafter\fR 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 \fIid\fR, and the second element is either
\fBidle\fR or \fBtimer\fR to indicate what kind of event
handler it is.
.LP
The \fBafter \fIms\fR and \fBafter idle\fR 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
\fBtclsh\fR, the event loop can be entered with the \fBvwait\fR
and \fBupdate\fR commands.
.SH "EXAMPLES"
This defines a command to make Tcl do nothing at all for \fIN\fR
seconds:
.PP
.CS
proc sleep {N} {
    \fBafter\fR [expr {int($N * 1000)}]
}
.CE
.PP
This arranges for the command \fIwake_up\fR to be run in eight hours
(providing the event loop is active at that time):
.PP
.CS
\fBafter\fR [expr {1000 * 60 * 60 * 8}] wake_up
.CE
.PP
The following command can be used to do long-running calculations (as
represented here by \fI::my_calc::one_step\fR, 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.
.PP
.CS
proc doOneStep {} {
    if {[::my_calc::one_step]} {
        \fBafter idle\fR [list \fBafter\fR 0 doOneStep]
    }
}
doOneStep
.CE
.SH "SEE ALSO"
concat(n), interp(n), update(n), vwait(n)
.SH KEYWORDS
cancel, delay, idle callback, sleep, time
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