Events
You are encouraged to solve this task according to the task description, using any language you may know.
Event is a synchronization object. An event has two states signaled and reset. A task may await for the event to enter the desired state, usually the signaled state. It is released once the state is entered. Releasing waiting tasks is called event notification. Programmatically controlled events can be set by a task into one of its states.
In concurrent programming event also refers to a notification that some state has been reached through an asynchronous activity. The source of the event can be:
- internal, from another task, programmatically;
- external, from the hardware, such as user input, timer, etc. Signaling an event from the hardware is accomplished by means of hardware interrupts.
Event is a low-level synchronization mechanism. It neither identify the state that caused it signaled, nor the source of, nor who is the subject of notification. Events augmented by data and/or publisher-subscriber schemes are often referred as messages, signals etc.
In the context of general programming event-driven architecture refers to a design that deploy events in order to synchronize tasks with the asynchronous activities they must be aware of. The opposite approach is polling sometimes called busy waiting, when the synchronization is achieved by an explicit periodic querying the state of the activity. As the name suggests busy waiting consumes system resources even when the external activity does not change its state.
Event-driven architectures are widely used in GUI design and SCADA systems. They are flexible and have relatively short response times. At the same time event-driven architectures suffer to the problems related to their unpredictability. They face race condition, deadlocking, live locks and priority inversion. For this reason real-time systems tend to polling schemes, trading performance for predictability in the worst case scenario.
Variants of events
Manual-reset event
This event changes its state by an explicit request of a task. I.e. once signaled it remains in this state until it will be explicitly reset.
Pulse event
A pulse event when signaled releases all tasks awaiting it and then is automatically reset.
Sample implementations / APIs
Show how a manual-reset event can be implemented in the language or else use an API to a library that provides events. Write a program that waits 1s and then signals the event to a task waiting for the event.
Ada
Ada provides higher-level concurrency primitives, which are complete in the sense that they also allow implementations of the lower-level ones, like event. Here is an implementation of the manual-reset event.
The event interface: <lang ada>protected type Event is
procedure Signal; procedure Reset; entry Wait;
private
Fired : Boolean := False;
end Event;</lang> The event implementation: <lang ada>protected body Event is
procedure Signal is begin Fired := True; end Signal; procedure Reset is begin Fired := False; end Reset; entry Wait when Fired is begin null; end Wait;
end Event;</lang> With the event defined above: <lang ada>with Ada.Text_IO; use Ada.Text_IO;
procedure Test_Events is
-- Place the event implementation here X : Event;
task A; task body A is begin Put_Line ("A is waiting for X"); X.Wait; Put_Line ("A received X"); end A;
begin
delay 1.0; Put_Line ("Signal X"); X.Signal;
end Test_Events;</lang> Sample output:
A is waiting for X Signal X A received X
AutoHotkey
<lang AutoHotkey>SetTimer, internal, 1000 Return
internal: ; fire on a timer
TrayTip, internal, internal event!`npress F2 for external event SetTimer, internal, off
Return
F2:: ; external event: fire on F2 key press
TrayTip, external, f2 key pressed
Return</lang>
BBC BASIC
API
This uses a Windows event object: <lang bbcbasic> INSTALL @lib$+"TIMERLIB"
WAIT_TIMEOUT = 258 SYS "CreateEvent", 0, 1, 0, 0 TO hEvent% timerID% = FN_ontimer(1000, PROCelapsed, 0) PRINT "Waiting for event..." REPEAT SYS "WaitForSingleObject", hEvent%, 1 TO res% UNTIL res% <> WAIT_TIMEOUT PRINT "Event signalled" END DEF PROCelapsed SYS "SetEvent", hEvent% ENDPROC</lang>
Native
This uses a simple variable as a semaphore: <lang bbcbasic> INSTALL @lib$+"TIMERLIB"
Event% = FALSE timerID% = FN_ontimer(1000, PROCelapsed, 0) PRINT "Waiting for event..." REPEAT WAIT 0 UNTIL Event% PRINT "Event signalled" END DEF PROCelapsed Event% = TRUE ENDPROC</lang>
C
Using pipe to communicate to fork
ed child. Since child will be blocking trying to read the other end of the pipe, this can be used for synchronization.
