WebReference.com - Part 2 of Chapter 6 from Dynamic HTML: The Definitive Reference, 2nd Edition. From O'Reilly (3/8). | WebReference

WebReference.com - Part 2 of Chapter 6 from Dynamic HTML: The Definitive Reference, 2nd Edition. From O'Reilly (3/8).

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Dynamic HTML: The Definitive Reference, 2E. Chapter 6: Scripting Events

Event Propagation

In some DHTML applications, it is not efficient to have target elements process events. For example, if you have a page that allows users to select and drag elements around the page, it is quite possible that one set of centralized functions can handle that operation for all elements. Rather than define event handlers for all of those elements, it is better to have the mouse-related events go directly to an object or element that has scope over all the draggable elements. In other words, one event handler can do the job of a dozen. For this kind of treatment to work, events must be able to propagate through the hierarchy of objects or nodes in the document. IE 5 and later and the W3C (Netscape 6) event models share some, but not all, event propagation schemes. For the most typical applications, you can easily equalize the small differences in implementation details and syntax you use to override the natural flow.

W3C DOM event propagation in Netscape 6 and later can be summarized thus: in response to a user or system action, an event starts at the outermost container and follows the most direct route ("trickles down") through the node container hierarchy to the intended target; after it reaches its target, the event reverses course and "bubbles upward" through the same node hierarchy back to the top, from which it disappears. The trickle-down portion of the journey is called the capture phase, while the return trip is called the bubbling phase. The IE propagation model consists only of the bubbling phase. While IE 5 and later has an event feature related to capture (described later in this chapter), its operation is not along the lines of the W3C capture phase of propagation.

Consider the following skeletal structure of an HTML document:

<html>
<body>
    <form>
        <div id="div1">
            <input id="txt1" type="text">
        </div>
        <div id="div2">
            <input id="txt2" type="text">
        </div>
    </form>
</body>
</html>

As the user types into the txt2 text input field, an onkeypress event begins its journey at an outermost container in Netscape 6, works its way through containers on its way to the text box (where IE's event starts), and then goes back to the outermost container. The precise top-level container varies with browser version. For Netscape 6, the window object is the master container for event propagation purposes; IE holds the line at the document node. Figure 6-1 depicts the onkeypress event propagation sequence through the objects of this document for three different browser versions.

Figure 6-1 Sample event propagation sequences
Figure 6-1 Sample event propagation sequences

You can assign an onkeypress event handler for any and all of the nodes in the hierarchy to process the event. By default, a Netscape 6 event handler "listens" for events only during the bubbling phase, which means that Netscape 6 behaves like IE. Event bubbling isn't as anarchic as it sounds. In fact, it's quite flexible if you're careful to avoid conflicts that may occur at higher containment levels.


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Created: September 9, 2002
Revised: September 9, 2002

URL: http://webreference.com/programming/javascript/dhtmlref/chap6/2/3.html