Accessibility Adventure

2024-08-05

As part of my effort to reduce Mutter dependencies, I finally found some time to focus on removing Cairo now that we can have a Wayland only build. Since the majority of the remaining usages are related to fonts, we would need to move CoglPango to be part of GNOME Shell's libst.

Merging Cally

Moving CoglPango also necessitates moving ClutterText and its corresponding accessibility implementation object, CallyText. The accessibility implementation is an object that inherits from AtkObject and can implement various interfaces to describe the type it corresponds to.

Inside Mutter, you would find the following types

ClutterActor -> CallyActor implements AtkComponent
ClutterClone -> CallyClone implements AtkComponent
ClutterStage -> CallyStage implements AtkComponent, AtkWindow
ClutterStageManager -> CallyRoot corresponds to the Atk.Role.APPLICATION
ClutterText -> CallyText implements AtkAction, AtkComponent, AtkEditableText, AtkText

Once you initialize accessibility inside Mutter, the first step is to set up the AtkUtil implementation, which overrides the get_root and returns a CallyRoot instance. The remaining types register an AtkObjectFactory using the provided macros.

CALLY_ACCESSIBLE_FACTORY (CALLY_TYPE_ACTOR, cally_actor, cally_actor_new);
CALLY_ACTOR_SET_FACTORY (CLUTTER_TYPE_ACTOR, cally_actor);

Allowing Atk to know which type to instantiate in case it encounters a CLUTTER_TYPE_CLONE, for example. That usually happens when you call atk_gobject_accessible_for_object(actor), which creates an instance of CallyActor using cally_actor_new. Although, the factories are nice, they don't allow out-of-tree AtkObject implementations. But wait, how is GNOME Shell accessible then?

Inside GNOME Shell StWidget, the widget abstraction, overrides the ClutterActorClass.get_accessible virtual function and replaces the default behavior that would call atk_gobject_accessible_for_object to:

Getting the corresponding accessibility type GType from another StWidgetClass.get_accessible_type virtual function
Create an instance of that type using g_object_new (T->get_accessible_type())
Call atk_object_initialize on it
Keep a weak reference to it

After "upstreaming" both StWidgetClass.get_accessible_type and their override of ClutterActorClass.get_accessible to become the default behavior, I managed to completely get rid of the factories. The remainder of the migration was mostly renaming things and trying to emit accessibility state changes when the Clutter type updated its state where possible.

The patches are available at:

Testing Accessibility

Now that my branches of Mutter and GNOME Shell are working, I decided to test whether the reshuffling I did has caused any regressions. Unfortunately, Accerciser from the Fedora Rawhide repositories doesn't start due to a Python forward compatibility issue.

I remembered that Georges Basile Stavracas Neto started a re-implementation in C, called Elevado, so between fixing Accerciser or adding some missing features I wanted in Elevado, the choice was easy.

And few hours later, I had managed to add some meaningful accessible names to various objects.

Accessibility tree from local development

Now that I have confirmed that my changes didn't break any obvious thing, my weekend fun is over.