Re: [webcomponents] Decoupling Custom Elements and Shadow DOM (Was Proposal for Cross Origin Use Case and Declarative Syntax)

On Dec 8, 2013, at 5:21 PM, Dominic Cooney <dominicc@google.com> wrote:
> On Fri, Dec 6, 2013 at 12:49 PM, Ryosuke Niwa <rniwa@apple.com> wrote:
> On Dec 5, 2013, at 7:32 PM, Scott Miles <sjmiles@google.com> wrote:
> > >> We don't think decoupling custom elements and shadow DOM completely is useful given that most important and natural use cases of custom elements involve the use of shadow DOM.
> >
> > Separating concerns is always useful, especially when it involves contentious standards.
> >
> > I also would like to point out that Mozilla's X-Tags/Brick custom elements library is built entirely without Shadow DOM.
> 
> Could you elaborate on what value does document.register add in that world?  Web developers could use mutation observers to manipulate elements of a given name and add a bunch of properties already.
> 
> Custom Elements adds a lot of value over Mutation Observers. Mutation Observers are great, but for other things. A couple of the benefits are:
> 
> Instances get callbacks/the right prototype more eagerly than they do with Mutation Observers. If element authors follow local discipline, they end up with a composition of elements that is consistent. This makes it easier to create more robust applications. Technically, more eager callbacks (more eager than typical uses of Mutation Observers, but less eager than Custom Elements) are possible with Mutation Observers through takeRecords. However it turns out this is infeasible because developers end up cargo-culting takeRecords. (It doesn't work? Add a call to the thing that flushes changes with takeRecords! Something else broke? ...)

That seems like a problem/limitation with mutation observers that we should fix regardless of custom elements.

> It is easier to get consistent availability of the element than it is with Mutation Observers. For example, Custom Elements are upgraded even if the element instance is not in the document. With Mutation Observers this is extremely difficult because it requires registering observers as subtrees are disconnected... and modifications including disconnections may have happened in the interim. With Custom Elements, the responsibility of understanding how operations affect elements lies with the UA, which has comprehensive knowledge of this now and into the future, and not with the author.

On the other hand, this is a good point.  Thanks for the clarification.

If we should probably include it in http://w3c.github.io/webcomponents/spec/custom/#motivations

- R. Niwa