clutter/gesture: Allow only a single gesture to recognize globally

Gestures are independent from each other when they are on different
actors and when they don't have a conflict over input with each other: For
example a gesture on one window and a gesture on another window will
recognize at the same time perfectly fine right now.

For those gestures (let's call them "independent gestures") we don't want
to control how the conflicting input should be handled, i.e. whether one
gesture wins over another or whether both can be recognizing using the
same touchpoint (e.g. zoom+rotate on the same actor). Instead we want
to control whether they are allowed to start while another one is running.

For now, make it impossible for two gestures to recognize globally at
the same time in all cases. This helps prevent subtle bugs and makes life
easier for users of the API. We can introduce API for fine grained control
over the behavior later.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2389>

clutter/clutter/clutter-gesture.c

@@ -121,6 +121,7 @@ typedef struct _ClutterGesturePrivate ClutterGesturePrivate;
 struct _ClutterGesturePrivate
 {
   GArray *sequences;
+  GPtrArray *stage_all_active_gestures;
 
   unsigned int latest_index;
 
@@ -389,11 +390,56 @@ out:
   maybe_move_to_waiting (self);
 }
 
+static gboolean
+other_gesture_allowed_to_start (ClutterGesture *self,
+                                ClutterGesture *other_gesture)
+{
+  /* Only a single gesture can be recognizing globally at a time */
+  return FALSE;
+}
+
+static gboolean
+new_gesture_allowed_to_start (ClutterGesture *self)
+{
+  ClutterGesturePrivate *priv = clutter_gesture_get_instance_private (self);
+  unsigned int i;
+
+  for (i = 0; i < priv->stage_all_active_gestures->len; i++)
+    {
+      ClutterGesture *existing_gesture =
+        g_ptr_array_index (priv->stage_all_active_gestures, i);
+      ClutterGesturePrivate *other_priv =
+        clutter_gesture_get_instance_private (existing_gesture);
+
+      if (existing_gesture == self)
+        continue;
+
+      /* For gestures in relationship we have different APIs */
+      if (g_hash_table_contains (other_priv->in_relationship_with, self))
+        continue;
+
+      if (other_priv->state == CLUTTER_GESTURE_STATE_RECOGNIZING)
+        {
+          if (!other_gesture_allowed_to_start (existing_gesture, self))
+            return FALSE;
+        }
+    }
+
+  return TRUE;
+}
+
 static gboolean
 gesture_may_start (ClutterGesture *self)
 {
   gboolean may_recognize;
 
+  if (!new_gesture_allowed_to_start (self))
+    {
+      debug_message (self,
+                    "gesture may not recognize, another gesture is already running");
+      return FALSE;
+    }
+
   g_signal_emit (self, obj_signals[MAY_RECOGNIZE], 0, &may_recognize);
   if (!may_recognize)
     {
@@ -405,6 +451,40 @@ gesture_may_start (ClutterGesture *self)
   return TRUE;
 }
 
+static void
+maybe_cancel_independent_gestures (ClutterGesture *self)
+{
+  ClutterGesturePrivate *priv = clutter_gesture_get_instance_private (self);
+  int i;
+
+  g_assert (priv->stage_all_active_gestures != NULL);
+
+  for (i = priv->stage_all_active_gestures->len - 1; i >= 0; i--)
+    {
+      if (i >= priv->stage_all_active_gestures->len)
+        continue;
+
+      ClutterGesture *other_gesture =
+        g_ptr_array_index (priv->stage_all_active_gestures, i);
+      ClutterGesturePrivate *other_priv =
+        clutter_gesture_get_instance_private (other_gesture);
+
+      if (other_gesture == self)
+        continue;
+
+      /* For gestures in relationship we have different APIs */
+      if (g_hash_table_contains (priv->in_relationship_with, other_gesture))
+        continue;
+
+      if (other_priv->state == CLUTTER_GESTURE_STATE_POSSIBLE &&
+          !other_gesture_allowed_to_start (self, other_gesture))
+        {
+          debug_message (self, "Cancelling independent gesture in POSSIBLE on recognize");
+          set_state_authoritative (other_gesture, CLUTTER_GESTURE_STATE_CANCELLED);
+        }
+    }
+}
+
 static void
 set_state (ClutterGesture      *self,
            ClutterGestureState  new_state)
@@ -445,6 +525,29 @@ set_state (ClutterGesture      *self,
       break;
     }
 
+  if (priv->state == CLUTTER_GESTURE_STATE_WAITING)
+    {
+      if (new_state == CLUTTER_GESTURE_STATE_POSSIBLE)
+        {
+          if (!priv->stage_all_active_gestures)
+            {
+              ClutterActor *actor;
+              ClutterStage *stage;
+
+              actor = clutter_actor_meta_get_actor (CLUTTER_ACTOR_META (self));
+              g_assert (actor);
+
+              stage = CLUTTER_STAGE (clutter_actor_get_stage (actor));
+              g_assert (stage);
+
+              priv->stage_all_active_gestures =
+                clutter_stage_get_active_gestures_array (stage);
+            }
+
+          g_ptr_array_add (priv->stage_all_active_gestures, self);
+        }
+    }
+
   if (priv->state == CLUTTER_GESTURE_STATE_POSSIBLE)
     {
       if (new_state == CLUTTER_GESTURE_STATE_RECOGNIZING ||
@@ -465,37 +568,45 @@ set_state (ClutterGesture      *self,
        new_state == CLUTTER_GESTURE_STATE_COMPLETED))
     {
       ClutterActor *actor;
+      ClutterStage *stage;
+      unsigned int i;
 
       actor = clutter_actor_meta_get_actor (CLUTTER_ACTOR_META (self));
-      if (actor)
+      g_assert (actor);
+
+      stage = CLUTTER_STAGE (clutter_actor_get_stage (actor));
+      g_assert (stage);
+
+      for (i = 0; i < priv->sequences->len; i++)
         {
-          ClutterStage *stage = CLUTTER_STAGE (clutter_actor_get_stage (actor));
+          GestureSequenceData *seq_data =
+            &g_array_index (priv->sequences, GestureSequenceData, i);
 
