Re: Custom widgets in ATK

[Alejandro Piñeiro <apinheiro igalia com>]

On 14/08/17 23:11, Eric Williams wrote:
> Hello,

Hi,

a quick answer for your email below. Sorry for not answering before, I
have just came back from holidays.

> I am an Eclipse SWT developer, working to improve accessibility
> support for SWT on Linux.
>
> Some background: SWT is a cross-platform Java UI toolkit which runs
> using GTK on Linux. Most SWT widgets match up to GTK widgets -- these
> are accessible by default via GTK. However we implement a custom GTK
> container, called SwtFixed, for custom widgets. These custom widgets
> do not map up to native GTK widgets and are drawn manually. The role
> of these widgets vary: from text editing to custom combos, etc. The
> main text editor in Eclipse is a custom widget, which means that
> accessibility for these widgets is important.
>
> The issue: at present time, Orca is unable to read these custom
> widgets. It works on GTK2 semi-reliably, but GTK3 is unusable. Orca
> either hangs, crashes, or fails to read events/information from the
> custom widgets.
>
> We currently have an accessibility implementation in SWT that was
> created long before GTK3 was around. From what I gather, it has not
> been regularly maintained since ~2012. The existing implementation
> uses ATK to enable accessibility in areas that GTK cannot (i.e., for
> custom widgets). I am virtually certain that the problem lies in the
> accessibility implementation, as the custom container implementation
> is accessible by default through GTK. I have confirmed this via testing.
>
> Here is some of the code to register a custom widget with ATK:
>
> (I've removed some of the Java typing to make it easier to read)
>
> widgetType = G_OBJECT_TYPE (widget);
> registry = atk_get_default_registry ();
> factory = atk_registry_get_factory (registry, widgetType);
> if (ATK_IS_NO_OP_OBJECT_FACTORY(factory)) return;
>
> factoryName = FACTORY_TYPENAME + getTypeName(widgetType);
>
> if (g_type_from_name (factoryName) == 0) {
>     /* register the factory */
>     GTypeInfo typeInfo = new GTypeInfo ();
>     typeInfo.base_init = GTypeInfo_base_init_factory.getAddress ();
>     typeInfo.class_size = (short)AtkObjectFactoryClass_sizeof ();
>     typeInfo.instance_size = (short)AtkObjectFactory_sizeof ();
>     
>     info = g_malloc (GTypeInfo.sizeof);
>     memmove (info, typeInfo, GTypeInfo.sizeof);
>     swtFactoryType = g_type_register_static
>         (ATK_TYPE_OBJECT_FACTORY(), factoryName, info, 0);
>     parentType = atk_object_factory_get_accessible_type(factory);
>     atk_registry_set_factory_type (registry, widgetType,
>         swtFactoryType);
> }

ATK Factories are not the way to go anymore to extend already existing
accessible objects/containers, now that most moder ATK implementations
are not modules anymore. In fact, now I then I'm tempted to mark them as
deprecated on ATK itself. The only reason I didn't do that is that there
are still plenty of implementations using them, without no plans to port
that code (AFAIK).

> Another thing: I tried investigating with Accerciser, but it hangs and
> gives me the following error message:
>
> WARNING **: AT-SPI: Error in GetItems, sender=(null), error=Did not
> receive a reply. Possible causes include: the remote application did
> not send a reply, the message bus security policy blocked the reply,
> the reply timeout expired, or the network connection was broken.
>
> I would appreciate it if someone could point me in the right
> direction. Is the code provided correct? Is there something additional
> we need to be doing to register with ATK?

On GTK3, the accessible implementation is public. We needed to ask that
when they stopped to use factories, so apps/toolkits providing custom
widgets could extend them. The poster boy at that moment was Evince. You
mention that you have a custom container, so I hope that you are just
extending GtkContainer. If that is the case this code could be used as
an example:

https://git.gnome.org/browse/evince/tree/libview/ev-view-accessible.c#n71

I hope this helps.

BR