<lang c>#include <stdio.h>
- include <unistd.h>
int main() { int p[2]; pipe(p); if (fork()) { close(p[0]); sleep(1); write(p[1], p, 1); wait(0); } else { close(p[1]); read(p[0], p + 1, 1); puts("received signal from pipe"); } return 0; }</lang>
C#
<lang csharp>using System; using System.Timers;
class Program {
static void Main() { var timer = new Timer(1000); timer.Elapsed += new ElapsedEventHandler(OnElapsed); Console.WriteLine(DateTime.Now); timer.Start(); Console.ReadLine(); }
static void OnElapsed(object sender, ElapsedEventArgs eventArgs) { Console.WriteLine(eventArgs.SignalTime); ((Timer)sender).Stop(); }
}</lang> Sample output:
10-11-2010 18:35:11 10-11-2010 18:35:12
Clojure
<lang lisp>(ns async-example.core
(:require [clojure.core.async :refer [>! <! >!! <!! go chan]]) (:require [clj-time.core :as time]) (:require [clj-time.format :as time-format]) (:gen-class))
- Helper functions (logging & time stamp)
- Time stamp format
(def custom-formatter (time-format/formatter "yyyy:MM:dd:ss.SS"))
(defn safe-println [& more]
" This function avoids interleaving of text output when using println due to race condition for multi-processes printing as discussed http://yellerapp.com/posts/2014-12-11-14-race-condition-in-clojure-println.html " (.write *out* (str (clojure.string/join " " more) "\n")))
(defn log [s]
" Outputs mesage with time stamp " (safe-println (time-format/unparse custom-formatter (time/now)) ":" s))
- Main code
(defn -main [& args]
(let [c (chan)] (log "Program start") (go (log "Task start") (log (str "Event received by task: "(<! c))))
(<!! (go (log "program sleeping") (Thread/sleep 1000) ; Wait 1 second (log "Program signaling event") (>! c "reset") ; Send message to task ))))
- Invoke -main function
(-main) </lang>
- Output:
2016:10:18:06.93 : Program start 2016:10:18:06.94 : task start 2016:10:18:06.94 : program sleeping 2016:10:18:07.94 : Program signaling event 2016:10:18:07.94 : Event received by task: reset
Delphi
<lang Delphi>program Events;
{$APPTYPE CONSOLE}
uses
SysUtils, Classes, Windows;
type
TWaitThread = class(TThread) private FEvent: THandle; public procedure Sync; procedure Execute; override; constructor Create(const aEvent: THandle); reintroduce; end;
{ TWaitThread }
constructor TWaitThread.Create(const aEvent: THandle); begin
inherited Create(False); FEvent := aEvent;
end;
procedure TWaitThread.Execute; var
res: Cardinal;
begin
res := WaitForSingleObject(FEvent, INFINITE); if res = 0 then Synchronize(Sync);
end;
procedure TWaitThread.Sync; begin
Writeln(DateTimeToStr(Now));
end;
var
event: THandle;
begin
Writeln(DateTimeToStr(Now)); event := CreateEvent(nil, False, False, 'Event'); with TWaitThread.Create(event) do try Sleep(1000); SetEvent(event) finally Free; end; Readln;
end.</lang> Sample output:
09.08.2011 0:27:43 09.08.2011 0:27:44
E
<lang e>def makeEvent() {
def [var fired, var firer] := Ref.promise() def event { to signal() { firer.resolveRace(null) # all current and future wait()s will resolve } to reset() { if (firer.isDone()) { # ignore multiple resets. If we didn't, then # reset() wait() reset() signal() would never # resolve that wait(). # create all fresh state def [p, r] := Ref.promise() fired := p firer := r } } to wait() { return fired } } return event
}</lang>
The event object has this behavior: the return value of .wait()
will be resolved after the time of the earliest .signal()
for which there is no intervening .reset()
.
