gtk2/gtk/gtkatcontext.c

/* gtkatcontext.c: Assistive technology context
 *
 * Copyright 2020  GNOME Foundation
 *
 * SPDX-License-Identifier: LGPL-2.1-or-later
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, see <http://www.gnu.org/licenses/>.
 */

/**
 * SECTION:gtkatcontext
 * @Title: GtkATContext
 * @Short_description: An object communicating to Assistive Technologies
 *
 * GtkATContext is an abstract class provided by GTK to communicate to
 * platform-specific assistive technologies API.
 *
 * Each platform supported by GTK implements a #GtkATContext subclass, and
 * is responsible for updating the accessible state in response to state
 * changes in #GtkAccessible.
 */

#include "config.h"

#include "gtkatcontextprivate.h"

#include "gtkaccessiblevalueprivate.h"
#include "gtkdebug.h"
#include "gtktestatcontextprivate.h"
#include "gtktypebuiltins.h"

#if defined(GDK_WINDOWING_X11) || defined(GDK_WINDOWING_WAYLAND)
#include "a11y/gtkatspicontextprivate.h"
#endif

G_DEFINE_ABSTRACT_TYPE (GtkATContext, gtk_at_context, G_TYPE_OBJECT)

enum
{
  PROP_ACCESSIBLE_ROLE = 1,
  PROP_ACCESSIBLE,
  PROP_DISPLAY,

  N_PROPS
};

enum
{
  STATE_CHANGE,

  LAST_SIGNAL
};

static GParamSpec *obj_props[N_PROPS];

static guint obj_signals[LAST_SIGNAL];

static void
gtk_at_context_finalize (GObject *gobject)
{
  GtkATContext *self = GTK_AT_CONTEXT (gobject);

  gtk_accessible_attribute_set_unref (self->properties);
  gtk_accessible_attribute_set_unref (self->relations);
  gtk_accessible_attribute_set_unref (self->states);

  G_OBJECT_CLASS (gtk_at_context_parent_class)->finalize (gobject);
}

static void
gtk_at_context_set_property (GObject      *gobject,
                             guint         prop_id,
                             const GValue *value,
                             GParamSpec   *pspec)
{
  GtkATContext *self = GTK_AT_CONTEXT (gobject);

  switch (prop_id)
    {
    case PROP_ACCESSIBLE_ROLE:
      self->accessible_role = g_value_get_enum (value);
      break;

    case PROP_ACCESSIBLE:
      self->accessible = g_value_get_object (value);
      break;

    case PROP_DISPLAY:
      self->display = g_value_get_object (value);
      break;

    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (gobject, prop_id, pspec);
    }
}

static void
gtk_at_context_get_property (GObject    *gobject,
                             guint       prop_id,
                             GValue     *value,
                             GParamSpec *pspec)
{
  GtkATContext *self = GTK_AT_CONTEXT (gobject);

  switch (prop_id)
    {
    case PROP_ACCESSIBLE_ROLE:
      g_value_set_enum (value, self->accessible_role);
      break;

    case PROP_ACCESSIBLE:
      g_value_set_object (value, self->accessible);
      break;

    case PROP_DISPLAY:
      g_value_set_object (value, self->display);
      break;

    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (gobject, prop_id, pspec);
    }
}

static void
gtk_at_context_real_state_change (GtkATContext                *self,
                                  GtkAccessibleStateChange     changed_states,
                                  GtkAccessiblePropertyChange  changed_properties,
                                  GtkAccessibleRelationChange  changed_relations,
                                  GtkAccessibleAttributeSet   *states,
                                  GtkAccessibleAttributeSet   *properties,
                                  GtkAccessibleAttributeSet   *relations)
{
}

static void
gtk_at_context_class_init (GtkATContextClass *klass)
{
  GObjectClass *gobject_class = G_OBJECT_CLASS (klass);

  gobject_class->set_property = gtk_at_context_set_property;
  gobject_class->get_property = gtk_at_context_get_property;
  gobject_class->finalize = gtk_at_context_finalize;

  klass->state_change = gtk_at_context_real_state_change;

