Event Handler (C# and JavaScript Cross Reference Guide)

In computer programming, an event handler is part of event driven programming where the events are created by the framework based on interpreting inputs. Each event allows you to add code to an application-level event generated by the underlying framework, typically GUI triggers such as a key press, mouse movement, action selection, and an expired timer. In addition, events can represent data changes, new data, etc. Specifically, an event handler is an asynchronous callback subroutine that handles inputs received in a program.

A custom event is a programmer created event. For example, you can contrast an event handler with a member event, an OOP concept where you add an event to a class.

Languages Focus

Many development environments and compilers provide for event driven programming, a standard set of application events such as startup, end, on click of a button, etc. This section documents the applicaton event handler or an overview for each language.

For OOP languages, do not confuse this section with class member events discussed in the OOP Details section of our Cross Reference Coding Encyclopedia.

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JavaScript:  The JavaScript event handler contains events centered around the Document Object Model (DOM). Common events include onMouseOver and onMouseOut, onFocus and onBlur, onClick and onDblClick, onChange and onSelect, onLoad and onUnload. For example, onMouseOver and onMouseOut are frequently used with websites to change an image when your mouse moves over it. The onClick event is used to trigger code upon a mouse click. Syntax Example: In the following example, we use a standard image tag set to an image, change it on mouse over, then set it back on mouse out. <img id="Image1" src="i_search.gif" onmouseover="document.images['Image1'].src='i_coat.gif';" onmouseout="document.images['Image1'].src='i_search.gif';">  JavaScript Event Model