<lang e>def e := makeEvent()
{
when (e.wait()) -> { println("[2] Received event.") } println("[2] Waiting for event...")
}
{
timer.whenPast(timer.now() + 1000, def _() { println("[1] Signaling event.") e.signal() }) println("[1] Waiting 1 second...")
}</lang>
Elixir
<lang elixir>defmodule Events do
def log(msg) do time = Time.utc_now |> to_string |> String.slice(0..7) IO.puts "#{time} => #{msg}" end def task do log("Task start") receive do :go -> :ok end log("Task resumed") end def main do log("Program start") {pid,ref} = spawn_monitor(__MODULE__,:task,[]) log("Program sleeping") Process.sleep(1000) log("Program signalling event") send(pid, :go) receive do {:DOWN,^ref,_,_,_} -> :task_is_down end end
end
Events.main</lang>
- Output:
06:27:05 => Program start 06:27:05 => Program sleeping 06:27:05 => Task start 06:27:06 => Program signalling event 06:27:06 => Task resumed
Erlang
Events can be implemented by using the selective receive expression and erlang's built in message passing. Here task waits for the message 'go' before it will continue. <lang erlang> -module(events). -compile(export_all).
log(Msg) ->
{H,M,S} = erlang:time(), io:fwrite("~2.B:~2.B:~2.B => ~s~n",[H,M,S,Msg]).
task() ->
log("Task start"), receive go -> ok end, log("Task resumed").
main() ->
log("Program start"), P = spawn(?MODULE,task,[]), log("Program sleeping"), timer:sleep(1000), log("Program signalling event"), P ! go, timer:sleep(100).
</lang> Output: <lang erlang> 66> events:main().
0: 0:57 => Program start 0: 0:57 => Program sleeping 0: 0:57 => Task start 0: 0:58 => Program signalling event 0: 0:58 => Task resumed
ok </lang>
F#
<lang fsharp>open System open System.Timers
let onElapsed (sender : obj) (eventArgs : ElapsedEventArgs) =
printfn "%A" eventArgs.SignalTime (sender :?> Timer).Stop()
[<EntryPoint>] let main argv =
let timer = new Timer(1000.) timer.Elapsed.AddHandler(new ElapsedEventHandler(onElapsed)) printfn "%A" DateTime.Now timer.Start() ignore <| Console.ReadLine() 0</lang>
Gambas
<lang gambas>Public Sub Timer1_Timer()
Print Str(Time(Now))
End</lang> Output:
16:14:18 16:14:19 16:14:20 16:14:21 16:14:22 16:14:23 16:14:24 16:14:25
Go
A Go channel can represent an manual-reset event, as described by the task. The two states of signaled and reset correspond to the presence or absence of a value on the channel. The program signals by sending a value on the channel. The event is reset when the waiting task explicitly executes the channel receive operation, <-event. <lang go>package main
import (
"log" "os" "time"
)
func main() {
l := log.New(os.Stdout, "", log.Ltime | log.Lmicroseconds) l.Println("program start") event := make(chan int) go func() { l.Println("task start") <-event l.Println("event reset by task") }() l.Println("program sleeping") time.Sleep(1 * time.Second) l.Println("program signaling event") event <- 0 time.Sleep(100 * time.Millisecond)
}</lang>
- Output:
01:27:21.862000 program start 01:27:21.862245 program sleeping 01:27:21.867269 task start 01:27:22.868294 program signaling event 01:27:22.868346 event reset by task
Haskell
<lang haskell>import Control.Concurrent (threadDelay, forkIO) import Control.Concurrent.SampleVar
-- An Event is defined as a SampleVar with no data. -- http://haskell.org/ghc/docs/latest/html/libraries/base/Control-Concurrent-SampleVar.html newtype Event = Event (SampleVar ())
newEvent = fmap Event (newEmptySampleVar) signalEvent (Event sv) = writeSampleVar sv () resetEvent (Event sv) = emptySampleVar sv waitEvent (Event sv) = readSampleVar sv</lang> <lang haskell>main = do e <- newEvent
forkIO (waitTask e) putStrLn "[1] Waiting 1 second..." threadDelay 1000000 {- µs -} putStrLn "[1] Signaling event." signalEvent e threadDelay 1000000 {- µs -} -- defer program exit for reception
waitTask e = do putStrLn "[2] Waiting for event..."