-          if (stage)
+          if (seq_data->ended)
-            {
+            continue;
-              unsigned int i;
 
-              for (i = 0; i < priv->sequences->len; i++)
+          clutter_stage_notify_action_implicit_grab (stage,
-                {
+                                                     seq_data->device,
-                  GestureSequenceData *seq_data =
+                                                     seq_data->sequence);
-                    &g_array_index (priv->sequences, GestureSequenceData, i);
-
-                  if (seq_data->ended)
-                    continue;
-
-                  clutter_stage_notify_action_implicit_grab (stage,
-                                                             seq_data->device,
-                                                             seq_data->sequence);
-                }
-            }
         }
+
+      /* Cancel gestures that are independent of ours and still in POSSIBLE:
+       * That's to prevent subtle UI bugs like a click gesture preemptively
+       * applying "pressed" style to a widget even though it most likely won't
+       * recognize anyway.
+       */
+      maybe_cancel_independent_gestures (self);
     }
 
   if (new_state == CLUTTER_GESTURE_STATE_WAITING)
     {
+      gboolean removed;
       GHashTableIter iter;
       ClutterGesture *other_gesture;
 
+      removed = g_ptr_array_remove (priv->stage_all_active_gestures, self);
+      g_assert (removed);
+
       g_array_set_size (priv->sequences, 0);
 
       g_hash_table_iter_init (&iter, priv->in_relationship_with);
@@ -503,7 +614,6 @@ set_state (ClutterGesture      *self,
         {
           ClutterGesturePrivate *other_priv =
             clutter_gesture_get_instance_private (other_gesture);
-          gboolean removed;
 
           removed = g_hash_table_remove (other_priv->in_relationship_with, self);
           g_assert (removed);
@@ -643,8 +753,7 @@ clutter_gesture_real_sequences_cancelled (ClutterGesture *self,
                                           unsigned int   *sequences,
                                           unsigned int    n_sequences)
 {
-  if (clutter_gesture_get_n_points (self) == n_sequences)
+  set_state_authoritative (self, CLUTTER_GESTURE_STATE_CANCELLED);
-    set_state_authoritative (self, CLUTTER_GESTURE_STATE_CANCELLED);
 }
 
 static void
@@ -752,6 +861,7 @@ clutter_gesture_handle_event (ClutterAction      *action,
   g_assert (priv->state != CLUTTER_GESTURE_STATE_WAITING);
 
   is_first_event = !seq_data->seen;
+
   should_emit =
     priv->state == CLUTTER_GESTURE_STATE_POSSIBLE ||
     priv->state == CLUTTER_GESTURE_STATE_RECOGNIZING;
@@ -769,6 +879,34 @@ clutter_gesture_handle_event (ClutterAction      *action,
         may_remove_point = should_emit = FALSE;
     }
 
+  if (priv->state == CLUTTER_GESTURE_STATE_POSSIBLE &&
+      priv->sequences->len == 1 && is_first_event)
+    {
+      /* We cancel independent gestures that are in POSSIBLE when a gesture is
+       * moving to RECOGNIZING, see maybe_cancel_independent_gestures().
+       *
+       * The other half of this behavior is implemented here: Bailing out
+       * on the first event and moving to CANCELLED when an independent one
+       * is already RECOGNIZING.
+       *
+       * Note that we could also return FALSE in register_sequence(), but this
+       * would mean we could't track the sequence and remain in CANCELLED until
+       * the sequence ends. We could also move to CANCELLED in register_sequence()
+       * while still returning TRUE, but then we'd be moving to CANCELLED before
+       * the influencing is fully set-up. So we do in the handle_event() stage
+       * instead.
+       */
+      if (!new_gesture_allowed_to_start (self))
+        {
+          debug_message (self,
+                         "Cancelling gesture on first event, another gesture is "
+                         "already running");
+
+          set_state_authoritative (self, CLUTTER_GESTURE_STATE_CANCELLED);
+          return CLUTTER_EVENT_PROPAGATE;
+        }
+    }
+
   if (should_emit)
     {
       if (seq_data->previous_event)
@@ -1011,6 +1149,9 @@ clutter_gesture_set_actor (ClutterActorMeta *meta,
       cancel_all_points (self);
     }
 
+  if (!actor)
+    priv->stage_all_active_gestures = NULL;
+
   CLUTTER_ACTOR_META_CLASS (clutter_gesture_parent_class)->set_actor (meta, actor);
 }
 
@@ -1085,11 +1226,16 @@ clutter_gesture_finalize (GObject *gobject)
 
   if (priv->state != CLUTTER_GESTURE_STATE_WAITING)
     {
+      gboolean removed;
+
       g_warning ("gesture <%s> [<%s>:%p]: Finalizing while in active state (%s), "
                  "implementation didn't move the gesture to an end state.",
                  clutter_actor_meta_get_name (CLUTTER_ACTOR_META (self)),
                  G_OBJECT_TYPE_NAME (self), self,
                  state_to_string[priv->state]);
+
+      removed = g_ptr_array_remove (priv->stage_all_active_gestures, self);
+      g_assert (removed);
     }
 
   g_array_unref (priv->sequences);