frame-clock: Add callback before the actual frame callback

Aimed to have the frame listener do things like processing events before
the actual frame. In between the before-frame and actual frame,
timelines will be advanced.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1285

clutter/clutter/clutter-frame-clock.c

@@ -255,6 +255,7 @@ static gboolean
 clutter_frame_clock_dispatch (gpointer user_data)
 {
   ClutterFrameClock *frame_clock = user_data;
+  int64_t frame_count;
   ClutterFrameResult result;
 
   COGL_TRACE_BEGIN_SCOPED (ClutterFrameCLockDispatch, "Frame Clock (dispatch)");
@@ -263,9 +264,20 @@ clutter_frame_clock_dispatch (gpointer user_data)
 
   frame_clock->state = CLUTTER_FRAME_CLOCK_STATE_DISPATCHING;
 
+  frame_count = frame_clock->frame_count++;
+
+  COGL_TRACE_BEGIN (ClutterFrameClockEvents, "Frame Clock (before frame)");
+  if (frame_clock->listener.iface->before_frame)
+    {
+      frame_clock->listener.iface->before_frame (frame_clock,
+                                                 frame_count,
+                                                 frame_clock->listener.user_data);
+    }
+  COGL_TRACE_END (ClutterFrameClockEvents);
+
   COGL_TRACE_BEGIN (ClutterFrameClockFrame, "Frame Clock (frame)");
   result = frame_clock->listener.iface->frame (frame_clock,
-                                               frame_clock->frame_count++,
+                                               frame_count,
                                                frame_clock->listener.user_data);
   COGL_TRACE_END (ClutterFrameClockFrame);
 

clutter/clutter/clutter-frame-clock.h

@@ -38,6 +38,9 @@ G_DECLARE_FINAL_TYPE (ClutterFrameClock, clutter_frame_clock,
 
 typedef struct _ClutterFrameListenerIface
 {
+  void (* before_frame) (ClutterFrameClock *frame_clock,
+                         int64_t            frame_count,
+                         gpointer           user_data);
   ClutterFrameResult (* frame) (ClutterFrameClock *frame_clock,
                                 int64_t            frame_count,
                                 gpointer           user_data);

src/tests/clutter/conform/frame-clock.c

@@ -458,10 +458,77 @@ frame_clock_schedule_update_now (void)
   g_source_unref (source);
 }
 
+static void
+before_frame_frame_clock_before_frame (ClutterFrameClock *frame_clock,
+                                       int64_t            frame_count,
+                                       gpointer           user_data)
+{
+  int64_t *expected_frame_count = user_data;
+
+  g_assert_cmpint (*expected_frame_count, ==, frame_count);
+}
+
+static ClutterFrameResult
+before_frame_frame_clock_frame (ClutterFrameClock *frame_clock,
+                                int64_t            frame_count,
+                                gpointer           user_data)
+{
+  int64_t *expected_frame_count = user_data;
+
+  g_assert_cmpint (*expected_frame_count, ==, frame_count);
+
+  (*expected_frame_count)++;
+
+  clutter_frame_clock_notify_presented (frame_clock, g_get_monotonic_time ());
+  clutter_frame_clock_schedule_update (frame_clock);
+
+  return CLUTTER_FRAME_RESULT_PENDING_PRESENTED;
+}
+
+static const ClutterFrameListenerIface before_frame_frame_listener_iface = {
+  .before_frame = before_frame_frame_clock_before_frame,
+  .frame = before_frame_frame_clock_frame,
+};
+
+static gboolean
+quit_main_loop_timeout (gpointer user_data)
+{
+  GMainLoop *main_loop = user_data;
+
+  g_main_loop_quit (main_loop);
+
+  return G_SOURCE_REMOVE;
+}
+
+static void
+frame_clock_before_frame (void)
+{
+  GMainLoop *main_loop;
+  ClutterFrameClock *frame_clock;
+
+  expected_frame_count = 0;
+
+  main_loop = g_main_loop_new (NULL, FALSE);
+  frame_clock = clutter_frame_clock_new (refresh_rate,
+                                         &before_frame_frame_listener_iface,
+                                         &expected_frame_count);
+
+  clutter_frame_clock_schedule_update (frame_clock);
+  g_timeout_add (100, quit_main_loop_timeout, main_loop);
+  g_main_loop_run (main_loop);
+
+  /* We should have at least processed a couple of frames within 100 ms. */
+  g_assert_cmpint (expected_frame_count, >, 2);
+
+  g_main_loop_unref (main_loop);
+  g_object_unref (frame_clock);
+}
+
 CLUTTER_TEST_SUITE (
   CLUTTER_TEST_UNIT ("/frame-clock/schedule-update", frame_clock_schedule_update)
   CLUTTER_TEST_UNIT ("/frame-clock/immediate-present", frame_clock_immediate_present)
   CLUTTER_TEST_UNIT ("/frame-clock/delayed-damage", frame_clock_delayed_damage)
   CLUTTER_TEST_UNIT ("/frame-clock/no-damage", frame_clock_no_damage)
   CLUTTER_TEST_UNIT ("/frame-clock/schedule-update-now", frame_clock_schedule_update_now)
+  CLUTTER_TEST_UNIT ("/frame-clock/before-frame", frame_clock_before_frame)
 )