Event (computing)

In computing, an event is an action or occurrence recognized by software, often originating asynchronously from the external environment, that may be handled by the software. Because an event is an entity which encapsulates the action and the contextual variables triggering the action we can use the acrostic mnemonic of an event as an "Execution Variable Encapsulating Named Trigger" to clarify the concept. Computer events can be generated or triggered by the system, by the user or in other ways. Typically, events are handled synchronously with the program flow, that is, the software may have one or more dedicated places where events are handled, frequently an event loop. A source of events includes the user, who may interact with the software by way of, for example, keystrokes on the keyboard. Another source is a hardware device such as a 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, often with the goal of being interactive.
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.
In computer programming, an event handler is a callback subroutine that handles inputs received in a program (called a listener in Java and JavaScript). Each event is a piece of application-level information from the underlying framework, typically the GUI toolkit. GUI events include key presses, mouse movement, action selections, and timers 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. On this level, interrupt handlers and signal handlers correspond to event handlers.