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 ATs 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 attributes

The fundamental concepts of an accessible widget are roles and attributes; each GTK control has a role, while its functionality is described by a set of attributes.

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_ACCESSIBLE_ROLE_BUTTON; an entry will have a role of GTK_ACCESSIBLE_ROLE_TEXTBOX; a check button will have a role of GTK_ACCESSIBLE_ROLE_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.

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

Attributes

Attributes provide specific information about an accessible UI control, and describe it for the assistive technology applications. GTK divides the accessible attributes into three categories:

• properties, described by the values of the GtkAccessibleProperty enumeration
• relations, described by the values of the GtkAccessibleRelation enumeration
• states, described by the values of the GtkAccessibleState enumeration

Each attribute accepts a value of a specific type.

Unlike roles, attributes may change over time, or in response to user action; for instance:

• a toggle button will change its GTK_ACCESSIBLE_STATE_CHECKED state every time it is toggled, either by the user or programmatically
• setting the mnemonic widget on a GtkLabel will update the GTK_ACCESSIBLE_RELATION_LABELLED_BY relation on the widget with a reference to the label
• changing the GtkAdjustment instance on a GtkScrollbar will change the GTK_ACCESSIBLE_PROPERTY_VALUE_MAX, GTK_ACCESSIBLE_PROPERTY_VALUE_MIN, and GTK_ACCESSIBLE_PROPERTY_VALUE_NOW properties with the upper, lower, and value properties of the GtkAdjustment

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

List of accessible states

Each state name is part of the GtkAccessibleState enumeration.

List of accessible properties

Each property name is part of the GtkAccessibleProperty enumeration.

List of accessible relations

Each relation name is part of the GtkAccessibleRelation enumeration.

Note: When using gtk_accessible_update_relation() with a relation that requires a list of GtkAccessible instances, you should pass every accessible object separately, followed by NULL.

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_ACCESSIBLE_ROLE_FORM role
• each text entry widget in the form has the GTK_ACCESSIBLE_RELATION_LABELLED_BY relation pointing to the label widget that describes it

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

• the container has a GTK_ACCESSIBLE_ROLE_TOOLBAR role
• each button has a GTK_ACCESSIBLE_PROPERTY_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 properties, for instance the accessible name will be taken from the label used by the UI control, or from its tooltip, if the GTK_ACCESSIBLE_PROPERTY_LABEL property or the GTK_ACCESSIBLE_RELATION_LABELLED_BY relation are unset. Similarly for the accessible description. 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. The GTK Inspector shows the accessible attributes of each widget, and also provides an overlay that can highlight accessibility issues.

It is possible to set accessible attributes in UI files as well:

<object class="GtkButton" id="button1">
  <accessibility>
    <property name="label">Download</property>
    <relation name="labelled-by">label1</relation>
  </accessibility>
</object>

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 GtkAccessible implementation must provide a GtkATContext instance, which acts as a proxy to the specific platform’s accessibility API:

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

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

Authoring practices

The authoring practices are aimed at application developers, as well as developers of GUI elements based on GTK.

Functionally, GtkAccessible roles, states, properties, and relations are analogous to a CSS for assistive technologies. For screen reader users, for instance, the various accessible attributes control the rendering of their non-visual experience. Incorrect roles and attributes may result in a completely inaccessible user interface.

A role is a promise

The following code:

gtk_widget_class_set_accessible_role (widget_class, GTK_ACCESSIBLE_ROLE_BUTTON);

is a promise that the widget being created will provide the same keyboard interactions expected for a button. An accessible role of a button will not turn automatically any widget into a GtkButton; but if your widget behaves like a button, using the GTK_ACCESSIBLE_ROLE_BUTTON will allow any assistive technology to handle it like they would a GtkButton.

For widgets that act as containers of other widgets, you should use GTK_ACCESSIBLE_ROLE_GROUP if the grouping of the children is semantic in nature; for instance, the children of a GtkHeaderBar are grouped together on the header of a window. For generic containers that only impose a layout on their children, you should use GTK_ACCESSIBLE_ROLE_GENERIC instead.

