after - Execute a command after a time delay
after ms
after ms ?
script script script ...?
after cancel id
after cancel script script script ...
after idle ?
script script script ...?
after info ?
id?
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.
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
concat(3tcl),
interp(3tcl),
update(3tcl),
vwait(3tcl)
cancel, delay, idle callback, sleep, time