distrib/share/tcl/help/tcl/events/vwait

   1 NAME
   2        vwait - Process events until a variable is written
   3
   4 SYNOPSIS
   5        vwait varName
   6
   7
   8 DESCRIPTION
   9        This  command enters the Tcl event loop to process events, blocking the
  10        application if no events are ready.   It  continues  processing  events
  11        until some event handler sets the value of variable varName.  Once var-
  12        Name has been set, the vwait command will return as soon as  the  event
  13        handler  that modified varName completes.  varName must globally scoped
  14        (either with a call to global for the varName, or with the full  names-
  15        pace path specification).
  16
  17        In  some  cases the vwait command may not return immediately after var-
  18        Name is set.  This can happen if the event handler  that  sets  varName
  19        does  not  complete immediately.  For example, if an event handler sets
  20        varName and then itself calls vwait to wait for a  different  variable,
  21        then it may not return for a long time.  During this time the top-level
  22        vwait is blocked waiting for the event handler to complete, so it  can-
  23        not return either.
  24
  25 EXAMPLES
  26        Run  the  event-loop continually until some event calls exit.  (You can
  27        use any variable not mentioned elsewhere, but the name forever  reminds
  28        you at a glance of the intent.)
  29               vwait forever
  30
  31        Wait  five seconds for a connection to a server socket, otherwise close
  32        the socket and continue running the script:
  33               # Initialise the state
  34               after 5000 set state timeout
  35               set server [socket -server accept 12345]
  36               proc accept {args} {
  37                  global state connectionInfo
  38                  set state accepted
  39                  set connectionInfo $args
  40               }
  41
  42               # Wait for something to happen
  43               vwait state
  44
  45               # Clean up events that could have happened
  46               close $server
  47               after cancel set state timeout
  48
  49               # Do something based on how the vwait finished...
  50               switch $state {
  51                  timeout {
  52                     puts "no connection on port 12345"
  53                  }
  54                  accepted {
  55                     puts "connection: $connectionInfo"
  56                     puts [lindex $connectionInfo 0] "Hello there!"
  57                  }
  58               }
  59
  60
  61 SEE ALSO
  62        global(n), update(n)
  63
  64
  65 KEYWORDS