x11: Add public API to handle X11 events

This API will be used by GNOME Shell to handle X11 events
in the relevant places, as a substitute to gdk_window_add_filter().

It is ATM still a bit ironic, since the Mutter X11 event handler
is itself a GdkFilterFunc, but it may move away from that eventually.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2779>

src/meta/meta-x11-display.h

@@ -27,6 +27,10 @@
 #include <meta/prefs.h>
 #include <meta/types.h>
 
+typedef void (* MetaX11DisplayEventFunc) (MetaX11Display *x11_display,
+                                          XEvent         *xev,
+                                          gpointer        user_data);
+
 #define META_TYPE_X11_DISPLAY (meta_x11_display_get_type ())
 
 META_EXPORT
@@ -67,4 +71,14 @@ void     meta_x11_display_set_stage_input_region (MetaX11Display *x11_display,
 META_EXPORT
 void     meta_x11_display_clear_stage_input_region (MetaX11Display *x11_display);
 
+META_EXPORT
+unsigned int meta_x11_display_add_event_func (MetaX11Display          *x11_display,
+                                              MetaX11DisplayEventFunc  event_func,
+                                              gpointer                 user_data,
+                                              GDestroyNotify           destroy_notify);
+
+META_EXPORT
+void meta_x11_display_remove_event_func (MetaX11Display *x11_display,
+                                         unsigned int    id);
+
 #endif /* META_X11_DISPLAY_H */

src/x11/events.c

@@ -1923,6 +1923,8 @@ meta_x11_display_handle_xevent (MetaX11Display *x11_display,
   meta_spew_event_print (x11_display, event);
 #endif
 
+  meta_x11_display_run_event_funcs (x11_display, event);
+
   if (meta_x11_startup_notification_handle_xevent (x11_display, event))
     {
       bypass_gtk = bypass_compositor = TRUE;

src/x11/meta-x11-display-private.h

@@ -121,6 +121,8 @@ struct _MetaX11Display
   GHashTable *xids;
   GHashTable *alarms;
 
+  GList *event_funcs;
+
   gboolean has_xinerama_indices;
 
   /* Managed by group.c */
@@ -282,4 +284,7 @@ void meta_x11_display_sync_input_focus (MetaX11Display *x11_display);
 
 MetaDisplay * meta_x11_display_get_display (MetaX11Display *x11_display);
 
+void meta_x11_display_run_event_funcs (MetaX11Display *x11_display,
+                                       XEvent         *xevent);
+
 #endif /* META_X11_DISPLAY_PRIVATE_H */

src/x11/meta-x11-display.c

@@ -77,6 +77,16 @@ G_DEFINE_TYPE (MetaX11Display, meta_x11_display, G_TYPE_OBJECT)
 
 static GQuark quark_x11_display_logical_monitor_data = 0;
 
+typedef struct _MetaX11EventFilter MetaX11EventFilter;
+
+struct _MetaX11EventFilter
+{
+  unsigned int id;
+  MetaX11DisplayEventFunc func;
+  gpointer user_data;
+  GDestroyNotify destroy_notify;
+};
+
 typedef struct _MetaX11DisplayLogicalMonitorData
 {
   int xinerama_index;
@@ -126,6 +136,14 @@ meta_x11_display_unmanage_windows (MetaX11Display *x11_display)
   g_list_free_full (windows, g_object_unref);
 }
 
+static void
+meta_x11_event_filter_free (MetaX11EventFilter *filter)
+{
+  if (filter->destroy_notify && filter->user_data)
+    filter->destroy_notify (filter->user_data);
+  g_free (filter);
+}
+
 static void
 meta_x11_display_dispose (GObject *object)
 {
@@ -135,6 +153,9 @@ meta_x11_display_dispose (GObject *object)
 
   g_clear_pointer (&x11_display->alarm_filters, g_ptr_array_unref);
 
+  g_clear_list (&x11_display->event_funcs,
+                (GDestroyNotify) meta_x11_event_filter_free);
+
   if (x11_display->frames_client_cancellable)
     {
       g_cancellable_cancel (x11_display->frames_client_cancellable);
@@ -2472,3 +2493,64 @@ meta_x11_display_clear_stage_input_region (MetaX11Display *x11_display)
   meta_x11_display_set_stage_input_region (x11_display,
                                            x11_display->empty_region);
 }
+
+/**
+ * meta_x11_display_add_event_func: (skip):
+ **/
+unsigned int
+meta_x11_display_add_event_func (MetaX11Display          *x11_display,
+                                 MetaX11DisplayEventFunc  event_func,
+                                 gpointer                 user_data,
+                                 GDestroyNotify           destroy_notify)
+{
+  MetaX11EventFilter *filter;
+  static unsigned int id = 0;
+
+  filter = g_new0 (MetaX11EventFilter, 1);
+  filter->func = event_func;
+  filter->user_data = user_data;
+  filter->destroy_notify = destroy_notify;
+  filter->id = ++id;
+
+  x11_display->event_funcs = g_list_prepend (x11_display->event_funcs, filter);
+
+  return filter->id;
+}
+
+/**
+ * meta_x11_display_remove_event_func: (skip):
+ **/
+void
+meta_x11_display_remove_event_func (MetaX11Display *x11_display,
+                                    unsigned int    id)
+{
+  MetaX11EventFilter *filter;
+  GList *l;
+
+  for (l = x11_display->event_funcs; l; l = l->next)
+    {
+      filter = l->data;
+
+      if (filter->id != id)
+        continue;
+
+      x11_display->event_funcs =
+        g_list_delete_link (x11_display->event_funcs, l);
+      meta_x11_event_filter_free (filter);
+      break;
+    }
+}
+
+void
+meta_x11_display_run_event_funcs (MetaX11Display *x11_display,
+                                  XEvent         *xevent)
+{
+  MetaX11EventFilter *filter;
+  GList *l;
+
+  for (l = x11_display->event_funcs; l; l = l->next)
+    {
+      filter = l->data;
+      filter->func (x11_display, xevent, filter->user_data);
+    }
+}