  /**
   * GtkATContext:accessible-role:
   *
   * The accessible role used by the AT context.
   *
   * Depending on the given role, different states and properties can be
   * set or retrieved.
   */
  obj_props[PROP_ACCESSIBLE_ROLE] =
    g_param_spec_enum ("accessible-role",
                       "Accessible Role",
                       "The accessible role of the AT context",
                       GTK_TYPE_ACCESSIBLE_ROLE,
                       GTK_ACCESSIBLE_ROLE_NONE,
                       G_PARAM_READWRITE |
                       G_PARAM_CONSTRUCT_ONLY |
                       G_PARAM_STATIC_STRINGS);

  /**
   * GtkATContext:accessible:
   *
   * The #GtkAccessible that created the #GtkATContext instance.
   */
  obj_props[PROP_ACCESSIBLE] =
    g_param_spec_object ("accessible",
                         "Accessible",
                         "The accessible implementation",
                         GTK_TYPE_ACCESSIBLE,
                         G_PARAM_READWRITE |
                         G_PARAM_CONSTRUCT_ONLY |
                         G_PARAM_STATIC_STRINGS);

  /**
   * GtkATContext:display:
   *
   * The #GdkDisplay for the #GtkATContext.
   */
  obj_props[PROP_DISPLAY] =
    g_param_spec_object ("display",
                         "Display",
                         "The display connection",
                         GDK_TYPE_DISPLAY,
                         G_PARAM_READWRITE |
                         G_PARAM_CONSTRUCT_ONLY |
                         G_PARAM_STATIC_STRINGS);

  /**
   * GtkATContext::state-change:
   * @self: the #GtkATContext
   *
   * Emitted when the attributes of the accessible for the
   * #GtkATContext instance change.
   */
  obj_signals[STATE_CHANGE] =
    g_signal_new ("state-change",
                  G_TYPE_FROM_CLASS (gobject_class),
                  G_SIGNAL_RUN_FIRST,
                  0,
                  NULL, NULL,
                  NULL,
                  G_TYPE_NONE, 0);

  g_object_class_install_properties (gobject_class, N_PROPS, obj_props);
}

#define N_PROPERTIES    (GTK_ACCESSIBLE_PROPERTY_VALUE_TEXT + 1)
#define N_RELATIONS     (GTK_ACCESSIBLE_RELATION_SET_SIZE + 1)
#define N_STATES        (GTK_ACCESSIBLE_STATE_SELECTED + 1)

static const char *property_attrs[] = {
  [GTK_ACCESSIBLE_PROPERTY_AUTOCOMPLETE]        = "autocomplete",
  [GTK_ACCESSIBLE_PROPERTY_DESCRIPTION]         = "description",
  [GTK_ACCESSIBLE_PROPERTY_HAS_POPUP]           = "haspopup",
  [GTK_ACCESSIBLE_PROPERTY_KEY_SHORTCUTS]       = "keyshortcuts",
  [GTK_ACCESSIBLE_PROPERTY_LABEL]               = "label",
  [GTK_ACCESSIBLE_PROPERTY_LEVEL]               = "level",
  [GTK_ACCESSIBLE_PROPERTY_MODAL]               = "modal",
  [GTK_ACCESSIBLE_PROPERTY_MULTI_LINE]          = "multiline",
  [GTK_ACCESSIBLE_PROPERTY_MULTI_SELECTABLE]    = "multiselectable",
  [GTK_ACCESSIBLE_PROPERTY_ORIENTATION]         = "orientation",
  [GTK_ACCESSIBLE_PROPERTY_PLACEHOLDER]         = "placeholder",
  [GTK_ACCESSIBLE_PROPERTY_READ_ONLY]           = "readonly",
  [GTK_ACCESSIBLE_PROPERTY_REQUIRED]            = "required",
  [GTK_ACCESSIBLE_PROPERTY_ROLE_DESCRIPTION]    = "roledescription",
  [GTK_ACCESSIBLE_PROPERTY_SORT]                = "sort",
  [GTK_ACCESSIBLE_PROPERTY_VALUE_MAX]           = "valuemax",
  [GTK_ACCESSIBLE_PROPERTY_VALUE_MIN]           = "valuemin",
  [GTK_ACCESSIBLE_PROPERTY_VALUE_NOW]           = "valuenow",
  [GTK_ACCESSIBLE_PROPERTY_VALUE_TEXT]          = "valuetext",
};

