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Re: Widget rendering in GNUstep

From: Sheldon Gill
Subject: Re: Widget rendering in GNUstep
Date: Fri, 16 Sep 2005 11:40:36 +0800
User-agent: Mozilla Thunderbird 1.0.6 (Windows/20050716)

Cocoa code that uses the NSAccessibility protocol can't currently be
ported - it hasn't been added to GNUstep-base yet, and touches enough
classes in interesting ways that may lead to some issues like additional
instance variables being required. I'd like to see this added, despite
any short-term pain. I'm also interested in finding out if there are any
free software assistive tools on Cocoa.

The first issue I came against comes from the cross-platform nature of GNUstep. There are a number of different Accessibility/Assitance APIs on different platforms. To make it worse on *nix there are rather different approaches on the *same* platform. There is some work going for compatibility but...

Adding support for *one* of these is a lot of work. Making that support available using other underlying APIs seems quite a design task.

I'm all for adding it.
{Don't forget I used to work with Special Needs...}

It's fairly straight-forward how this would play out on GNUstep on MacOS
X - but I'm more interested in how we'd interact with other existing host
platform capabilities. As a non-Win32 guy, I have no idea if there's any
simple or useful mapping into the Windows-native functionality. Any

There is no simple mapping. There is a useful one but its a lot of work and touches on a number of other issues in implementing it...

Basically, adding support requires:

1) Implementing IAccessible:: properties and methods for each custom UI element. For GNUstep this is all the controls.

2) Adding calls to NotifyWinEvent() into the event handling system so clients can be appropriately notified.

3) Handle Win32 native WM_GETOBJECT messages to provide access to the objects.

A lot of work but it doesn't seem too bad?

Getting to step (1) reasonably requires good COM support. That is: IUnknown, IDispatch, VARIANT structure passing and string support. Attendant memory handling and threading issues.

Mostly this won't be terribly useful for accessibility support elsewhere. So I think we'd end up with Bundles and back-ends, assuming we can manage a good front-end design. Otherwise it'll be all platform/API specific.


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