From:  Trevor Saunders <>
Date:  27 Mar 2014 02:49:14 Hong Kong Time

Re: IPC plans


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.

> 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.

> 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.


>     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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