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((Short description|Action or occurrence recognized by software))

In [[Computer programming|programming]] and [[software design]], an '''event''' is an action or occurrence recognized by [[software]], often originating [[asynchronous I/O|asynchronously]] from the external environment, that may be [[Event handler|handled]] by the software.  Computer events can be generated or triggered by the system, by the [[User (computing)|user]], or in other ways. Typically, events are handled [[synchronisation|synchronously]] with the [[Control flow|program flow]]; that is, the software may have one or more dedicated places where events are handled, frequently an [[event loop]]. 

The user can be the source of an event. The user may [[Human–computer interaction|interact]] with the software through the computer's [[peripheral]]s - for example, by typing on a [[Keyboard (computing)|akeyboard]] or clicking with a mouse. Another source is a [[Computer hardware|hardware]] device such as a [[Timer#Software_timers|timer]]. Software can also trigger its own set of events into the event loop, e.g. to communicate the completion of a task. Software that changes its behavior in response to events is said to be [[Event-driven programming|event-driven]], often with the goal of being [[Interactivity|interactive]].

== Description ==
Event driven systems are typically used when there is some asynchronous external activity that needs to be handled by a program; for example, a user who presses a button on their mouse. An event driven system typically runs an [[event loop]], that keeps waiting for such activities, e.g. input from devices or internal alarms. When one of these occurs, it collects data about the event and dispatches the event to the ''event handler'' software that will deal with it.

A program can choose to ignore events, and there may be libraries to dispatch an event to multiple handlers that may be programmed to ''listen'' for a particular event. The data associated with an event at a minimum specifies what type of event it is, but may include other information such as when it occurred, who or what caused it to occur, and extra data provided by the event ''source'' to the handler about how the event should be processed.

Events are typically used in user interfaces, where actions in the outside world (mouse clicks, window-resizing, keyboard presses, messages from other programs, etc.) are handled by the program as a series of events. Programs written for many windowing environments consist predominantly of event handlers.

Events can also be used at [[instruction set]] level, where they complement [[interrupt]]s. Compared to interrupts, events are normally implemented synchronously: the program explicitly waits for an event to be generated and handled (typically by calling an instruction that dispatches the next event), whereas an interrupt can demand immediate service.

== User-generated events ==
There are a large number of situations or events that a program or system may generate or respond to. Some common user generated events include:

===Mouse events===
<!--[[Mouse clicks]] redirects here-->
A [[pointing device]] can generate a number of software recognisable [[pointing device gesture]]s. A mouse can generate a number of mouse events, such as mouse move (including direction of move and distance), mouse left/right button up/down<ref name="event">[http://msdn2.microsoft.com/en-us/library/aa983670(VS.71).aspx Mouse and Keyboard Events in Windows Forms]. Microsoft. Retrieved on February 12, 2008.</ref> and [[Scroll wheel|mouse wheel]] motion, or a combination of these gestures. For example, [[double-click]]s commonly select words and characters within boundary, and [[triple-click]]s entire paragraphs.

===Keyboard events===
Pressing a key on a keyboard or a combination of keys generates a keyboard event, enabling the program currently running to respond to the introduced data such as which key/s the user pressed.<ref name="event"/>

===Joystick events===
Moving a [[joystick]] generates an X-Y analogue signal. They often have multiple buttons to trigger events. Some [[gamepad]]s for popular game boxes use joysticks. 

===Touchscreen events===
The events generated using a [[touchscreen]] are commonly referred to as [[touch event]]s or [[Gesture recognition|gesture]]s.

===Device events===
Device events include action by or to a device, such as a shake, tilt, rotation, move etc.

== Delegate event model ==
[[Image:GUI - Delegate Event Model.PNG|thumb|Delegate event model. <code>clickme</code> is the event source &ndash;a button in this example&ndash;, and it contains a list of listeners.]]
A common variant in [[object-oriented programming]] is the [[Delegation (programming)|delegate event model]], which is provided by some [[graphic user interface]]s. This model is based on three entities:
* a control, which is the event source
* [[observer pattern|listeners]], also called [[event handler]]s, that receive the event notification from the source
* [[Interface (computer science)|interfaces]] (in the broader meaning of the term) that describe the [[Protocol (object-oriented programming)|protocol]] by which the event is to be communicated.

Furthermore, the model requires that:
* every listener must implement the interface for the [[event-driven programming|event]] it wants to listen to
* every listener must register with the source to declare its desire to listen to the event
* every time the source generates the event, it communicates it to the registered listeners, following the protocol of the interface.

