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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 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 names-
       pace, 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  var-
       Name  is set.  This happens if the event handler that sets varName does
       not complete immediately.  For example, if an event handler  sets  var-
       Name 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 can-
       not return either. (See the NESTED VWAITS BY EXAMPLE below.)

       To be clear, multiple vwait calls will nest and will not happen in par-
       allel.  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 possi-
       ble, 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.


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!"
           } }

       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 }}

   NESTED VWAITS BY EXAMPLE
       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  run-
       ning  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 "set-
       ting 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 until after it has printed 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  wait-
       time 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


SEE ALSO

       global(n), update(n)


KEYWORDS

       asynchronous I/O, event, variable, wait



Tcl                                   8.0                             vwait(n)

tcl 8.6.0 - Generated Sat Jan 12 18:55:00 CST 2013
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