YUI 3 Cookbook

Chapter 4. Events

Browser applications operate inside an event loop. The event loop is a browser thread that collects events such as mouse actions and keypresses and passes these events back to the JavaScript engine. To handle events, applications must register callback functions that listen or subscribe to different event types. Since events are the only way for applications to respond to user actions, they are a fundamental component in almost any client-side JavaScript program.

Event handling has evolved over the years, with browsers accreting different behaviors and quirks around working with events. Despite well-meaning attempts to clean things up, many of these inconsistencies persist to the present day. In fact, the Event API is arguably even more volatile between different browsers than the Node API. Any application that relies on events needs to protect itself against this volatility.

YUI addresses the problem using the same strategy described in Chapter 2: by wrapping event objects in a consistent façade that replicates the W3C DOM Level 2 Event object, with the exception that all native HTMLElement references are replaced with YUI Node references. The YUI event façade normalizes all sorts of browser inconsistencies around event propagation and event object properties.

Beyond offering normalization and more pleasant APIs, the YUI event façade opens up the possibility of defining entirely new event types. YUI supports four basic categories of events:

• DOM events, which enable ...