waitEvent e putStrLn "[2] Received event."</lang>
Note: Because there is no serialization of the text output, there is a chance that it will appear interleaved.
Icon and Unicon
The following only works in Unicon. The example illustrates the multiple tasks can receive the same event: <lang unicon>record Event(cond, value)
procedure main()
event := Event(condvar()) t1 := thread { write("Task one waiting for event....") critical event.cond: while /(event.value) do wait(event.cond) write("Task one received event.") } t2 := thread { write("Task two waiting for event....") critical event.cond: while /(event.value) do wait(event.cond) write("Task two received event.") } delay(1000) # Let main thread post the event. event.value := "yes" write("Signalling event.") signal(event.cond,0) every wait(t1|t2)
end</lang>
Sample run:
->event Task two waiting for event.... Task one waiting for event.... Signalling event. Task two received event. Task one received event. ->
JavaScript
An example using the YUI library: <lang javascript>YUI().use('event-custom', function(Y) {
// add a custom event: Y.on('my:event', function () { alert("Event fired"); }); // fire the event after one second: setTimeout(function () { Y.fire('my:event'); }, 1000);
});</lang> An example simulating DOM events: <lang javascript>YUI().use('node-event-simulate', function(Y) {
// add a click event handler to a DOM node with id "button": Y.one("#button").on("click", function (e) { alert("Button clicked"); }); // simulate the click after one second: setTimeout(function () { Y.one("#button").simulate("click"); }, 1000);
});</lang>
Julia
Julia provides a variety of high and low level functions and macros for multitasking and events. The code below uses a Condition() event semaphore created in the base thread for communication between two child threads.
<lang julia> function dolongcomputation(cond)
det(rand(4000, 4000)) Base.notify(cond)
end
function printnotice(cond)
Base.wait(cond) println("They are finished.")
end
function delegate()
println("Starting task, sleeping...") condition = Base.Condition() Base.@async(printnotice(condition)) Base.@async(dolongcomputation(condition))
end
delegate() sleep(5) println("Done sleeping.") </lang>
- Output:
Starting task, sleeping... They are finished. Done sleeping.
LFE
Paste in the REPL:
<lang lisp> (defun log (msg)
(let ((`#(,h ,m ,s) (erlang:time))) (lfe_io:format "~2.B:~2.B:~2.B => ~s~n" `(,h ,m ,s ,msg))))
(defun task ()
(log "Task start") (receive ('go 'ok)) (log "Task resumed"))
(defun run ()
(log "Program start") (let ((pid (spawn (lambda () (task))))) (progn (log "Program sleeping") (timer:sleep 1000) (log "Program signalling event") (! pid 'go) (timer:sleep 100))))
</lang>
Usage:
> (run) 18:34:53 => Program start 18:34:53 => Program sleeping 18:34:53 => Task start 18:34:54 => Program signalling event 18:34:54 => Task resumed ok
OTP comes with a gen_even
t behavior that is more robust and resilient than this version. That is what should be used for any non-toy example or project.
Lingo
Lingo/Director uses (stateless) events for system/application state change notifications, user action notifications and inter-sprite communication.