Attributes can both hide and enhance

Accessible attributes can be used to override the content of a UI element, for instance:

gtk_label_set_text (GTK_LABEL (label), "Some text");
gtk_accessible_update_property (GTK_ACCESSIBLE (label),
                GTK_ACCESSIBLE_PROPERTY_LABEL,
                "Assistive technologies users will perceive "
                "this text, not the contents of the label",
                -1);

In the example above, the “label” property will override the contents of the label widget.

The attributes can also enhance the UI:

gtk_button_set_label (GTK_BUTTON (button), "Download");
gtk_box_append (GTK_BOX (button), button);

gtk_label_set_text (GTK_LABEL (label), "Final report.pdf");
gtk_box_append (GTK_BOX (box), label);

gtk_accessible_update_relation (GTK_ACCESSIBLE (button),
                GTK_ACCESSIBLE_RELATION_LABELLED_BY,
                g_list_append (NULL, label),
                -1);

In the example above, an assistive technology will read the button’s accessible label as “Download Final report.pdf”.

The power of hiding and enhancing can be a double-edged sword, as it can lead to inadvertently overriding the accessible semantics of existing widgets.

Hiding UI elements from the accessible tree

The accessibility API is mainly used to express semantics useful for assistive technologies, but it can also be used to hide elements. The canonical way to do so is to use the GTK_ACCESSIBLE_ROLE_PRESENTATION, which declares that a UI element is purely meant for presentation purposes, and as such it has no meaningful impact on the accessibility of the interface.

A “presentation” role should not be confused with the GTK_ACCESSIBLE_STATE_HIDDEN state; the “hidden” state is transient, and is typically controlled by showing and hiding a widget using the GtkWidget API.

Design patterns and custom widgets

When creating custom widgets, following established patterns can help ensuring that the widgets work well for users of accessible technologies as well.

Buttons

A button is a widget that enables users to trigger an action. While it is recommended you use GtkButton for anything that looks and behaves like a button, it is possible to apply a button behavior to UI elements like images by using a GtkGestureClick gesture. When doing so, you should:

• Give your widget the role GTK_ACCESSIBLE_ROLE_BUTTON
• Install an action with no parameters, which will activate the widget

Custom entries

For custom entries, it is highly recommended that you implement the GtkEditable interface by using a GtkText widget as delegate. If you do this, GTK will make your widgets text editing functionality accessible in the same way as a GtkSpinButton or GtkSearchEntry.

Tab-based UI

If you make a tab-based interface, you should consider using GtkStack as the core, and just make a custom tab widget to control the active stack page. When doing so, the following extra steps will ensure that your tabs are accessible in the same way as GtkStackSwitcher or GtkNotebook:

• Give your tab container the role GTK_ACCESSIBLE_ROLE_TAB_LIST
• Give your tab widgets the role GTK_ACCESSIBLE_ROLE_TAB
• Set up the GTK_ACCESSIBLE_RELATION_CONTROLS relation between each tab and the GtkStackPage object for its page
• Set the GTK_ACCESSIBLE_PROPERTY_SELECTED property on each tab, with the active tab getting the value TRUE, all others FALSE

To allow changing the active tab via accessible technologies, you can export actions. Since the accessibility interfaces only support actions without parameters, you can either provide previous-tab and next-tab actions on the tab container that let users step through the tabs one-by-one, or add a activate-tab action on each tab.

Value controls

A value control (ie a widget that controls a one-dimensional quantity that can be represented by a GtkAdjustment) can be represented to accessible technologies by setting the GTK_ACCESSIBLE_PROPERTY_VALUE_MIN, GTK_ACCESSIBLE_PROPERTY_VALUE_MAX, and GTK_ACCESSIBLE_PROPERTY_VALUE_NOW properties.

To allow changing the value via accessible technologies, you can export actions. Since the accessibility interfaces only support actions without parameters, you should provide actions such as increase-value and decrease-value.

Since GTK 4.10, the best way to suppose changing the value is by implementing the GtkAccessibleRange interface.