EventHandling

EventHandling is the mechanism by which software components respond to events. It is central to event-driven programming, where program flow is determined by events such as user actions, system messages, or sensor data.

An event source generates events. Listeners or handlers are functions or objects registered to be notified when specific event types occur. When an event occurs, an event object may be created containing metadata (type, source, payload) and passed to the handler.

A dispatcher or event loop routes events from sources to appropriate handlers. In many systems, handlers execute asynchronously, enabling responsiveness. In synchronous models, handlers run in the same thread and may block.

Common approaches include callbacks, the observer pattern, and publish/subscribe. Event delegation is a technique where a single handler manages events from multiple elements. In the browser, events bubble up through a hierarchy and can be captured or bubbled.

Applications span graphical user interfaces, web development, and server-side platforms like Node.js. Design considerations include performance, thread safety, ordering guarantees, error handling, and the possibility of cancelable or cancellable events.