To catch an event, a corresponding event handler - a function with a predefined name - has to be definined in the code. Examples for such event handlers are: <lang lingo>-- the current window was closed on closeWindow ... end
-- the left mouse button was pressed by the user on mouseDown ... end</lang> Also "Sprites" (visual elements) receive events by setting up such event handlers in scripts attached to them. Both predefined and custom events can be sent programmatically to sprites, e.g. using: <lang lingo>-- send event #mouseDown programmatically to sprite 1 sendSprite(1, #mouseDown)
-- send custom event #foo to named sprite "bar" sendSprite("bar", #foo)
-- send custom event #fooBar to all existing sprites sendAllSprites(#fooBar)</lang>
Using a binary plugin ("Xtra"), in Windows also lower level window messages can be both sent and received:
<lang lingo>mx = xtra("Msg").new()
-- send message WM_LBUTTONDOWN to a specific window identified by HWND hwnd WM_LBUTTONDOWN = 513 MK_LBUTTON = 1 lParam = 65536*y + x mx.send_msg (hwnd, WM_LBUTTONDOWN, MK_LBUTTON, lParam)
-- listen for WM_COPYDATA and WM_MOUSEWHEEL messages sent to current application -- window, notify Lingo callback function 'msgReceived' when such messages occur. -- This callback function will receive hwnd, message, wParam and lParam as arguments -- (and for WM_COPYDATA messages also the data that was sent as ByteArray). WM_COPYDATA = 74 WM_MOUSEWHEEL = 522 mx.msg_listen([WM_COPYDATA, WM_MOUSEWHEEL], VOID, #msgReceived)</lang>
Mathematica
Mathematica supports events from timers (via Pause[]), task schedule descriptors. This will print a message after 4 seconds, then terminate the program. <lang Mathematica>Print["Will exit in 4 seconds"]; Pause[4]; Quit[] ->Will exit in 4 seconds</lang>
Nim
<lang nim>import posix
var p: array[2, cint] discard pipe p if fork() > 0:
discard close p[0] discard sleep 1 discard p[1].write(addr p[0], 1) var x: cint = 0 discard wait x
else:
discard close p[1] discard p[0].read(addr p[1], 1) echo "received signal from pipe"</lang>
Stdlib Semaphore
This version using locks module for signaling the condition.
<lang nim>import locks from os import sleep from times import cpuTime from strformat import fmt
var
# condition variable which shared across threads cond: Cond lock: Lock threadproc: Thread[void]
proc waiting {.thread.} =
echo "spawned waiting proc" let start = cpuTime() cond.wait lock echo fmt"thread ended after waiting: {cpuTime() - start} seconds."
proc main =
initCond cond initLock lock threadproc.createThread waiting echo "in main proc" os.sleep 1000 echo "send signal/event notification" signal cond joinThread threadproc deinitCond cond deinitLock lock
main()</lang>
Compile and run:
nim c -r --threads:on events_cond.nim
- Output:
in main proc spawned waiting proc send signal/event notification thread ended after waiting: 1.001 seconds.
Oforth
An event is often implemented with a control channel. A task is waiting for an object on the channel. When the event occurs, another task sends an object on this channel.
