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Outline for the a11y rework

A preliminary document for the accessibility API overview, to be dumped in the GTK reference.

GTK Accessibility

The standard accessibility interface

The GtkAccessible interface provides the accessibility information about an application's user interface elements. Assistive technology (AT) applications, like Orca, convey this information to users with disabilities, or reduced abilities, to help them use the application.

Standard GTK controls implement the GtkAccessible interface and are thus accessible to AT by default. This means that if you use GTK controls such as GtkButton, GtkEntry, or GtkListView, you only need to supply application-specific details when the defaults values are incomplete. You can do this by setting the appropriate properties in your GtkBuilder template and UI definition files, or by setting the properties defined by the GtkAccessible interface.

If you are implementing your own GtkWidget derived type, you will need to set the GtkAccessible properties yourself, and provide an implementation of the GtkAccessible virtual functions.

Accessible roles and states

The fundamental concepts of an accessible widget are roles and states; each GTK control fills out a role and its functionality is described by a set of states.

Roles

Roles define the taxonomy and semantics of a UI control to any assistive technology application; for instance, a button will have a role of Gtk.AccessibleRole.BUTTON; an entry will have a role of Gtk.AccessibleRole.TEXTBOX; a toggle button will have a role of Gtk.AccessibleRole.CHECKBOX; etc.

Each role is part of the widget's instance, and CANNOT be changed over time or as the result of a user action. Roles allows assistive technology applications to identify a UI control and decide how to present it to a user; if a part of the application's UI changes role, the control needs to be removed and replaced with another one with the appropriate role.

List of accessible roles

Each role name is part of the GtkAccessibleRole enumeration.

Role name Description
ALERT A message with important information
BUTTON A control that performs an action when pressed
CHECKBOX A control that has three possible value: true, false, or undefined
COLUMNHEADER The header of a column in a list or grid
COMBOBOX A control that can be expanded to show a list of possible values to select
... …

See the WAI-ARIA list of roles for additional information.

States

States, or properties, provide specific information about an accessible UI control, and describe it for the assistive technology applications.

Unlike roles, states may change over time, or in response to user action; for instance, a toggle button will change its Gtk.AccessibleState.CHECKED state every time it is toggled, either by the user or programmatically.

List of accessible states

Each state name is part of the GtkAccessibleState enumeration.

State name Description
... …

See the WAI-ARIA list of states for additional information.

Application development rules

Even if standard UI controls provided by GTK have accessibility information out of the box, there are some additional properties and considerations for application developers. For instance, if your application presents the user with a form to fill out, you should ensure that:

  • the container of the form has a Gtk.AccessibleRole.FORM role
  • each text entry widget in the form has the GtkAccessible:labelled-by property pointing to the label widget that describes it

Another example: if you create a tool bar containing buttons with only icons, you should ensure that:

  • the container has a Gtk.AccessibleRole.TOOLBAR role
  • each button has a GtkAccessible:label property set with the user readable and localised action performed when pressed; for instance "Copy", "Paste", "Add layer", or "Remove"

GTK will try to fill in some information by using ancillary UI control property, for instance the accessible label will be taken from the label or placeholder text used by the UI control, or from its tooltip, if the GtkAccessible:label or the GtkAccessible:labelled-by properties are unset. Nevertheless, it is good practice and project hygiene to explicitly specify the accessible properties, just like it's good practice to specify tooltips and style classes.

Application developers using GTK SHOULD ensure that their UI controls are accessible as part of the development process. When using GtkBuilder templates and UI definition files, GTK provides a validation tool that verifies that each UI element has a valid role and properties; this tool can be used as part of the application's test suite to avoid regressions.

Implementations

Each UI control implements the GtkAccessible interface to allow widget and application developers to specify the roles, state, and relations between UI controls. This API is purely descriptive.

Each GtkWidget has a GtkAccessibleImpl (actual name TBD), which acts as a proxy to the specific platform's accessibility API:

  • AT-SPI on Linux/BSD
  • NSAccessibility on macOS
  • Active Accessibility on Windows
  • ARIA on Broadway

Additionally, an ad hoc accessibility backend is available for the GTK testsuite, to ensure reproducibility of issues in the CI pipeline.