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Quoting MSAA doc about implementing child IDs [1]: > # How Servers Implement Child IDs > > Server developers can assign child IDs to both simple elements and > accessible objects. However, the recommended approach is to support the > standard Component Object Model (COM) interface IEnumVARIANT in every > accessible object that has children. > > If you implement IEnumVARIANT, you must: > > * Enumerate all children, both simple elements and accessible objects. > Provide child IDs for all simple elements and provide the IDispatch to > each accessible object. > * For accessible objects, set the vt member of the VARIANT to > VT_DISPATCH. The pdispVal member must contain a pointer to the IDispatch > interface. Note that the VARIANT is allocated and freed by the client. > * For simple elements, the child ID is any 32-bit positive integer. > Note that zero and negative integers are reserved by Microsoft Active > Accessibility. Set the VARIANT structure vt member to VT_I4 and the lVal > member to the child ID. > > If you do not support IEnumVARIANT, you must assign child IDs and > number the children in each object sequentially starting with one. So far, LibreOffice was returning negative "child IDs" instead of pointers to accessible objects, which were not conformant to the MSAA specification and not accepted by NVDA as valid child IDs (s.a. discussion on the first version of my related NVDA pull request to fix the announcement of a single selected cell in Calc, [2]). Adapt that to return pointers to accessible objects and drop the now unused 'CMAccessible::Get_XAccChildID'. [1] https://docs.microsoft.com/en-us/windows/win32/winauto/how-servers-implement-child-ids [2] https://github.com/nvaccess/nvda/pull/13277 Change-Id: I52a6f637adf334dee66627e6992451e6d81a7c9a Reviewed-on: https://gerrit.libreoffice.org/c/core/+/129201 Tested-by: Jenkins Reviewed-by: Michael Weghorn <m.weghorn@posteo.de> |
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| .. | ||
| inc | ||
| source | ||
| CustomTarget_ia2_idl.mk | ||
| Library_uacccom.mk | ||
| Library_winaccessibility.mk | ||
| Makefile | ||
| Module_winaccessibility.mk | ||
| README.md | ||
| WinResTarget_uacccom.mk | ||
Windows Accessibility Bridge
This code provides a bridge between our internal Accessibility
interfaces (implemented on all visible 'things' in the suite: eg.
windows, buttons, entry boxes etc.) - and the Windows MSAA /
IAccessible2 COM interfaces that are familiar to windows users and
Accessible Technologies (ATs) such as the NVDA screen reader.
The code breaks into three bits:
-
source/service/- the UNO service providing the accessibility bridge. It essentially listens to events from the LibreOffice core and creates and synchronises COM peers for our internal accessibility objects when events arrive.
-
source/UAccCom/- COM implementations of the
MSAA/IAccessible2interfaces to provide native peers for the accessibility code.
- COM implementations of the
-
source/UAccCOMIDL/- COM Interface Definition Language (IDL) for UAccCom.
Here is one way of visualising the code / control flow
VCL <-> UNO toolkit <-> UNO a11y <-> win a11y <-> COM / IAccessible2
vcl/ <-> toolkit/ <-> accessibility/ <-> winaccessibility/ <-> UAccCom/
Threading
It's possible that the UNO components are called from threads other
than the main thread, so they have to be synchronized. It would be nice
to put the component into a UNO apartment (and the COM components into STA)
but UNO would spawn a new thread for it so it's not possible.
The COM components also call into the same global AccObjectWinManager
as the UNO components do so both have to be synchronized in the same way.
So we use the SolarMutex for all synchronization since anything else
would be rather difficult to make work. Unfortunately there is a
pre-existing problem in vcl with Win32 Window creation and destruction
on non-main threads where a synchronous SendMessage is used while
the SolarMutex is locked that can cause deadlocks if the main thread is
waiting on the SolarMutex itself at that time and thus not handing the
Win32 message; this is easy to trigger with JunitTests but hopefully
not by actual end users.
Debugging / Playing with winaccessibility
If NVDA is running when soffice starts, IA2 should be automatically enabled and work as expected. In order to use 'accprobe' to debug it is necessary to override the check for whether an AT (like NVDA) is running; to do that use:
SAL_FORCE_IACCESSIBLE2=1 soffice.exe -writer
Then you can use accprobe to introspect the accessibility hierarchy remotely, checkout:
http://accessibility.linuxfoundation.org/a11yweb/util/accprobe/
But often it's more useful to look at NVDA's text output window.