<lang Oforth>: anEvent | ch |
Channel new ->ch #[ ch receive "Ok, event is signaled !" println ] & System sleep(1000) ch send($myEvent) ;</lang>
An emitter is a general implementation for handling events : an emitter waits for events emitted and launches listeners that are waiting for those events. <lang Oforth>import: emitter
- anEvent2
| e i |
Emitter new(null) ->e e onEvent($myEvent, #[ "Event is signaled !" println ]) 10 loop: i [ 1000 System sleep $myEvent e emit ] e close ;</lang>
Oz
Events can be implemented as mutable references to dataflow variables: <lang oz>declare
fun {NewEvent} {NewCell _} end
proc {SignalEvent Event} @Event = unit end
proc {ResetEvent Event} Event := _ end
proc {WaitEvent Event} {Wait @Event} end
E = {NewEvent}
in
thread {System.showInfo "[2] Waiting for event..."} {WaitEvent E} {System.showInfo "[2] Received event."} end
{System.showInfo "[1] Waiting 1 second..."} {Delay 1000} {System.showInfo "[1] Signaling event."} {SignalEvent E}</lang>
However, this code is quite unidiomatic. If we need to wait for an event just once (like in this example), we can simply use a dataflow variable, i.e. an event that cannot be reset: <lang oz>declare
E
in
thread {System.showInfo "[2] Waiting for event..."} {Wait E} {System.showInfo "[2] Received event."} end
{System.showInfo "[1] Waiting 1 second..."} {Delay 1000} {System.showInfo "[1] Signaling event."} E = unit</lang>
If we want to synchronize two threads repeatedly and exchange data, it is natural to use ports and streams. Streams are just lists with an unbound tail. A port is basically a pointer to the tail of a list, i.e. it keeps track of where the next event can be written to: <lang oz>declare
MyPort
in
thread MyStream in {NewPort ?MyStream ?MyPort} {System.showInfo "[2] Waiting for event..."} for Event in MyStream do
{System.showInfo "[2] Received event."} {System.showInfo "[2] Waiting for event again..."}
end end
for do {System.showInfo "[1] Waiting 1 second..."} {Delay 1000} {System.showInfo "[1] Signaling event."} {Port.send MyPort unit} end</lang>
It is important to limit the scope of a stream as much as possible to ensure that the already read part of the stream is garbage-collected.
Perl
This is an example of using the AnyEvent module. The result is this: it prints "Hello world!" after one second, then after another second prints "Hi!" four times every quarter of a second and then immediately prints "Bye!" and quits: <lang Perl>use AnyEvent;
- a new condition with a callback:
my $quit = AnyEvent->condvar(
cb => sub { warn "Bye!\n"; }
);
- a new timer, starts after 2s and repeats every 0.25s:
my $counter = 1; my $hi = AnyEvent->timer(
after => 2, interval => 0.25, cb => sub { warn "Hi!\n"; # flag the condition as ready after 4 times: $quit->send if ++$counter > 4; }
);
- another timer, runs the callback once after 1s:
my $hello = AnyEvent->timer(
after => 1, cb => sub { warn "Hello world!\n"; }
);
- wait for the $quit condition to be ready:
$quit->recv();</lang> This is the same using AnyEvent simplified API: <lang Perl>use AnyEvent;
my $quit = AE::cv sub { warn "Bye!\n" };
my $counter = 1; my $hi = AE::timer 2, 0.25, sub {
warn "Hi!\n"; $quit->send if ++$counter > 4;
};
my $hello = AE::timer 1, 0, sub {
warn "Hello world!\n";
};
$quit->recv;</lang>
Phix
The primary synchronisation primitive in phix is the critical section, in the following the leave_cs() in main() acts as signalling an event, and the one in echo() from whichever goes first acts to signal that the other can/should resume. <lang Phix>constant lock = init_cs() include timedate.e
procedure showtime()
puts(1,format_timedate(date()," h:m:s\n"))
end procedure
procedure echo(string s)
sleep(rnd()/10) -- see note enter_cs(lock) puts(1,s) sleep(1) showtime() leave_cs(lock)
end procedure
procedure main()
enter_cs(lock) sequence threads = {create_thread(routine_id("echo"),{"job1"}), create_thread(routine_id("echo"),{"job2"})} puts(1,"main") showtime() sleep(1) puts(1,"free") showtime() leave_cs(lock) wait_thread(threads) puts(1,"done\n")
end procedure main()</lang>
- Output:
Typically the first thread to attempt enter_cs() is released first, but there is no guarantee of that. The sleep(rnd()/10) above evens out the likelihood, by pausing for up to 0.1s, but otherwise isn't necessary.
