vwait - Process events until a variable is written
vwait varName
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
varName. Once
varName has been set, the
vwait command will return as soon as
the event handler that modified
varName completes. The
varName
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.
In some cases the
vwait command may not return immediately after
varName is set. This happens 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. (See the
NESTED VWAITS BY EXAMPLE below.)
To be clear,
multiple vwait calls will nest and will not happen
in parallel. The outermost call to
vwait will not return
until all the inner ones do. It is recommended that code should never nest
vwait calls (by avoiding putting them in event callbacks) but when that
is not possible, care should be taken to add interlock variables to the code
to prevent all reentrant calls to
vwait that are not
strictly
necessary. Be aware that the synchronous modes of operation of some Tcl
packages (e.g.,
http) use
vwait internally; if using the
event loop, it is best to use the asynchronous callback-based modes of
operation of those packages where available.
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.)
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 $connectionInfo 0] "Hello there!"
}
}
A command that will wait for some time delay by waiting for a namespace variable
to be set. Includes an interlock to prevent nested waits.
namespace eval example {
variable v done
proc wait {delay} {
variable v
if {$v ne "waiting"} {
set v waiting
after $delay [namespace code {set v done}]
vwait [namespace which -variable v]
}
return $v
}
}
When running inside a
coroutine, an alternative to using
vwait is
to
yield to an outer event loop and to get recommenced when the
variable is set, or at an idle moment after that.
coroutine task apply {{} {
# simulate [after 1000]
after 1000 [info coroutine]
yield
# schedule the setting of a global variable, as normal
after 2000 {set var 1}
# simulate [ vwait var]
proc updatedVar {task args} {
after idle $task
trace remove variable ::var write "updatedVar $task"
}
trace add variable ::var write "updatedVar [info coroutine]"
yield
}}
This example demonstrates what can happen when the
vwait command is
nested. The script will never finish because the waiting for the
a
variable never finishes; that
vwait command is still waiting for a
script scheduled with
after to complete, which just happens to be
running an inner
vwait (for
b) even though the event that the
outer
vwait was waiting for (the setting of
a) has occurred.
after 500 {
puts "waiting for b"
vwait b
puts "b was set"
}
after 1000 {
puts "setting a"
set a 10
}
puts "waiting for a"
vwait a
puts "a was set"
puts "setting b"
set b 42
If you run the above code, you get this output:
waiting for a
waiting for b
setting a
The script will never print “a was set” until after it has printed
“b was set” because of the nesting of
vwait commands, and
yet
b will not be set until after the outer
vwait returns, so
the script has deadlocked. The only ways to avoid this are to either structure
the overall program in continuation-passing style or to use
coroutine
to make the continuations implicit. The first of these options would be
written as:
after 500 {
puts "waiting for b"
trace add variable b write {apply {args {
global a b
trace remove variable ::b write \
[lrange [info level 0] 0 1]
puts "b was set"
set ::done ok
}}}
}
after 1000 {
puts "setting a"
set a 10
}
puts "waiting for a"
trace add variable a write {apply {args {
global a b
trace remove variable a write [lrange [info level 0] 0 1]
puts "a was set"
puts "setting b"
set b 42
}}}
vwait done
The second option, with
coroutine and some helper procedures, is done
like this:
# A coroutine-based wait-for-variable command
proc waitvar globalVar {
trace add variable ::$globalVar write \
[list apply {{v c args} {
trace remove variable $v write \
[lrange [info level 0] 0 3]
after 0 $c
}} ::$globalVar [info coroutine]]
yield
}
# A coroutine-based wait-for-some-time command
proc waittime ms {
after $ms [info coroutine]
yield
}
coroutine task-1 eval {
puts "waiting for a"
waitvar a
puts "a was set"
puts "setting b"
set b 42
}
coroutine task-2 eval {
waittime 500
puts "waiting for b"
waitvar b
puts "b was set"
set done ok
}
coroutine task-3 eval {
waittime 1000
puts "setting a"
set a 10
}
vwait done
global(3tcl),
update(3tcl)
asynchronous I/O, event, variable, wait