/*< private >
 * gtk_accessible_property_get_attribute_name:
 * @property: a #GtkAccessibleProperty
 *
 * Retrieves the name of a #GtkAccessibleProperty.
 *
 * Returns: (transfer none): the name of the accessible property
 */
const char *
gtk_accessible_property_get_attribute_name (GtkAccessibleProperty property)
{
  g_return_val_if_fail (property >= GTK_ACCESSIBLE_PROPERTY_AUTOCOMPLETE &&
                        property <= GTK_ACCESSIBLE_PROPERTY_VALUE_TEXT,
                        "<none>");

  return property_attrs[property];
}

static const char *relation_attrs[] = {
  [GTK_ACCESSIBLE_RELATION_ACTIVE_DESCENDANT]   = "activedescendant",
  [GTK_ACCESSIBLE_RELATION_COL_COUNT]           = "colcount",
  [GTK_ACCESSIBLE_RELATION_COL_INDEX]           = "colindex",
  [GTK_ACCESSIBLE_RELATION_COL_INDEX_TEXT]      = "colindextext",
  [GTK_ACCESSIBLE_RELATION_COL_SPAN]            = "colspan",
  [GTK_ACCESSIBLE_RELATION_CONTROLS]            = "controls",
  [GTK_ACCESSIBLE_RELATION_DESCRIBED_BY]        = "describedby",
  [GTK_ACCESSIBLE_RELATION_DETAILS]             = "details",
  [GTK_ACCESSIBLE_RELATION_ERROR_MESSAGE]       = "errormessage",
  [GTK_ACCESSIBLE_RELATION_FLOW_TO]             = "flowto",
  [GTK_ACCESSIBLE_RELATION_LABELLED_BY]         = "labelledby",
  [GTK_ACCESSIBLE_RELATION_OWNS]                = "owns",
  [GTK_ACCESSIBLE_RELATION_POS_IN_SET]          = "posinset",
  [GTK_ACCESSIBLE_RELATION_ROW_COUNT]           = "rowcount",
  [GTK_ACCESSIBLE_RELATION_ROW_INDEX]           = "rowindex",
  [GTK_ACCESSIBLE_RELATION_ROW_INDEX_TEXT]      = "rowindextext",
  [GTK_ACCESSIBLE_RELATION_ROW_SPAN]            = "rowspan",
  [GTK_ACCESSIBLE_RELATION_SET_SIZE]            = "setsize",
};

/*< private >
 * gtk_accessible_relation_get_attribute_name:
 * @relation: a #GtkAccessibleRelation
 *
 * Retrieves the name of a #GtkAccessibleRelation.
 *
 * Returns: (transfer none): the name of the accessible relation
 */
const char *
gtk_accessible_relation_get_attribute_name (GtkAccessibleRelation relation)
{
  g_return_val_if_fail (relation >= GTK_ACCESSIBLE_RELATION_ACTIVE_DESCENDANT &&
                        relation <= GTK_ACCESSIBLE_RELATION_SET_SIZE,
                        "<none>");

  return relation_attrs[relation];
}

static const char *state_attrs[] = {
  [GTK_ACCESSIBLE_STATE_BUSY]           = "busy",
  [GTK_ACCESSIBLE_STATE_CHECKED]        = "checked",
  [GTK_ACCESSIBLE_STATE_DISABLED]       = "disabled",
  [GTK_ACCESSIBLE_STATE_EXPANDED]       = "expanded",
  [GTK_ACCESSIBLE_STATE_HIDDEN]         = "hidden",
  [GTK_ACCESSIBLE_STATE_INVALID]        = "invalid",
  [GTK_ACCESSIBLE_STATE_PRESSED]        = "pressed",
  [GTK_ACCESSIBLE_STATE_SELECTED]       = "selected",
};