main 10:00:57 free 10:00:58 job2 10:00:59 job1 10:01:00 done
External events such as timers and user input are handled in pGUI, eg <lang Phix>function timer_cb(Ihandle /*ih*/)
IupUpdate(canvas) return IUP_IGNORE
end function
Ihandle timer = IupTimer(Icallback("timer_cb"), 1000)
function key_cb(Ihandle /*ih*/, atom c)
if c=K_ESC then return IUP_CLOSE end if if c=K_F5 then iteration = 0 IupSetInt(timer,"RUN",1) -- (restart timer) end if return IUP_CONTINUE
end function
IupSetCallback(dlg, "K_ANY", Icallback("key_cb"))</lang>
PicoLisp
PicoLisp supports events from timers (via 'task' and 'alarm'), file descriptors (also 'task') and various 'signals'. This will print a message after one second, then terminate the program after another four seconds: <lang PicoLisp>(alarm 1
(prinl "Exit in 4 seconds") (alarm 4 (bye)) )</lang>
PowerShell
<lang PowerShell> $timer = New-Object -TypeName System.Timers.Timer -Property @{Enabled=$true; Interval=1000; AutoReset=$true}
$action = {
$global:counter += 1 Write-Host “Event counter is ${counter}: $((Get-Date).ToString("hh:mm:ss"))” if ($counter -ge $event.MessageData) { Write-Host “Timer stopped” $timer.Stop() }
}
$job = Register-ObjectEvent -InputObject $timer -MessageData 5 -SourceIdentifier Count -EventName Elapsed -Action $action
$global:counter = 0 & $job.Module {$global:counter} </lang>
- Output:
Event counter is 1: 04:58:04 Event counter is 2: 04:58:05 Event counter is 3: 04:58:06 Event counter is 4: 04:58:07 Event counter is 5: 04:58:08 Timer stopped
PureBasic
<lang Purebasic>OpenWindow (0, 10, 10, 150, 40, "Event Demo") ButtonGadget (1, 10, 10, 35, 20, "Quit")
Repeat
Event = WaitWindowEvent() If Event = #PB_Event_Gadget And EventGadget() = 1 End EndIf
ForEver</lang>
Python
<lang Python> import threading import time
def wait_for_event(event):
event.wait() print("Thread: Got event")
e = threading.Event()
t = threading.Thread(target=wait_for_event, args=(e,)) t.start()
print("Main: Waiting one second") time.sleep(1.0) print("Main: Setting event") e.set() time.sleep(1.0) print("Main: Done") t.join() </lang>
Racket
Racket comes with events as part of its implementation; various types of events are used for different purposes: there are events that become ready when some input is available in a port, when a TCP connection is made, when a thread is dead, etc etc. Here we use a simple alarm event as requested, even though it's a odd to send the actual event result to the task (since it's a useless value):
<lang racket>
- lang racket
(define task (thread (lambda () (printf "Got: ~s\n" (thread-receive)))))
(thread-send task ; wait for it, then send it
(sync (alarm-evt (+ 1000 (current-inexact-milliseconds)))))
(void (sync task)) ; wait for the task to be done before exiting </lang>
Raku
(formerly Perl 6)
<lang perl6>note now, " program start"; my $event = Channel.new;
my $todo = start {
note now, " task start"; $event.receive; note now, " event reset by task";
}
note now, " program sleeping"; sleep 1; note now, " program signaling event"; $event.send(0); await $todo;</lang>
- Output:
Instant:1403880984.089974 program start Instant:1403880984.095400 program sleeping Instant:1403880984.095491 task start Instant:1403880985.099381 program signaling event Instant:1403880985.109395 event reset by task
See also Handle_a_signal#Perl_6 for an example of using Supplies to do reactive programming based on events (Unix signals in this case).
REXX
Although REXX can be event driven, most events would probably have to be actively checked to see if the event occurs.
Here is a time-driven example of events happening, based on specific timer ticks.
