[webkit-dev] DOMAttrModified events in WebKit

[Chris Marrin]

There is a bug posted (https://bugs.webkit.org/show_bug.cgi?id=8191) about the implementation of DOMAttrModified events. The implementation is quite far along and there are many posts begging for it, but it is stalled because of one comment (https://bugs.webkit.org/show_bug.cgi?id=8191#c17) which is opposed to it on the grounds that it would be slow and buggy, and that there is an alternative proposal.

I'm interested in this issue because of WebGL. There is currently an implementation of a subset of X3D in WebGL (http://x3dom.org), which allows you to add a 3D scene as a hierarchy of nodes rendered by WebGL. They have an example of using mutation events to allow the DOM to change the attributes of nodes in the hierarchy and have that redraw the rendered scene. That's just one of the many uses of mutation events. Others are sprinkled throughout the bug log.

I don't think the discussion here should be whether or not mutation events a A Good Thing or not. They are being put to good use in Firefox already and they are part of DOM Level 2 already. The question should be whether they would add value to WebKit without penalizing performance when they are not used. I think there is enough evidence to say that they would be useful for many purposes. And from a cursory look at the patch it appears that, while there would be overhead in doing the checks, it would be fairly minor unless an event were actually attached to an attribute. And more might be done to short circuit earlier and further reduce the overhead. But I haven't actually tried the patch to see what the baseline overhead is.

If we reach consensus that we should add this feature, I would be happy to get some numbers about the overhead.

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~Chris
cmarrin at apple.com