Introduction

This chapter is focused on events, which is an important concept in any JavaScript program. Although this chapter is brief, the concepts explained here are very important to understand when working with OpenLayers.

Events are the heart of JavaScript. They are the impulses that allow us to produce a reaction. As programmers of a mapping application, we are interested in reacting when the map zoom changes, when a layer is loaded, or when a feature is added to a layer. Every class susceptible to emit events is responsible for managing its listeners (those interested in being notified when an event is fired) and also to emit events under certain circumstances.

For example, we can register a function listening for the zoomend event on the OpenLayers.Map instance. Every time the map instance changes its zoom, it is responsible to trigger the zoomend event, so all its listeners will be notified by the new event.

To help in all this process, OpenLayers has the OpenLayers.Events class...