[[C Sharp (programming language)|C#]] uses events as special delegates that can only be fired by the class that declares it. This allows for better [[Abstraction (computer science)|abstraction]], for example:<ref>
((cite web
| access-date = 2011-08-05
| date       = 2002-03-25
| first      = Hanspeter
| last       = Mössenböck
| page       = 26
| publisher  = Institut für Systemsoftware, Johannes Kepler Universität Linz, Fachbereich Informatik
| title      = Advanced C#: Variable Number of Parameters
| url        = http://ssw.jku.at/Teaching/Lectures/CSharp/Tutorial/Part2.pdf))
</ref>

<syntaxhighlight lang="csharp">
delegate void Notifier (string sender);

class Model
{
    public event Notifier notifyViews;
    public void Change() { ... notifyViews("Model"); }
}

class View1
{
    public View1(Model model)
    {
        model.notifyViews += new Notifier(this.Update1);
    }

    void Update1(string sender)
    {
        Console.WriteLine($"{sender} was changed during update"); 
    }
}

class View2
{
    public View2(Model model)
    {
        model.notifyViews += new Notifier(this.Update2); 
    }

    void Update2(string sender)
    {
        Console.WriteLine($"{sender} was changed"); 
    }
}

class Test
{
    static void Main()
    {
        Model model = new Model();

        new View1(model);
        new View2(model);
        model.Change();
    }
}
</syntaxhighlight>

== Event handler ==
((see|Event-driven programming#Event handlers))
In computer programming, an ''event handler'' may be implemented using a [[callback (computer science)|callback]] [[subroutine]] that handles inputs received in a program (called a '''listener''' in [[Java (programming language)|Java]] and [[JavaScript]]<ref>((Cite web|url=https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener|title=EventTarget.addEventListener() - Web APIs &#124; MDN|website=developer.mozilla.org|date=11 March 2024 ))</ref>). Each ''event'' is a piece of application-level information from the underlying [[Application framework|framework]], typically the [[GUI toolkit]]. GUI events include [[computer keyboard|key]] presses, [[computer mouse|mouse]] movement, action selections, and [[timer]]s expiring. On a lower level, events can represent availability of new data for reading a file or network stream. Event handlers are a central concept in [[event-driven programming]].

The events are created by the framework based on interpreting lower-level inputs, which may be lower-level events themselves. For example, mouse movements and clicks are interpreted as menu selections. The events initially originate from actions on the operating system level, such as [[interrupts]] generated by hardware devices, software interrupt instructions, or state changes in [[Polling (computer science)|polling]]. On this level, [[interrupt handler]]s and [[signal handler]]s correspond to event handlers.

Created events are first processed by an ''event dispatcher'' within the framework. It typically manages the associations between events and event handlers, and may queue event handlers or events for later processing. Event dispatchers may call event handlers directly, or wait for events to be dequeued with information about the handler to be executed.

== Event notification ==

((Unreferenced section|date=June 2007))
''Event notification'' is a term used in conjunction with [[Telecommunication|communication]]s [[software]] for linking applications that generate small [[messages]] (the "events") to applications that monitor the associated conditions and may take actions triggered by events.

Event notification is an important feature in modern [[database]] systems (used to inform applications when conditions they are watching for have occurred), modern operating systems (used to inform applications when they should take some action, such as refreshing a window), and modern distributed systems, where the producer of an event might be on a different machine than the consumer, or consumers. Event notification platforms are normally designed so that the application producing events does not need to know which applications will consume them, or even how many applications will monitor the event stream.

It is sometimes used as a synonym for [[publish-subscribe]], a term that relates to one class of products supporting event notification in networked settings. The [[virtual synchrony]] model is sometimes used to endow event notification systems, and publish-subscribe systems, with stronger fault-tolerance and consistency guarantees.

== See also ==
* [[Callback (computer programming)]]
* [[Database trigger]]
* [[DOM events]]
* [[Event-driven programming]]
* [[Exception handling]]
* [[Interrupt handler]]
* [[Interrupt]]s
* [[Observer pattern]] (e.g., Event listener)
* [[Reactor pattern]] vs. [[Proactor pattern]]
* [[Signal programming]]
* [[Virtual synchrony]]

== References ==
((Reflist))

== External links ==
* Article ''[http://w3future.com/html/stories/callbacks.xml Event Handlers and Callback Functions]''
* [https://web.archive.org/web/20070211111543/http://atddoc.cern.ch/Atlas/Notes/061/Note061-1.html A High Level Design of the Sub-Farm Event Handler]
* [http://www.w3.org/TR/xml-events/Overview.html#section-eventhandlers An Events Syntax for XML]
* [https://web.archive.org/web/20110720015340/http://www.jini.org/wiki/Jini_Distributed_Events_Specification#Distributed_Events_and_Notifications Distributed Events and Notifications]
* [http://www.quirksmode.org/js/events_order.html Event order]
* ((Javadoc:SE|name=Java DOM Interface Event|org/w3c/dom/events|Event|module=java.xml)) [[Javadoc]] documentation
* ((Javadoc:SE|package=java.awt.event|java/awt/event|module=java.desktop)) Java package Javadoc API documentation
* ((Javadoc:SE|package=javax.swing.event|javax/swing/event|module=java.desktop)) [[Java package]] Javadoc API documentation
* [https://web.archive.org/web/20110719030547/http://hikwww2.fzk.de/hik/orga/verdi/rs/Dokumentation/Cpp/Cpp/ioc/tasks/t90us014.htm Write an Event Handler]

((DEFAULTSORT:Event (Computing)))
<!--Categories-->
[[Category:Computer programming]]
[[Category:Events (computing)| ]]
[[Category:Subroutines]]

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