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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:

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 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:

But often it’s more useful to look at NVDA’s text output window.