/*< private >
 * gtk_accessible_state_get_attribute_name:
 * @state: a #GtkAccessibleState
 *
 * Retrieves the name of a #GtkAccessibleState.
 *
 * Returns: (transfer none): the name of the accessible state
 */
const char *
gtk_accessible_state_get_attribute_name (GtkAccessibleState state)
{
  g_return_val_if_fail (state >= GTK_ACCESSIBLE_STATE_BUSY &&
                        state <= GTK_ACCESSIBLE_STATE_SELECTED,
                        "<none>");

  return state_attrs[state];
}

static void
gtk_at_context_init (GtkATContext *self)
{
  self->accessible_role = GTK_ACCESSIBLE_ROLE_NONE;

  self->properties =
    gtk_accessible_attribute_set_new (N_PROPERTIES,
                                      property_attrs,
                                      (GtkAccessibleAttributeDefaultFunc) gtk_accessible_value_get_default_for_property);
  self->relations =
    gtk_accessible_attribute_set_new (N_RELATIONS,
                                      relation_attrs,
                                      (GtkAccessibleAttributeDefaultFunc) gtk_accessible_value_get_default_for_relation);
  self->states =
    gtk_accessible_attribute_set_new (N_STATES,
                                      state_attrs,
                                      (GtkAccessibleAttributeDefaultFunc) gtk_accessible_value_get_default_for_state);
}

/**
 * gtk_at_context_get_accessible:
 * @self: a #GtkATContext
 *
 * Retrieves the #GtkAccessible using this context.
 *
 * Returns: (transfer none): a #GtkAccessible
 */
GtkAccessible *
gtk_at_context_get_accessible (GtkATContext *self)
{
  g_return_val_if_fail (GTK_IS_AT_CONTEXT (self), NULL);

  return self->accessible;
}

/**
 * gtk_at_context_get_accessible_role:
 * @self: a #GtkATContext
 *
 * Retrieves the accessible role of this context.
 *
 * Returns: a #GtkAccessibleRole
 */
GtkAccessibleRole
gtk_at_context_get_accessible_role (GtkATContext *self)
{
  g_return_val_if_fail (GTK_IS_AT_CONTEXT (self), GTK_ACCESSIBLE_ROLE_NONE);

  return self->accessible_role;
}

/*< private >
 * gtk_at_context_get_display:
 * @self: a #GtkATContext
 *
 * Retrieves the #GdkDisplay used to create the context.
 *
 * Returns: (transfer none): a #GdkDisplay
 */
GdkDisplay *
gtk_at_context_get_display (GtkATContext *self)
{
  g_return_val_if_fail (GTK_IS_AT_CONTEXT (self), NULL);

  return self->display;
}

static const struct {
  const char *name;
  GtkATContext * (* create_context) (GtkAccessibleRole accessible_role,
                                     GtkAccessible    *accessible,
                                     GdkDisplay       *display);
} a11y_backends[] = {
#if defined(GDK_WINDOWING_WAYLAND)
  { "AT-SPI (Wayland)", gtk_at_spi_create_context },
#endif
#if defined(GDK_WINDOWING_X11)
  { "AT-SPI (X11)", gtk_at_spi_create_context },
#endif
  { NULL, NULL },
};

/**
 * gtk_at_context_create: (constructor)
 * @accessible_role: the accessible role used by the #GtkATContext
 * @accessible: the #GtkAccessible implementation using the #GtkATContext
 * @display: the #GdkDisplay used by the #GtkATContext
 *
 * Creates a new #GtkATContext instance for the given accessible role,
 * accessible instance, and display connection.
 *
 * The #GtkATContext implementation being instantiated will depend on the
 * platform.
 *
 * Returns: (nullable) (transfer full): the #GtkATContext
 */
GtkATContext *
gtk_at_context_create (GtkAccessibleRole  accessible_role,
                       GtkAccessible     *accessible,
                       GdkDisplay        *display)
{
  static const char *gtk_test_accessible;
  static const char *gtk_no_a11y;

  if (G_UNLIKELY (gtk_test_accessible == NULL))
    {
      const char *env = g_getenv ("GTK_TEST_ACCESSIBLE");

      if (env != NULL && *env !='\0')
        gtk_test_accessible = "1";
      else
        gtk_test_accessible = "0";
    }

  if (G_UNLIKELY (gtk_no_a11y == NULL))
    {
      const char *env = g_getenv ("GTK_NO_A11Y");

      if (env != NULL && *env != '\0')
        gtk_no_a11y = "1";
      else
        gtk_no_a11y = "0";
    }

  /* Shortcut everything if we're running with the test AT context */
  if (gtk_test_accessible[0] == '1')
    return gtk_test_at_context_new (accessible_role, accessible);

  if (gtk_no_a11y[0] == '1')
    return NULL;

