Simplify getting the accessible role when checking if an accessible
implementation should present itself; this avoids going through
GtkAccessible twice to get the same data.
Since GtkATContexts are now lazily realized, we need to go through the
GtkAccessible's implementation to access the :accessible-role property,
in case there are fallbacks.
The ARIA spec is clear on this: when an element has the
HIDDEN state, it should not be presented in the accessible
tree.
This change is incomplete, we also need to emit child-added/
removed signals when the state changes, but that needs to
wait for the child added infrastructure to land.
platform change is called from gtk_widget_set_focusable
which is likely to be called early on in init(). We don't
want to create an AT context that early if we can help
it, e.g. since it makes it impossible to override the
accessible-role with a construct property.
As a companion to go with the platform_change api,
add a gtk_accessible_get_platform_state() function
that can be used by backends to get the platform
state.
This is in preparation for making entries inherit
their focus states from the text widget within.
Similar to gtk_widget_should_layout(), add a
gtk_accessible_should_present() function that backends can
use to determine whether an accessible should be presented
or not.
Ways to make a widget not presented in a11y:
- hide the widget
- set its role to NONE
- make it have a NULL AT context
We will use this in future to hide the GtkText inside
an entry, since the Text implementation will be done
by the wrapper.
Add an enum for 'platform changes' to the at context
change notification mechanism. This will let us pass
along things that ARIA considers 'platform state' such
as focus or editability. The difference between the
platform state and other ARIA states is that we don't
keep the platform state separately in the at context
- backends are expected to just query the widgets.
This is just about avoiding notify listeners for
change notification.
In some cases we explicitly want to unset an accessible attribute; for
instance, an accessible property is gated on a widget property, and if
the widget property gets unset, the accessible property should be reset.
We're currently overloading NULL to mean both "this value is undefined,
and should be reset to its default" and "the value collection failed".
Let's do error reporting right, by using GError to mean "the collection
failed, for this specific reason"; then, we can use a NULL return value
to signal that the accessible attribute should be reset to its default
value.
This is only relevant for pointer-sized attribute values: strings,
references, and reference lists; numeric, boolean, tristate, and token
values either cannot be undefined, or have a specific "undefined" value.
Some widgets have different roles after they are constructed, so we need
to allow changing the role defined by the class. We should still avoid
setting a role after the GtkATContext has been created.
Only GtkWidget should use GTK_ACCESSIBLE_ROLE_WIDGET as its default
accessible role; the default for GtkAccessible and GtkATContext should
be GTK_ACCESSIBLE_ROLE_NONE, meaning "an element whose implicit native
role semantics will not be mapped to the accessibility API", according
to the WAI-ARIA specification.
It's pointless, we can use an explicit value of `-1` everywhere.
Additionally, it complicates all code that uses the state enumeration as
an array index, since now we need to guard against a negative offset.
Some widgets have different accessible roles depending on some
parameter, so we cannot set the role at class init time. For those
widgets, we add an "accessible-role" property to GtkAccessible, and we
allow setting it (only) at construction time.
An Accessible implementation must create an ATContext object. UI
elements are supposed to interact with the GtkAccessible API, but we
expose GtkATContext to allow patterns like delegation.
GtkAccessible is an interface for accessible UI elements.
Currently, it doesn't do much except exist as a type; in the future, it
will be the entry point for all accessible state in GTK.
To build a better world sometimes means having to tear the old one down.
-- Alexander Pierce, "Captain America: The Winter Soldier"
ATK served us well for nearly 20 years, but the world has changed, and
GTK has changed with it. Now ATK is mostly a hindrance towards improving
the accessibility stack:
- it maps to a very specific implementation, AT-SPI, which is Linux and
Unix specific
- it requires implementing the same functionality in three different
layers of the stack: AT-SPI, ATK, and GTK
- only GTK uses it; every other Linux and Unix toolkit and application
talks to AT-SPI directly, including assistive technologies
Sadly, we cannot incrementally port GTK to a new accessibility stack;
since ATK insulates us entirely from the underlying implementation, we
cannot replace it piecemeal. Instead, we're going to remove everything
and then incrementally build on a clean slate:
- add an "accessible" interface, implemented by GTK objects directly,
which describe the accessible role and state changes for every UI
element
- add an "assistive technology context" to proxy a native accessibility
API, and assign it to every widget
- implement the AT context depending on the platform
For more information, see: https://gitlab.gnome.org/GNOME/gtk/-/issues/2833
Remove all the old 2.x and 3.x version annotations.
GTK+ 4 is a new start, and from the perspective of a
GTK+ 4 developer all these APIs have been around since
the beginning.
Unsetting the widget causes events to get emitted and we want to
(a) really emit that event to our signal handlers
(b) still be recognized as a real GObject while doing that
https://bugzilla.gnome.org/show_bug.cgi?id=669794
I expect them to be used a lot, so this approach seems better than
requiring signals that connect to "notify::widget". Also, we can't use
regular functions (like dispose or constructed), becaiuse those assume
that (un)setting of the widget only happens once and with the current
design (a puble set_widget() function) we can't really guarantee that.
Also, I split them into two separate functions as one function is part
of construction and the other part of destruction of the object. And it
doesn't sound like a good idea to have that both be part of one
function.
