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vwait(3tcl) | Tcl Built-In Commands | vwait(3tcl) |
NAME¶
vwait - Process events until a variable is writtenSYNOPSIS¶
vwait varNameDESCRIPTION¶
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.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
# 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 $connectionInfo 0] "Hello there!" } }
namespace eval example { variable v done proc wait {delay} { variable v if {$v ne "waiting"} { set v waiting after $delay [namespace code {set v done}] vwait [namespace which -variable v] } return $v } }
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 $task" } trace add variable ::var write "updatedVar [info coroutine]" yield }}
NESTED VWAITS BY EXAMPLE¶
This example demonstrates what can happen when the vwait command is nested. The script will never finish because the waiting for the a variable never finishes; that vwait command is still waiting for a script scheduled with after to complete, which just happens to be running an inner vwait (for b) even though the event that the outer vwait was waiting for (the setting of a) has occurred.after 500 { puts "waiting for b" vwait b puts "b was set" } after 1000 { puts "setting a" set a 10 } puts "waiting for a" vwait a puts "a was set" puts "setting b" set b 42
waiting for a waiting for b setting a
after 500 { puts "waiting for b" trace add variable b write {apply {args { global a b trace remove variable ::b write \ [lrange [info level 0] 0 1] puts "b was set" set ::done ok }}} } after 1000 { puts "setting a" set a 10 } puts "waiting for a" trace add variable a write {apply {args { global a b trace remove variable a write [lrange [info level 0] 0 1] puts "a was set" puts "setting b" set b 42 }}} vwait done
# A coroutine-based wait-for-variable command proc waitvar globalVar { trace add variable ::$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
SEE ALSO¶
global(3tcl), update(3tcl)KEYWORDS¶
asynchronous I/O, event, variable, wait8.0 | Tcl |