From:  Trevor Saunders <trev.saunders@gmail.com>
Date:  27 Mar 2014 03:34:32 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 03:13:58PM -0400, Alexander Surkov wrote:
> 
> 
> 
> On Wed, Mar 26, 2014 at 2:49 PM, Trevor Saunders 
> 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.

>     > 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"

>     > 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.

> specific code then it's worth to consider the option (it seems Hub agrees on
> it).

I haven't seen him say anything...

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
>     >
>     >     _______________________________________________
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>     >     accessibility@lists.mozilla.org
>     >     https://lists.mozilla.org/listinfo/accessibility
>     >
>     >
>     >
> 
>