On 10/12/2011 02:00 PM, Sean Hogan wrote:
On 12/10/11 3:26 AM, Tab Atkins Jr. wrote:
On Mon, Oct 10, 2011 at 7:51 PM, Sean Hogan<shogu...@westnet.com.au>
On 24/09/11 7:16 AM, Adam Klein wrote:
- Is free of the faults of the existing Mutation Events mechanism
(enumerated in detail here:

A simpler solution that is free from the faults listed in that email
be to have (at max) one mutation observer for the whole page context. I
guess this would be called at the end of the task or immediately
before page

If a js lib (or multiple libs) want to provide finer grained mutation
handling then let them work out the details.
That seems unworkably restrictive. It's very easy to imagine multiple
libraries listening for different kinds of things at the same time.
Libraries would just end up re-implementing event distribution, which
is something we can avoid by doing it correctly now.

This proposal doesn't entirely avoid the issue of event distribution.
There is no equivalent of event.stopPropagation() and hence no way to
prevent mutation records being delivered to observers. The observers may
have to be written with this is in mind.

For example, what if two observers can potentially handle the same
mutation - which one should handle it?
Both. Or the observers need to somehow communicate with each others to decide who handles it. This is no different to event listeners. Event listeners don't know if there are other listeners before them or after them. You can have several listeners in the same target and different script libraries may have added them without knowing about each others.

Alternatively, some code might respond to an attribute by adding content
to the DOM. What if there are mutation listeners that could respond to
that added content? Is it desired that they ignore or handle it?

Another pattern that doesn't seem to be reliably handled is mutations
within DOM fragments that are temporarily removed from the document.
That is:
- if the fragment always remains in the document then all mutations can
be monitored by observers on the document (or document.body), but
- if the fragment is removed from the document followed by mutation
observers being called, then any further mutations won't be delivered to
the observers, even when the fragment is reinserted into the document.
> The exact behavior in this scenario depends on whether mutations
> complete within one microtask or more than one

If the modifications to the fragment are done during the same microtask, then the observer will just get notified about those modifications. If in different microtask, then observer should observe that fragment (so when the fragment is removed from document, observer.observe(root_of_fragement, options) should be called.).

If there was just a global - per document observers, those wouldn't
handle all the cases when node is adopted to and from other documents.
Also, such observers would make all the DOM mutations slower, since
the callback would need to be called all the time.

The proposed API allows one to restrict mutation observing to certain set of nodes. Mutations outside that set can be kept as fast as having no mutationobservers at all. And also, since the observed set can expand, and isn't limited to same document handling, it can easily handle cases when nodes are moved to some other document.



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