From:  Alexander Surkov <surkov.alexander@gmail.com>
Date:  27 Mar 2014 03:59:35 Hong Kong Time
Newsgroup:  news.mozilla.org/mozilla.accessibility
Subject:  

Re: IPC plans

NNTP-Posting-Host:  63.245.216.66

On Wed, Mar 26, 2014 at 3:34 PM, Trevor Saunders wrote:

> On Wed, Mar 26, 2014 at 03:13:58PM -0400, Alexander Surkov wrote:
> >
> >
> >
> > On Wed, Mar 26, 2014 at 2:49 PM, Trevor Saunders <
> trev.saunders@gmail.com>
> > wrote:
> >
> >     On Wed, Mar 26, 2014 at 09:04:36AM -0400, Alexander Surkov wrote:
> >     > Hi.
> >     >
> >     > 1) If you plan to make AT to talk to the main process only then it
> means
> >     the AT
> >     > should see a whole MSAA tree as unique tree, the same trick can be
> done
> >     for
> >     > XPCOM. I.e. the way XPCOM tree is implemented shouldn't be
> different from
> >     MSAA
> >     > tree implementation.
> >
> >     I agree the xpcom tree *can* all be in one process, but I'm not sure
> I
> >     agree its either desirable or easier than the alternative.
> >
> >
> > AccessFu is not quite different from platform screen readers by its
> nature. I'm
> > pretty sure they will need a mechanism to cross the boundaries between
> > processes. So either it'll be one tree or multiple trees with
> cross-platform
> > bridges.
>

> aiui they already deal with multiple processes because they work on b2g
> and they deal with it with content scripts and stuff in JS.  So since
> AcessFoo seems to already work in a multiprocess world I just assume
> leave it as is unless we have a reason to change it.
>

If it works then fine (I'd love to hear Eitan or Yura on this). One tree
might be useful for tools like DOMi though. But it can be an emulation of
the unique tree I guess on DOMi side.

>     > 2) I assume that you can have sync access to content process DOM
> tree
> >     from the
> >
> >     you can't unless we add it, and we'd need to add sync access to the
> >     frame tree too...
> >
> >
> >     > main process, so if the main process doesn't dupe DOM trees of
> content
> >     > processes then it's good to explain why accessibility tree is
> different.
> >
> >     there's just no reason to have general access to the dom in the child
> >     process from the parent.
> >
> >
> > I'm curious though how content plugins are implemented. Their head
> should be in
> > chrome process but they operate on content.
>
> I'm not sure what you mean "content plugins"
>

Firefox add-ons that deal with content. I can see they took a different way
on their implementation so it doesn't look like it requires sync
communications (
https://developer.mozilla.org/en-US/Add-ons/SDK/Guides/Content_Scripts)


>
> >     > 3) Didn't you consider a scenario when AT communicates with each
> process?
> >
> >     not really, it would be hard to sand box, and it would require a
> bunch
> >     of tricky platform specific code for each platform, and its not even
> >     clear to me its possible on mac.
> >
> >
> > The stake is to win in memory and in performance. Even if it requires
> platfrom
>
> it looses on security and it would be a lot of pretty evil platform
> specific code assuming it even works on mac and I'm pretty sure the
> android stuff would need its own special IPC stuff anyway.
>

ATK makes one-tree magic on their side afaik, so if we had disconnected
trees in each process then it should be ok for ATK. Windows is HWND based
so they are quite different. If we learned how to let AT to interact with
HWNDs from content processes then it'd be awesome since we could have
disconnected trees again. If that's a security treat then we need to work
with security team to figure out how that's bad because otherwise we can
end up with unperformant and memory heavy solution.


>
> > specific code then it's worth to consider the option (it seems Hub
> agrees on
> > it).
>
> I haven't seen him say anything...
>

He said:
"I'm also concerned that using more memory might be a problem on memory
constrained platform like mobile phone (we have less than half a GB of
RAM and then we have "containers" for each app). Just to keep that in mind."


>
> Trev
>
> >
> >
> >
> >     Trev
> >
> >     >     Hi,
> >     >
> >     >      This mail is going to attempt to describe how accessibility
> will
> >     work
> >     >      in e10ns Gecko. follow ups to dev-accessibility is probably
> best.
> >     >
> >     >     tldr:
> >     >     - the operating system level accessibility API will only talk
> to the
> >     >       main process
> >     >     - accessibility information will be computed in the same
> process as
> >     the
> >     >       content it is for.
> >     >     - Where it is advantagious we will cache information in the
> parent
> >     >       process instead of blocking on IPC.
> >     >
> >     >     details:
> >     >     The main process will have a tree caching data for each sub
> tree in a
> >     >     content process.  The tree of documents in in each of these
> subtrees
> >     >     will be updated with PContent::PDocAccessibleConstructor() and
> >     >     PDocAccessible::__delete__(), and the tree of accessibles in
> each
> >     >     document will be kept up to date with show and hide events.
>  This
> >     means
> >     >     caching the arrangement of accessibles in the tree is easy, and
> >     probably
> >     >     makes things simpler in addition to faster.  At least in
> theory we
> >     can
> >     >     cache everything that always fires an event when it changes,
> but I
> >     >     expect at first we'll only cache the tree and then add more
> caching
> >     >     based on performance data.
> >     >
> >     >     I plan on having a set of ipdl actors per document and then
> using
> >     >     integer ids to refer to accessibles within that document, we
> can
> >     >     repurpose the mAccessiblecache which maps pointers to
> accessibles to
> >     >     themselves to map ids to accessibles in a manor that is safe.
>  This
> >     >     saves the over head we'd have if we had an actor per
> accessible which
> >     >     would result in thousands of actors for sessions with many
> tabs.
> >     >
> >     >     unresolved issues:
> >     >     - xpcom API should it be one tree or tree per process? afaik
> >     AccessFoo
> >     >       assumes tree per process and our test suite assumes one tree
> for
> >     >     everything, so we need to reconsile this somehow, but I'm not
> sure it
> >     >     needs to happen immediately.
> >     >
> >     >     - we need to use the ipc infrastructure in the platform layer
> to
> >     handle
> >     >       accessible objects for content in child processes I'm not
> exactly
> >     sure
> >     >     how this should look some ideas would include sub classing
> accessible
> >     /
> >     >     making it more generic and having a proxy and local
> implementation,
> >     or
> >     >     maybe it would be better for platform API methods to explicitly
> >     handle
> >     >     remote accessibles with something like if accWrap
> accWrap->FOobar()
> >     else
> >     >     remoteDoc->Foobar(accWrapId) but I figure we can figure this
> out once
> >     we
> >     >     have the plumbing either case will need to call sorted out.
> >     >
> >     >     question / comments / whatever welcome!
> >     >
> >     >     Trev
> >     >
> >     >     _______________________________________________
> >     >     accessibility mailing list
> >     >     accessibility@lists.mozilla.org
> >     >     https://lists.mozilla.org/listinfo/accessibility
> >     >
> >     >
> >     >
> >
> >
>