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The DOM event flow

The DOM introduces a new concept for detecting events and assigning corresponding event handlers to react to them. Naturally, it also supports the 2 conventional techniques discussed earlier.

In order to fully understand and appreciate this new method, we must first understand how events are handled in the DOM.

-A roundtrip ticket- event capture and event bubbling

The DOM interprets all user action very differently from the user. To you, clicking on a link is just that, but to the DOM, an entire process is invoked that takes the action on a trip around the block and back. Ok, more explanation is definitely needed.

Let's take the simple user action of moving the mouse over a link as an example. To the DOM, the following events in fact took place, and in that order:

1) You moved your mouse over the document
2) You moved your mouse over any tags containing the target <A> tag
3) You moved your mouse over the <A> tag in question

No, we're not quite yet done. From here the event then travels back:

4) You moved your mouse over any tags containing the target <A> tag
5) You moved your mouse over the document. (the end).

Yes, the DOM likes to get very technical. Using a diagram, here's what just happened:

Mouse over document -->Mouse over any containment tags of <A> --> Mouse over destination <A> -->
Mouse over any containment tags of <A> --> Mouse over document

The above "roundtrip" model is used by the DOM to interpret most events. We call the event as it travels to the intended element (part in blue) event capture, and as it travels back event bubble.

-Event Capture

So, event capture basically refers to the process of an event as it travels to its destination element. Actually, it further refers to the ability to "capture", or intercept this event.

If event capture sounds suspiciously familiar, it should. NS4 has long supported event capture, though the syntax:

element.captureEvents()

In other words, when you click on a link in NS4 for example, this event first travels through the document, then incrementally downwards to all containing elements of the link until eventually reaching the link itself.

-Event bubble

Event bubble is the exact opposite of event capture, and points to the traveling upwards of an event from the source element to the topmost, or document. IE4 revolutionized event handling by being the first major browser to support event bubble. An event such as clicking on a link causes it to travel upwards, visiting all of the link's container tags until eventually reaching the top of the document source.

Consider the following HTML code:

<table onClick="alert('Hello!')"> 
<b><a href="#">This is some text</a></b> 
</table> 

Here we define an onClick event handler inside the TABLE. With event bubbling, merely clicking on the containing link fires this event (and subsequently the alert message), since the event traverses upwards from A to B and eventually reaching TABLE.

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