Re: Imperative API for Node Distribution in Shadow DOM (Revisited)

On Fri, May 1, 2015 at 2:59 AM Ryosuke Niwa <rniwa@apple.com> wrote:

>
> > On Apr 30, 2015, at 6:00 AM, Domenic Denicola <d@domenic.me> wrote:
> >
> >> This essentially forces distribution to happen since you can observe
> the result of distribution this way. Same with element.offsetWidth etc. And
> that's not necessarily problematic,
> >
> > OK. So the claim that the current spec cannot be interoperably
> implemented is false? (Not that I am a huge fan of <content select>, but I
> want to make sure we have our arguments against it lined up and on solid
> footing.)
> >
> >> but it is problematic if you want to do an imperative API as I tried to
> explain in the bit you did not quote back.
> >
> > Sure, let's dig in to that claim now. Again, this is mostly clarifying
> probing.
> >
> > Let's say we had an imperative API. As far as I understand from the
> gist, one of the problems is when do we invoke the distributedCallback. If
> we use MutationObserve time, then inconsistent states can be observed, etc.
> >
> > Why can't we say that this distributedCallback must be invoked at the
> same time that the current spec updates the distribution result? Since it
> sounds like there is no interop problem with this timing, I don't
> understand why this wouldn't be an option.
>
> There will be an interop problem. Consider a following example:
>
>
The return value of (2) is the same in either case. There is no observable
difference. No interop issue.

Please file a bug for the spec with a concrete example if you can find a
observable difference due to the lazy-evaluation of the distribution.



> ```js
> someNode = ~
> myButton.appendChild(someNode); // (1)
> absolutelyPositionElement.offsetTop; // (2)
> ```
>
> Now suppose absolutelyPositionElement.offsetTop is a some element that's
> in a disjoint subtree of the document. Heck, it could even in a separate
> iframe. In some UAs, (2) will trigger style resolution and update of the
> layout. Because UAs can't tell redistribution of myButton can affect (2),
> such UAs will update the distribution per spec text that says "the
> distribution result must be updated before any _use_ of the distribution
> result".
>
> Yet in other UAs, `offsetTop` may have been cached and UA might be smart
> enough to detect that (1) doesn't affect the result of
> `absolutelyPositionElement.offsetTop` because they're in a different parts
> of the tree and they're independent for the purpose of style resolution and
> layout. In such UAs, (2) does not trigger redistribution because it does
> not use the distribution result in order to compute this value.
>
> In general, there are thousands of other DOM and CSS OM APIs that may or
> may not _use_ the distribution result depending on implementations.
>
> - R. Niwa
>
>
>