Michael Weghorn
c6b7c9a7a7
Window::GetAccessible returns an OAccessible. Use that more specific type instead of the abstract XAccessible UNO interface. OAccessible also implements the XAccessibleContext interface itself, so there is no need to request that one separately. Change-Id: Ia40a9e5684ce321f0e5aff7ccd34bd3c488e1ab9 Reviewed-on: https://gerrit.libreoffice.org/c/core/+/210437 Reviewed-by: Michael Weghorn <m.weghorn@posteo.de> Tested-by: Jenkins
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 an assistive technology like NVDA is running when soffice starts, IA2 should be automatically enabled and work as expected.
'accprobe' can be used 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.
Another tool is Accessibility Insights for Windows: https://accessibilityinsights.io/ It does not support IAccessible2, but the Microsoft Active Accessibility to Microsoft UIA proxy makes some properties, methods and events available to UIA via the LegacyIAccessible pattern: https://learn.microsoft.com/en-us/windows/win32/winauto/uiauto-implementinglegacyiaccessible