*/ 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 0 /*stick a fork in it, we're all done. */ if timeEvent = '' then timeEvent = 5 /*Not specified? Then use the default.*/Įvent ?: do forever /*determine if an event has occurred. *REXX program demonstrates a method of handling events (this is a time─driven pgm).*/ signal on halt /*allow user to HALT (Break) the pgm.*/ parse arg timeEvent /*allow the "event" to be specified. Use AnyEvent my $quit = AE:: cv sub puts ( 1, "main" ) showtime () sleep ( 1 ) puts ( 1, "free" ) showtime () leave_cs ( lock ) wait_thread ( threads ) puts ( 1, "done\n" ) end procedure main () To catch an event, a corresponding event handler - a function with a predefined name - has to be definined in the code. Lingo/Director uses (stateless) events for system/application state change notifications, user action notifications and inter-sprite communication. That is what should be used for any non-toy example or project. OTP comes with a gen_event behavior that is more robust and resilient than this version. ( 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 )))) ( ns re ( :require [ :refer [>! !! ! c "reset" ) Send message to task )))) Invoke -main function ( -main ) Here is an implementation of the manual-reset event. Write a program that waits 1s and then signals the event to a task waiting for the event.Īda provides higher-level concurrency primitives, which are complete in the sense that they also allow implementations of the lower-level ones, like event. Show how a manual-reset event can be implemented in the language or else use an API to a library that provides events. once signaled it remains in this state until it will be explicitly reset.Ī pulse event when signaled releases all tasks awaiting it and then is automatically reset. This event changes its state by an explicit request of a task. For this reason real-time systems tend to polling schemes, trading performance for predictability in the worst case scenario. They face race condition, deadlocking, live locks and priority inversion. At the same time event-driven architectures suffer to the problems related to their unpredictability. They are flexible and have relatively short response times. As the name suggests busy waiting consumes system resources even when the external activity does not change its state.Įvent-driven architectures are widely used in GUI design and SCADA systems. The opposite approach is polling sometimes called busy waiting, when the synchronization is achieved by an explicit periodic querying the state of the activity. 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. Events augmented by data and/or publisher-subscriber schemes are often referred as messages, signals etc. It neither identify the state that caused it signaled, nor the source of, nor who is the subject of notification. Signaling an event from the hardware is accomplished by means of hardware interrupts.Įvent is a low-level synchronization mechanism. external, from the hardware, such as user input, timer, etc.internal, from another task, programmatically.In concurrent programming event also refers to a notification that some state has been reached through an asynchronous activity. Programmatically controlled events can be set by a task into one of its states. Releasing waiting tasks is called event notification. It is released once the state is entered. A task may await for the event to enter the desired state, usually the signaled state. An event has two states signaled and reset. You are encouraged to solve this task according to the task description, using any language you may know.Įvent is a synchronization object.
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