Event handler - Definition and Overview

An event handler is a part of a computer program created to tell the program how to act in response to a specific event (e.g. the clicking of a mouse or the dragging of a scrollbar or pressing a button). The programmer's custom event handling functions will be executed by the event dispatcher. The event dispatcher is part of the operating system that detects Graphical User Interface (GUI) events, and calls functions in the executing program to handle those events.

Without event handlers nothing would happen when you interacted with an event driven program, as most GUI programs are.

How event handlers function is highly dependent on the operating system, programming language, and GUI framework used. A programmer will need to consult the documentation provided with the language used to get the exact syntax and usage of event handling. A user only needs to know how their operating system functions. Users do not need to have any knowledge of the underlying event handler code.