0c7928dc10
As described in the IAccessible2 spec [1], -1 can be used as a
special text offset:
> Using IA2_TEXT_OFFSET_LENGTH (-1) as an offset in any of the
> IAccessibleText or IAccessibleEditableText methods is the same
> as specifying the length of the string.
Replace -1 by the text length *before* doing the
check whether end offset is smaller than the start offset.
Otherwise, trying to query the whole text of a Writer paragraph
containing the text "hello" in NVDA's Python console would
incorrectly trigger an error:
>>> focus.IAccessibleTextObject.text(0,-1)
Traceback (most recent call last):
File "<console>", line 1, in <module>
File "comtypes\__init__.pyc", line 856, in __call__
File "monkeyPatches\comtypesMonkeyPatches.pyc", line 32, in __call__
_ctypes.COMError: (-2147467259, 'Unspecified error', (None, None, None, 0, None))
With this commit in place, it works as expected:
>>> focus.IAccessibleTextObject.text(0,-1)
'hello'
[1] https://accessibility.linuxfoundation.org/a11yspecs/ia2/docs/html/_general_info.html#_specialOffsets
Change-Id: I489a42270a56178cc8ee0564eec3dc82e15969c4
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/168853
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 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.