01-04-2008
Because by the time that a variable is expanded, some other processing has already been done. Now you that processing to happen a second time. That's what eval does.
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vwait(n) Tcl Built-In Commands vwait(n)
__________________________________________________________________________________________________________________________________________________
NAME
vwait - Process events until a variable is written
SYNOPSIS
vwait varName
_________________________________________________________________
DESCRIPTION
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 variable varName. Once varName has been set, the vwait command will return as soon as the event
handler that modified varName completes. varName must be globally scoped (either with a call to global for the varName, or with the full
namespace path specification).
In some cases the vwait command may not return immediately after varName is set. This can happen if the event handler that sets varName
does not complete immediately. For example, if an event handler sets varName and then itself calls vwait to wait for a different variable,
then it may not return for a long time. During this time the top-level vwait is blocked waiting for the event handler to complete, so it
cannot return either.
EXAMPLES
Run the event-loop continually until some event calls exit. (You can use any variable not mentioned elsewhere, but the name forever
reminds you at a glance of the intent.)
vwait forever
Wait five seconds for a connection to a server socket, otherwise close the socket and continue running the script:
# 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
vwait 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!"
}
}
SEE ALSO
global(n), update(n)
KEYWORDS
event, variable, wait
Tcl 8.0 vwait(n)