  GtkATContext *res = NULL;

  for (guint i = 0; i < G_N_ELEMENTS (a11y_backends); i++)
    {
      if (a11y_backends[i].name == NULL)
        break;

      GTK_NOTE (A11Y, g_message ("Trying %s a11y backend", a11y_backends[i].name));
      if (a11y_backends[i].create_context != NULL)
        {
          res = a11y_backends[i].create_context (accessible_role, accessible, display);
          if (res != NULL)
            break;
        }
    }

  /* Fall back to the test context, so we can get debugging data */
  if (res == NULL)
    res = g_object_new (GTK_TYPE_TEST_AT_CONTEXT,
                        "accessible_role", accessible_role,
                        "accessible", accessible,
                        "display", display,
                        NULL);

  /* FIXME: Add GIOExtension for AT contexts */
  return res;
}

/*< private >
 * gtk_at_context_update:
 * @self: a #GtkATContext
 *
 * Notifies the AT connected to this #GtkATContext that the accessible
 * state and its properties have changed.
 */
void
gtk_at_context_update (GtkATContext *self)
{
  g_return_if_fail (GTK_IS_AT_CONTEXT (self));

  /* There's no point in notifying of state changes if there weren't any */
  if (self->updated_properties == 0 &&
      self->updated_relations == 0 &&
      self->updated_states == 0)
    return;

  GtkAccessibleStateChange changed_states =
    gtk_accessible_attribute_set_get_changed (self->states);
  GtkAccessiblePropertyChange changed_properties =
    gtk_accessible_attribute_set_get_changed (self->properties);
  GtkAccessibleRelationChange changed_relations =
    gtk_accessible_attribute_set_get_changed (self->relations);

  GTK_AT_CONTEXT_GET_CLASS (self)->state_change (self,
                                                 changed_states, changed_properties, changed_relations,
                                                 self->states, self->properties, self->relations);
  g_signal_emit (self, obj_signals[STATE_CHANGE], 0);

  self->updated_properties = 0;
  self->updated_relations = 0;
  self->updated_states = 0;
}

/*< private >
 * gtk_at_context_set_accessible_state:
 * @self: a #GtkATContext
 * @state: a #GtkAccessibleState
 * @value: (nullable): #GtkAccessibleValue
 *
 * Sets the @value for the given @state of a #GtkATContext.
 *
 * If @value is %NULL, the state is unset.
 *
 * This function will accumulate state changes until gtk_at_context_update()
 * is called.
 */
void
gtk_at_context_set_accessible_state (GtkATContext       *self,
                                     GtkAccessibleState  state,
                                     GtkAccessibleValue *value)
{
  g_return_if_fail (GTK_IS_AT_CONTEXT (self));

  gboolean res = FALSE;

  if (value != NULL)
    res = gtk_accessible_attribute_set_add (self->states, state, value);
  else
    res = gtk_accessible_attribute_set_remove (self->states, state);

  if (res)
    self->updated_states |= (1 << state);
}

/*< private >
 * gtk_at_context_has_accessible_state:
 * @self: a #GtkATContext
 * @state: a #GtkAccessibleState
 *
 * Checks whether a #GtkATContext has the given @state set.
 *
 * Returns: %TRUE, if the accessible state is set
 */
gboolean
gtk_at_context_has_accessible_state (GtkATContext       *self,
                                     GtkAccessibleState  state)
{
  g_return_val_if_fail (GTK_IS_AT_CONTEXT (self), FALSE);

  return gtk_accessible_attribute_set_contains (self->states, state);
}

/*< private >
 * gtk_at_context_get_accessible_state:
 * @self: a #GtkATContext
 * @state: a #GtkAccessibleState
 *
 * Retrieves the value for the accessible state of a #GtkATContext.
 *
 * Returns: (transfer none): the value for the given state
 */
GtkAccessibleValue *
gtk_at_context_get_accessible_state (GtkATContext       *self,
                                     GtkAccessibleState  state)
{
  g_return_val_if_fail (GTK_IS_AT_CONTEXT (self), NULL);

