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'\"
'\" Copyright (c) 1995-1996 Sun Microsystems, Inc.
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\"
'\" RCS: @(#) $Id: vwait.n,v 1.9 2009/04/04 17:32:00 dkf Exp $
'\"
.so man.macros
.TH vwait n 8.0 Tcl "Tcl Built-In Commands"
.BS
'\" Note: do not modify the .SH NAME line immediately below!
.SH NAME
vwait \- Process events until a variable is written
.SH SYNOPSIS
\fBvwait\fR \fIvarName\fR
.BE
.SH DESCRIPTION
.PP
This command enters the Tcl event loop to process events, blocking
the application if no events are ready. It continues processing
events until some event handler sets the value of the global variable
\fIvarName\fR. Once \fIvarName\fR has been set, the \fBvwait\fR
command will return as soon as the event handler that modified
\fIvarName\fR completes. The \fIvarName\fR argument is always interpreted as
a variable name with respect to the global namespace, but can refer to any
namespace's variables if the fully-qualified name is given.
.PP
In some cases the \fBvwait\fR command may not return immediately
after \fIvarName\fR is set. This can happen if the event handler
that sets \fIvarName\fR does not complete immediately. For example,
if an event handler sets \fIvarName\fR and then itself calls
\fBvwait\fR to wait for a different variable, then it may not return
for a long time. During this time the top-level \fBvwait\fR is
blocked waiting for the event handler to complete, so it cannot
return either.
.SH EXAMPLES
.PP
Run the event-loop continually until some event calls \fBexit\fR.
(You can use any variable not mentioned elsewhere, but the name
\fIforever\fR reminds you at a glance of the intent.)
.PP
.CS
\fBvwait\fR forever
.CE
.PP
Wait five seconds for a connection to a server socket, otherwise
close the socket and continue running the script:
.PP
.CS
# Initialise the state
after 5000 set state timeout
set server [socket -server accept 12345]
proc accept {args} {
global state connectionInfo
set state accepted
set connectionInfo $args
}
# Wait for something to happen
\fBvwait\fR state
# Clean up events that could have happened
close $server
after cancel set state timeout
# Do something based on how the vwait finished...
switch $state {
timeout {
puts "no connection on port 12345"
}
accepted {
puts "connection: $connectionInfo"
puts [lindex $connectionInfo 0] "Hello there!"
}
}
.CE
.PP
A command that will wait for some time delay by waiting for a namespace
variable to be set. Includes an interlock to prevent nested waits.
.PP
.CS
namespace eval example {
variable v done
proc wait {delay} {
variable v
if {$v ne "waiting"} {
set v waiting
after $delay [namespace code {set v done}]
\fBvwait\fR [namespace which -variable v]
}
return $v
}
}
.CE
.SH "SEE ALSO"
global(n), update(n)
.SH KEYWORDS
event, variable, wait
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