LibreOffice » winaccessibility
View module in: git
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
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:
IAccessible2interfaces to provide native peers for the accessibility code.
Here is one way of visualising the code / control flow
VCL <-> UNO toolkit <-> UNO a11y <-> win a11y <-> COM / IAccessible2 vcl/ <-> toolkit/ <-> accessibility/ <-> winaccessibility/ <-> UAccCom/
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
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
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.
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:
But often it’s more useful to look at NVDA’s text output window.