  return gtk_accessible_attribute_set_get_value (self->states, state);
}

/*< private >
 * gtk_at_context_set_accessible_property:
 * @self: a #GtkATContext
 * @property: a #GtkAccessibleProperty
 * @value: (nullable): #GtkAccessibleValue
 *
 * Sets the @value for the given @property of a #GtkATContext.
 *
 * If @value is %NULL, the property is unset.
 *
 * This function will accumulate property changes until gtk_at_context_update()
 * is called.
 */
void
gtk_at_context_set_accessible_property (GtkATContext          *self,
                                        GtkAccessibleProperty  property,
                                        GtkAccessibleValue    *value)
{
  g_return_if_fail (GTK_IS_AT_CONTEXT (self));

  gboolean res = FALSE;

  if (value != NULL)
    res = gtk_accessible_attribute_set_add (self->properties, property, value);
  else
    res = gtk_accessible_attribute_set_remove (self->properties, property);

  if (res)
    self->updated_properties |= (1 << property);
}

/*< private >
 * gtk_at_context_has_accessible_property:
 * @self: a #GtkATContext
 * @property: a #GtkAccessibleProperty
 *
 * Checks whether a #GtkATContext has the given @property set.
 *
 * Returns: %TRUE, if the accessible property is set
 */
gboolean
gtk_at_context_has_accessible_property (GtkATContext          *self,
                                        GtkAccessibleProperty  property)
{
  g_return_val_if_fail (GTK_IS_AT_CONTEXT (self), FALSE);

  return gtk_accessible_attribute_set_contains (self->properties, property);
}

/*< private >
 * gtk_at_context_get_accessible_property:
 * @self: a #GtkATContext
 * @property: a #GtkAccessibleProperty
 *
 * Retrieves the value for the accessible property of a #GtkATContext.
 *
 * Returns: (transfer none): the value for the given property
 */
GtkAccessibleValue *
gtk_at_context_get_accessible_property (GtkATContext          *self,
                                        GtkAccessibleProperty  property)
{
  g_return_val_if_fail (GTK_IS_AT_CONTEXT (self), NULL);

  return gtk_accessible_attribute_set_get_value (self->properties, property);
}

/*< private >
 * gtk_at_context_set_accessible_relation:
 * @self: a #GtkATContext
 * @relation: a #GtkAccessibleRelation
 * @value: (nullable): #GtkAccessibleValue
 *
 * Sets the @value for the given @relation of a #GtkATContext.
 *
 * If @value is %NULL, the relation is unset.
 *
 * This function will accumulate relation changes until gtk_at_context_update()
 * is called.
 */
void
gtk_at_context_set_accessible_relation (GtkATContext          *self,
                                        GtkAccessibleRelation  relation,
                                        GtkAccessibleValue    *value)
{
  g_return_if_fail (GTK_IS_AT_CONTEXT (self));

  gboolean res = FALSE;

  if (value != NULL)
    res = gtk_accessible_attribute_set_add (self->relations, relation, value);
  else
    res = gtk_accessible_attribute_set_remove (self->relations, relation);

  if (res)
    self->updated_relations |= (1 << relation);
}

/*< private >
 * gtk_at_context_has_accessible_relation:
 * @self: a #GtkATContext
 * @relation: a #GtkAccessibleRelation
 *
 * Checks whether a #GtkATContext has the given @relation set.
 *
 * Returns: %TRUE, if the accessible relation is set
 */
gboolean
gtk_at_context_has_accessible_relation (GtkATContext          *self,
                                        GtkAccessibleRelation  relation)
{
  g_return_val_if_fail (GTK_IS_AT_CONTEXT (self), FALSE);

  return gtk_accessible_attribute_set_contains (self->relations, relation);
}

/*< private >
 * gtk_at_context_get_accessible_relation:
 * @self: a #GtkATContext
 * @relation: a #GtkAccessibleRelation
 *
 * Retrieves the value for the accessible relation of a #GtkATContext.
 *
 * Returns: (transfer none): the value for the given relation
 */
GtkAccessibleValue *
gtk_at_context_get_accessible_relation (GtkATContext          *self,
                                        GtkAccessibleRelation  relation)
{
  g_return_val_if_fail (GTK_IS_AT_CONTEXT (self), NULL);

  return gtk_accessible_attribute_set_get_value (self->relations, relation);
}