Events

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Task
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[edit]

Manual-reset event[edit]

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[edit]

A pulse event when signaled releases all tasks awaiting it and then is automatically reset.

Sample implementations / APIs[edit]

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[edit]

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:

protected type Event is
procedure Signal;
procedure Reset;
entry Wait;
private
Fired : Boolean := False;
end Event;

The event implementation:

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;

With the event defined above:

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;

Sample output:

A is waiting for X
Signal X
A received X

AutoHotkey[edit]

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

BBC BASIC[edit]

API[edit]

This uses a Windows event object:

      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

Native[edit]

This uses a simple variable as a semaphore:

      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

C[edit]

Using pipe to communicate to forked child. Since child will be blocking trying to read the other end of the pipe, this can be used for synchronization.

#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;
}

C#[edit]

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();
}
}

Sample output:

10-11-2010 18:35:11
10-11-2010 18:35:12

Clojure[edit]

Translation of: Go
(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)
 
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[edit]

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.

Sample output:

09.08.2011 0:27:43
09.08.2011 0:27:44

E[edit]

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
}

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().

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...")
}

Elixir[edit]

Translation of: Erlang
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
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[edit]

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.

 
-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).
 

Output:

 
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
 

F#[edit]

Translation of: C#
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

Gambas[edit]

Public Sub Timer1_Timer()
 
Print Str(Time(Now))
 
End

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[edit]

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.

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)
}
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[edit]

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
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."

Note: Because there is no serialization of the text output, there is a chance that it will appear interleaved.

Icon and Unicon[edit]

The following only works in Unicon. The example illustrates the multiple tasks can receive the same event:

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

Sample run:

->event
Task two waiting for event....
Task one waiting for event....
Signalling event.
Task two received event.
Task one received event.
->

Julia[edit]

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.

 
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()
[email protected](printnotice(condition))
[email protected](dolongcomputation(condition))
end
 
delegate()
sleep(5)
println("Done sleeping.")
 
Output:

Starting task, sleeping... They are finished. Done sleeping.

LFE[edit]

Translation of: Erlang

Paste in the REPL:

 
(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))))
 

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_event behavior that is more robust and resilient than this version. That is what should be used for any non-toy example or project.

Lingo[edit]

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:

-- the current window was closed
on closeWindow
...
end
 
-- the left mouse button was pressed by the user
on mouseDown
...
end

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:

-- 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)

Using a binary plugin ("Xtra"), in Windows also lower level window messages can be both sent and received:

Library: Msg Xtra
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)

JavaScript[edit]

An example using the YUI library:

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);
});

An example simulating DOM events:

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);
});

Mathematica[edit]

Mathematica supports events from timers (via Pause[]), task schedule descriptors. This will print a message after 4 seconds, then terminate the program.

Print["Will exit in 4 seconds"]; Pause[4]; Quit[]
->Will exit in 4 seconds

Nim[edit]

Translation of: C
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"


Oforth[edit]

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.

: anEvent
| ch |
Channel new ->ch
#[ ch receive "Ok, event is signaled !" println ] &
System sleep(1000)
ch send($myEvent) ;

An emitter is a general implementation for handling events : an emitter waits for events emitted and launches listeners that are waiting for those events.

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 ;

Oz[edit]

Translation of: Haskell

Events can be implemented as mutable references to dataflow variables:

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}

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:

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

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:

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

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[edit]

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:

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();

This is the same using AnyEvent simplified API:

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;

Perl 6[edit]

Translation of: Go
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;
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).

PicoLisp[edit]

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:

(alarm 1
(prinl "Exit in 4 seconds")
(alarm 4 (bye)) )

PowerShell[edit]

 
$timer = New-Object -TypeName System.Timers.Timer -Property @{Enabled=$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}
 
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[edit]

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

Racket[edit]

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
 
(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
 

REXX[edit]

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.

/*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. */

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.

Tcl[edit]

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:

Works with: Tcl version 8.6
# 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

Output:

waiting for event
signalling X
received event

Of course, the classic way of writing this is much shorter, but intermingles the tasks:

after 1000 set X signalled
puts "waiting for event"
vwait X
puts "received event"

zkl[edit]

zkl provides an Atomics library for things like this. Events are async, waiting for an event doesn't poll.

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")
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.