evdev: add callback to constrain the pointer position

Add a new callback that is called prior to emitting pointer
motion events and that can modify the new pointer position.
The main purpose is allowing multiscreen apps to prevent the
pointer for entering the dead area that exists when the screens
are not the same size, but it could also used to implement
pointer barriers.
A callback is needed to make sure that the hook is called early
enough and the Clutter state is always consistent.

https://bugzilla.gnome.org/show_bug.cgi?id=706652

clutter/evdev/clutter-device-manager-evdev.c

@@ -73,6 +73,10 @@ struct _ClutterDeviceManagerEvdevPrivate
   ClutterInputDevice *core_pointer;
   ClutterInputDevice *core_keyboard;
 
+  ClutterPointerConstrainCallback constrain_callback;
+  gpointer                        constrain_data;
+  GDestroyNotify                  constrain_data_notify;
+
   ClutterStageManager *stage_manager;
   guint stage_added_handler;
   guint stage_removed_handler;
@@ -259,8 +263,17 @@ notify_relative_motion (ClutterEventSource *source,
   clutter_input_device_get_coords (manager_evdev->priv->core_pointer, NULL, &point);
   new_x = point.x + dx;
   new_y = point.y + dy;
-  new_x = CLAMP (new_x, 0.f, stage_width - 1);
+
-  new_y = CLAMP (new_y, 0.f, stage_height - 1);
+  if (manager_evdev->priv->constrain_callback)
+    {
+      manager_evdev->priv->constrain_callback (manager_evdev->priv->core_pointer, time_, &new_x, &new_y,
+					       manager_evdev->priv->constrain_data);
+    }
+  else
+    {
+      new_x = CLAMP (new_x, 0.f, stage_width - 1);
+      new_y = CLAMP (new_y, 0.f, stage_height - 1);
+    }
 
   event->motion.time = time_;
   event->motion.stage = stage;
@@ -1029,6 +1042,9 @@ clutter_device_manager_evdev_finalize (GObject *object)
     }
   g_slist_free (priv->event_sources);
 
+  if (priv->constrain_data_notify)
+    priv->constrain_data_notify (priv->constrain_data);
+
   G_OBJECT_CLASS (clutter_device_manager_evdev_parent_class)->finalize (object);
 }
 
@@ -1313,3 +1329,36 @@ clutter_evdev_set_keyboard_map (ClutterDeviceManager *evdev,
   for (i = 0; i < priv->keys->len; i++)
     xkb_state_update_key (priv->xkb, g_array_index (priv->keys, guint32, i), XKB_KEY_DOWN);
 }
+
+/**
+ * clutter_evdev_set_pointer_constrain_callback:
+ * @evdev: the #ClutterDeviceManager created by the evdev backend
+ * @callback: the callback
+ * @user_data:
+ * @user_data_notify:
+ *
+ * Sets a callback to be invoked for every pointer motion. The callback
+ * can then modify the new pointer coordinates to constrain movement within
+ * a specific region.
+ */
+void
+clutter_evdev_set_pointer_constrain_callback (ClutterDeviceManager            *evdev,
+					      ClutterPointerConstrainCallback  callback,
+					      gpointer                         user_data,
+					      GDestroyNotify                   user_data_notify)
+{
+  ClutterDeviceManagerEvdev *manager_evdev;
+  ClutterDeviceManagerEvdevPrivate *priv;
+
+  g_return_if_fail (CLUTTER_IS_DEVICE_MANAGER_EVDEV (evdev));
+
+  manager_evdev = CLUTTER_DEVICE_MANAGER_EVDEV (evdev);
+  priv = manager_evdev->priv;
+
+  if (priv->constrain_data_notify)
+    priv->constrain_data_notify (priv->constrain_data);
+
+  priv->constrain_callback = callback;
+  priv->constrain_data = user_data;
+  priv->constrain_data_notify = user_data_notify;
+}

clutter/evdev/clutter-evdev.h

@@ -51,6 +51,33 @@ void  clutter_evdev_set_open_callback (ClutterOpenDeviceCallback callback,
 void  clutter_evdev_release_devices (void);
 void  clutter_evdev_reclaim_devices (void);
 
+/**
+ * ClutterPointerConstrainCallback:
+ * @device: the core pointer device
+ * @time: the event time in milliseconds
+ * @x: (inout): the new X coordinate
+ * @y: (inout): the new Y coordinate
+ * @user_data:
+ *
+ * This callback will be called for all pointer motion events, and should
+ * update (@x, @y) to constrain the pointer position appropriately.
+ * The subsequent motion event will use the updated values as the new coordinates.
+ * Note that the coordinates are not clamped to the stage size, and the callback
+ * must make sure that this happens before it returns.
+ * Also note that the event will be emitted even if the pointer is constrained
+ * to be in the same position.
+ */
+typedef void (*ClutterPointerConstrainCallback) (ClutterInputDevice *device,
+						 guint32             time,
+						 float              *x,
+						 float              *y,
+						 gpointer            user_data);
+
+void  clutter_evdev_set_pointer_constrain_callback (ClutterDeviceManager            *evdev,
+						    ClutterPointerConstrainCallback  callback,
+						    gpointer                         user_data,
+						    GDestroyNotify                   user_data_notify);
+
 struct xkb_state * clutter_evdev_get_keyboard_state (ClutterDeviceManager *evdev);
 void               clutter_evdev_set_keyboard_map   (ClutterDeviceManager *evdev,
 						     struct xkb_keymap    *keymap);