d15163eceb
Change-Id: Ideafa411b53fe4a5f2e6559be10c4cb82b58a256 Reviewed-on: https://gerrit.libreoffice.org/24543 Tested-by: Jenkins <ci@libreoffice.org> Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
58 lines
2.4 KiB
Text
58 lines
2.4 KiB
Text
Windows Accessibility Bridge.
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This code provides a bridge between our internal Accessibility
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interfaces (implemented on all visible 'things' in the suite: eg.
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windows, buttons, entry boxes etc.) - and the Windows MSAA /
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IAccessible2 COM interfaces that are familiar to windows users and
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Accessible Technologies (ATs) such as the NVDA screen reader.
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The code breaks into three bits:
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source/service/
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+ the UNO service providing the accessibility bridge.
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It essentially listens to events from the LibreOffice
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core and creates and synchronises COM peers for our
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internal accessibility objects when events arrive.
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source/UAccCom/
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+ COM implementations of the MSAA / IAccessible2 interfaces
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to provide native peers for the accessibility code.
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source/UAccCOMIDL/
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+ COM Interface Definition Language (IDL) for UAccCom.
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Here is one way of visualising the code / control flow
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VCL <-> UNO toolkit <-> UNO a11y <-> win a11y <-> COM / IAccessible2
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vcl/ <-> toolkit/ <-> accessibility/ <-> winaccessibility/ <-> UAccCom/
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Threading
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It's possible that the UNO components are called from threads other
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than the main thread, so they have to be synchronized. It would be nice
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to put the component into an UNO apartment (and the COM components into STA)
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but UNO would spawn a new thread for it so it's not possible.
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The COM components also call into the same global AccObjectWinManager
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as the UNO components do so both have to be synchronized in the same way.
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So we use the SolarMutex for all synchronization since anything else
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would be rather difficult to make work. Unfortunately there is a
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pre-exising problem in vcl with Win32 Window creation and destruction
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on non-main threads where a synchronous SendMessage is used while
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the SolarMutex is locked that can cause deadlocks if the main thread is
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waiting on the SolarMutex itself at that time and thus not handing the
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Win32 message; this is easy to trigger with JunitTests but hopefully
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not by actual end users.
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Debugging / playing with winaccessibility
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You need to enable 'experimental mode' in Tools->Options. After
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that NVDA should work as expected. In order to use 'accprobe' to debug
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it is necessary to override the check for whether an AT (like NVDA) is
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running; to do that use:
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SAL_FORCE_IACCESSIBLE2=1 soffice.exe -writer
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Then you can use accprobe to introspect the accessibility hierarchy
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remotely, checkout:
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http://accessibility.linuxfoundation.org/a11yweb/util/accprobe/
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