<lang rexx>/*REXX program demonstrates a method of handling events (this is a time─driven pgm).*/
signal on halt /*allow the user to HALT the program.*/
parse arg timeEvent /*allow the "event" to be specified. */
if timeEvent= then timeEvent=5 /*Not specified? Then use the default.*/
event?: do forever /*determine if an event has occurred. */
theEvent=right(time(),1) /*maybe it's an event, ─or─ maybe not.*/ if pos(theEvent,timeEvent)\==0 then signal happening end /*forever*/
say 'Control should never get here!' /*This is a logic can─never─happen ! */ halt: say '════════════ program halted.'; exit /*stick a fork in it, we're all done. */ /*──────────────────────────────────────────────────────────────────────────────────────*/ happening: say 'an event occurred at' time()", the event is:" theEvent
do while theEvent==right(time(),1) nop /*replace NOP with the "process" code.*/ end /*while*/ /*NOP is a special REXX statement. */
signal event? /*see if another event has happened. */</lang> output when using the input of: 1 3 5 0 7 9
an event occurred at 16:13:29, the event is: 9 an event occurred at 16:13:30, the event is: 0 an event occurred at 16:13:31, the event is: 1 an event occurred at 16:13:33, the event is: 3 an event occurred at 16:13:35, the event is: 5 an event occurred at 16:13:37, the event is: 7 an event occurred at 16:13:39, the event is: 9 an event occurred at 16:13:40, the event is: 0 an event occurred at 16:13:41, the event is: 1 an event occurred at 16:13:43, the event is: 3 an event occurred at 16:13:45, the event is: 5 an event occurred at 16:13:47, the event is: 7 an event occurred at 16:13:49, the event is: 9 an event occurred at 16:13:50, the event is: 0 an event occurred at 16:13:51, the event is: 1 an event occurred at 16:13:53, the event is: 3 ════════════ program halted.
Rust
Rust ensures memory safety at compile-time without needing a garbage collector or runtime. There are several concurrency primitives in it's standard library.
<lang Rust> use std::{sync::mpsc, thread, time::Duration};
fn main() -> Result<(), Box<dyn std::error::Error>> {
println!("[1] Starting"); let (tx, rx) = mpsc::channel(); thread::spawn(move || { println!("[2] Waiting for event"); rx.recv(); println!("[2] Received event"); }); thread::sleep(Duration::from_secs(1)); println!("[1] Sending event"); tx.send(())?; thread::sleep(Duration::from_secs(1));
Ok(())
} </lang>
Tcl
Tcl has been event-driven since 7.5, but only supported channel and timer events (plus variable traces, which can be used to create event-like entitites). With the addition of coroutines, it becomes much simpler to create general events:
<lang tcl># Simple task framework built from coroutines proc pause ms {
after $ms [info coroutine];yield
} proc task {name script} {
coroutine $name apply [list {} \ "set ::tasks(\[info coro]) 1;$script;unset ::tasks(\[info coro])"]
} proc waitForTasksToFinish {} {
global tasks while {[array size tasks]} {
vwait tasks
}
}
- Make an Ada-like event class
oo::class create Event {
variable waiting fired constructor {} {
set waiting {} set fired 0
} method wait {} {
while {!$fired} { lappend waiting [info coroutine] yield }
} method signal {} {
set wake $waiting set waiting {} set fired 1 foreach task $wake { $task }
} method reset {} {
set fired 0
}
}
- Execute the example
Event create X task A {
puts "waiting for event" X wait puts "received event"
} task B {
pause 1000 puts "signalling X" X signal
} waitForTasksToFinish</lang> Output:
waiting for event signalling X received event
Of course, the classic way of writing this is much shorter, but intermingles the tasks: <lang tcl>after 1000 set X signalled puts "waiting for event" vwait X puts "received event"</lang>
zkl
zkl provides an Atomics library for things like this. Events are async, waiting for an event doesn't poll. <lang zkl>var event=Atomic.Bool(); // False
// create thread waiting for event
fcn(event){event.wait(); println(vm," ping!")}.launch(event); Atomic.sleep(1); event.set(); println("done")</lang>
- Output:
// snooze done // setting is fast, receiving maybe not so VM#4 ping! // and thread 4 exits
I ran this from the REPL so I didn't have to worry about the main thread exiting and nuking the child thread.
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