17.6. The onload Event
Example 17-7. Portable event registration for onload event handlers
17.7. Synthetic Events
Both the DOM Level 2 event model and the IE event model allow you to create synthetic event objects and dispatch them to event handlers registered on document elements. In essence, this is a technique for tricking browsers into invoking the event handlers registered on an element (and, in the case of bubbling events, the handlers registered on the ancestors of the element). With the Level 0 event model, synthetic events are not necessary because event handlers are available through the various event handler properties. In the advanced event models, however, there is no way to query the set of handlers registered with addEventListener or attachEvent, and those handlers can only be invoked using the techniques demonstrated in this section.
In the DOM event model, you create a synthetic event with Document.createEvent( ), initialize that event with Event.initEvent( ), UIEvent.initUIEvent( ), or MouseEvent.initMouseEvent( ) and then dispatch the event with the dispatchEvent method of the node to which it is to be dispatched. In IE, you create a new event object with Document.createEventObject and then dispatch it with the fireEvent( ) method of the target element. Example 17-8 demonstrates these methods. It defines a cross-platform function for dispatching synthetic dataavailable events and a function for registering event handlers for events of that type.
It is important to understand that synthetic events dispatched with dispatchEvent( ) and fireEvent( ) are not queued and handled asynchronously. Instead, they are dispatched immediately, and their handlers are invoked synchronously before the call to dispatchEvent( ) or fireEvent( ) returns. This means that dispatching a synthetic event is not a technique for deferring execution of code until the browser has handled all pending events. For that, it is necessary to call setTimeout( ) with a timeout value of 0 milliseconds.
It is possible to synthesize and dispatch low-level raw events such as mouse events, but it is not well specified how document elements respond to these events. It is typically more useful to use this functionality for higher-level semantic events to which the browser does not have a default response. This is why Example 17-8 uses the dataavailable event type.
Example 17-8. Dispatching synthetic events