Chapter 12. Events and Event Handling

In general terms, an event is a noteworthy runtime occurrence that has the potential to trigger a response in a program. In ActionScript, events can be broken into two categories: built-in events, which describe changes to the state of the runtime environment, and custom events, which describe changes to the state of a program. For example, a built-in event might be the clicking of the mouse or the completion of a file-load operation. By contrast, a custom event might be the ending of a game or the submission of an answer in a quiz.

Events are ubiquitous in ActionScript. In fact, in a pure ActionScript program, once the main-class constructor method has finished executing, all subsequent code is triggered by events. Accordingly, ActionScript supports a rich event architecture that provides the foundation for both built-in and custom events.