vwait(3tcl)

NAME

   vwait - Process events until a variable is written

SYNOPSIS

   vwait varName
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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(3tcl), update(3tcl)

KEYWORDS

   event, variable, wait

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