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

I'm feeling that there is a misunderstanding about the relation between DOM
tree and Composed Tree in this discussion.
If you understand the difference, the discussion might be more productive.

In short,
- Composed Tree DOES NOT replace DOM tree. Most of existing APIs work for
DOM tree. Composed Tree doesn't have any affect on (most of) existing APIs.
- Composed Tree is used in resolving CSS inheritance. That's all, except
for a few exception.


On Thu, May 7, 2015 at 10:18 AM Hayato Ito <hayato@chromium.org> wrote:

> On Wed, May 6, 2015 at 10:22 AM Ryosuke Niwa <rniwa@apple.com> wrote:
>
>>
>> > On May 5, 2015, at 11:55 AM, Tab Atkins Jr. <jackalmage@gmail.com>
>> wrote:
>> >
>> > On Tue, May 5, 2015 at 11:20 AM, Ryosuke Niwa <rniwa@apple.com> wrote:
>> >>> On May 4, 2015, at 10:20 PM, Anne van Kesteren <annevk@annevk.nl>
>> wrote:
>> >>>
>> >>>> On Tue, May 5, 2015 at 6:58 AM, Elliott Sprehn <esprehn@chromium.org>
>> wrote:
>> >>>> We can solve this
>> >>>> problem by running the distribution code in a separate scripting
>> context
>> >>>> with a restricted (distribution specific) API as is being discussed
>> for
>> >>>> other extension points in the platform.
>> >>>
>> >>> That seems like a lot of added complexity, but yeah, that would be an
>> >>> option I suppose. Dimitri added something like this to the imperative
>> >>> API proposal page a couple of days ago.
>> >>>
>> >>>
>> >>>> One thing to consider here is that we very much consider
>> distribution a
>> >>>> style concept. It's about computing who you inherit style from and
>> where you
>> >>>> should be in the box tree. It just so happens it's also leveraged in
>> event
>> >>>> dispatch too (like pointer-events). It happens asynchronously from
>> DOM
>> >>>> mutation as needed just like style and reflow though.
>> >>>
>> >>> I don't really see it that way. The render tree is still computed from
>> >>> the composed tree. The composed tree is still a DOM tree, just
>> >>> composed from various other trees. In the "open" case you can access
>> >>> it synchronously through various APIs (e.g. >>> if we keep that for
>> >>> querySelector() selectors and also deepPath).
>> >>
>> >> I agree. I don't see any reason node distribution should be considered
>> as a style concept. It's a DOM concept. There is no CSS involved here.
>> >
>> > Yes there is.  As Elliot stated in the elided parts of his quoted
>> > response above, most of the places where we update distribution are
>> > for CSS or related concerns:
>> >
>> > # 3 event related
>> > # 3 shadow dom JS api
>>
>> These two are nothing to do with styles or CSS.
>>
>>
> I'd like to inform all guys in this thread that Composed Tree is for
> resolving CSS inheritance by the definition.
> See the "Section 2.4 Composed Trees" in the spec:
> http://w3c.github.io/webcomponents/spec/shadow/#composed-trees
>
> Let me quote:
> > If an element doesn't participate in a composed tree whose root node is
> a document, the element must not appear in the formating structure [CSS21]
> nor create any CSS box. This behavior must not be overridden by setting the
> 'display' property.
>
> > In resolving CSS inheritance, an element must inherit from the parent
> node in the composed tree, if applicable.
>
> The motivation of a composed tree is to determine the parent node in
> resolving CSS inheritance. There is no other significant usages, except for
> event path.
>
>
>
>> >> I have issues with the argument that we should do it lazily.  On one
>> hand, if node distribution is so expensive that we need to do it lazily,
>> then it's unacceptable to make event dispatching so much slower.  On the
>> other hand, if node distribution is fast, as it should be, then there is no
>> reason we need to do it lazily.
>> >>
>> >> The problem is really the redistributions. If we instead had explicit
>> insertion points under each shadow host, then we wouldn't really need
>> redistributions at all, and node distribution can happen in O(1) per child
>> change.
>> >
>> > As repeatedly stated, redistribution appears to be a necessity for
>> > composition to work in all but the most trivial cases.
>>
>> Where?  I have not yet to see a use case for which selective
>> redistribution of nodes (i.e. redistributing only a non-empty strict subset
>> of nodes from an insertion point) are required.
>>
>> - R. Niwa
>>
>>