From 4ea4dc9176a3f33cf2752cc65290d98810e0f2bd Mon Sep 17 00:00:00 2001
From: Emmanuele Bassi <ebassi@gnome.org>
Date: Fri, 24 May 2024 19:15:09 +0100
Subject: [PATCH] a11y: Watch EventListenerRegistered/Deregistered signals

Assistive technologies using AT-SPI typically register themselves on the
accessibility bus through the org.a11y.atspi.Registry.RegisterEvent
method, which will emit the EventListenerRegistered signal. We can use
that signal (and its corresponding EventListenerDeregistered sibling) to
know whether there is at least an AT on the other side of the
accessibility bus.
---
 gtk/a11y/gtkatspiroot.c        | 110 +++++++++++++++++++++++++++++++++
 gtk/a11y/gtkatspirootprivate.h |   3 +
 2 files changed, 113 insertions(+)

diff --git a/gtk/a11y/gtkatspiroot.c b/gtk/a11y/gtkatspiroot.c
index 8727851add..de4e875d1d 100644
--- a/gtk/a11y/gtkatspiroot.c
+++ b/gtk/a11y/gtkatspiroot.c
@@ -45,6 +45,7 @@
 #define ATSPI_PATH_PREFIX       "/org/a11y/atspi"
 #define ATSPI_ROOT_PATH         ATSPI_PATH_PREFIX "/accessible/root"
 #define ATSPI_CACHE_PATH        ATSPI_PATH_PREFIX "/cache"
+#define ATSPI_REGISTRY_PATH     ATSPI_PATH_PREFIX "/registry"
 
 struct _GtkAtSpiRoot
 {
@@ -71,6 +72,9 @@ struct _GtkAtSpiRoot
   GtkAtSpiCache *cache;
 
   GListModel *toplevels;
+
+  /* HashTable<str, str> */
+  GHashTable *event_listeners;
 };
 
 enum
@@ -90,6 +94,7 @@ gtk_at_spi_root_finalize (GObject *gobject)
   GtkAtSpiRoot *self = GTK_AT_SPI_ROOT (gobject);
 
   g_clear_handle_id (&self->register_id, g_source_remove);
+  g_clear_pointer (&self->event_listeners, g_hash_table_unref);
 
   g_free (self->bus_address);
   g_free (self->base_path);
@@ -541,6 +546,81 @@ on_registration_reply (GObject      *gobject,
   g_free (data);
 }
 
+static void
+on_event_listener_registered (GDBusConnection *connection,
+                              const char *sender_name,
+                              const char *object_path,
+                              const char *interface_name,
+                              const char *signal_name,
+                              GVariant *parameters,
+                              gpointer user_data)
+{
+  GtkAtSpiRoot *self = user_data;
+
+  if (g_strcmp0 (object_path, ATSPI_REGISTRY_PATH) == 0 &&
+      g_strcmp0 (interface_name, "org.a11y.atspi.Registry") == 0 &&
+      g_strcmp0 (signal_name, "EventListenerRegistered") == 0)
+    {
+      char *sender = NULL;
+      char *event_name = NULL;
+      char **event_types = NULL;
+
+      if (self->event_listeners == NULL)
+        self->event_listeners = g_hash_table_new_full (g_str_hash, g_str_equal,
+                                                       g_free,
+                                                       NULL);
+
+      g_variant_get (parameters, "(ssas)", &sender, &event_name, &event_types);
+
+      GTK_DEBUG (A11Y, "Registering event listener (%s, %s) on the a11y bus",
+                 sender,
+                 event_name[0] != 0 ? event_name : "(none)");
+
+      g_hash_table_add (self->event_listeners, sender);
+
+      g_free (event_name);
+      g_strfreev (event_types);
+    }
+}
+
+static void
+on_event_listener_deregistered (GDBusConnection *connection,
+                                const char *sender_name,
+                                const char *object_path,
+                                const char *interface_name,
+                                const char *signal_name,
+                                GVariant *parameters,
+                                gpointer user_data)
+{
+  GtkAtSpiRoot *self = user_data;
+
+  if (g_strcmp0 (object_path, ATSPI_REGISTRY_PATH) == 0 &&
+      g_strcmp0 (interface_name, "org.a11y.atspi.Registry") == 0 &&
+      g_strcmp0 (signal_name, "EventListenerDeregistered") == 0)
+    {
+      const char *sender = NULL;
+      const char *event = NULL;
+
+      if (self->event_listeners == NULL)
+        {
+          g_critical ("Received org.a11y.atspi.Registry::EventListenerDeregistered without "
+                      "a corresponding EventListenerRegistered signal.");
+          return;
+        }
+
+      g_variant_get (parameters, "(&s&s)", &sender, &event);
+
+      if (g_hash_table_contains (self->event_listeners, sender))
+        {
+          GTK_DEBUG (A11Y, "Deregistering event listener (%s, %s) on the a11y bus",
+                     sender,
+                     event[0] != 0 ? event : "(none)");
+
+          g_hash_table_remove (self->event_listeners, sender);
+        }
+    }
+}
+
 static gboolean
 root_register (gpointer user_data)
 {
@@ -587,6 +667,27 @@ root_register (gpointer user_data)
                                      NULL,
                                      NULL);
 
+  g_dbus_connection_signal_subscribe (self->connection,
+                                      "org.a11y.atspi.Registry",
+                                      "org.a11y.atspi.Registry",
+                                      "EventListenerRegistered",
+                                      ATSPI_REGISTRY_PATH,
+                                      NULL,
+                                      G_DBUS_SIGNAL_FLAGS_NONE,
+                                      on_event_listener_registered,
+                                      self,
+                                      NULL);
+  g_dbus_connection_signal_subscribe (self->connection,
+                                      "org.a11y.atspi.Registry",
+                                      "org.a11y.atspi.Registry",
+                                      "EventListenerDeregistered",
+                                      ATSPI_REGISTRY_PATH,
+                                      NULL,
+                                      G_DBUS_SIGNAL_FLAGS_NONE,
+                                      on_event_listener_deregistered,
+                                      self,
+                                      NULL);
+
   GTK_DEBUG (A11Y, "Registering (%s, %s) on the a11y bus",
                    unique_name,
                    self->root_path);
@@ -818,3 +919,12 @@ gtk_at_spi_root_get_base_path (GtkAtSpiRoot *self)
 
   return self->base_path;
 }
+
+gboolean
+gtk_at_spi_root_has_event_listeners (GtkAtSpiRoot *self)
+{
+  g_return_val_if_fail (GTK_IS_AT_SPI_ROOT (self), FALSE);
+
+  return self->event_listeners != NULL &&
+         g_hash_table_size (self->event_listeners) != 0;
+}
diff --git a/gtk/a11y/gtkatspirootprivate.h b/gtk/a11y/gtkatspirootprivate.h
index 7b626fc28e..627cf3f5b1 100644
--- a/gtk/a11y/gtkatspirootprivate.h
+++ b/gtk/a11y/gtkatspirootprivate.h
@@ -63,4 +63,7 @@ gtk_at_spi_root_child_changed (GtkAtSpiRoot             *self,
                                GtkAccessibleChildChange  change,
                                GtkAccessible            *child);
 
+gboolean
+gtk_at_spi_root_has_event_listeners (GtkAtSpiRoot *self);
+
 G_END_DECLS