[gnome-shell] scroll-view: Add proper smooth scrolling

[Jasper St. Pierre <jstpierre src gnome org>]

commit 06dc12e217f0c2e94c761544b113ea8d24d0f19c
Author: Jasper St. Pierre <jstpierre mecheye net>
Date:   Sun Nov 4 10:10:50 2012 -0500

    scroll-view: Add proper smooth scrolling
    
    The code here before was added as dummy code to satisfy an error
    in the missing switch, and wasn't ever tested due to the lack of XI2
    in mutter. Use the same math as GtkRange does to calculate scroll bar
    positions from raw XI2 deltas to allow for proper smooth scrolling.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=687573

 src/st/st-adjustment.c  |   28 +++++++++++++++++++
 src/st/st-adjustment.h  |    4 +++
 src/st/st-scroll-view.c |   70 +++++++++++++++++++++++------------------------
 3 files changed, 66 insertions(+), 36 deletions(-)
---
diff --git a/src/st/st-adjustment.c b/src/st/st-adjustment.c
index e2ef9b2..e6b3367 100644
--- a/src/st/st-adjustment.c
+++ b/src/st/st-adjustment.c
@@ -566,3 +566,31 @@ st_adjustment_get_values (StAdjustment *adjustment,
     *page_size = priv->page_size;
 }
 
+/**
+ * st_adjustment_adjust_for_scroll_event:
+ * @adjustment: An #StAdjustment
+ * @delta: A delta, retrieved directly from clutter_event_get_scroll_delta()
+ *   or similar.
+ *
+ * Adjusts the adjustment using delta values from a scroll event.
+ * You should use this instead of using st_adjustment_set_value()
+ * as this method will tweak the values directly using the same
+ * math as GTK+, to ensure that scrolling is consistent across
+ * the environment.
+ */
+void
+st_adjustment_adjust_for_scroll_event (StAdjustment *adjustment,
+                                       gdouble       delta)
+{
+  StAdjustmentPrivate *priv;
+  gdouble new_value, scroll_unit;
+
+  g_return_if_fail (ST_IS_ADJUSTMENT (adjustment));
+
+  priv = adjustment->priv;
+
+  scroll_unit = pow (priv->page_size, 2.0 / 3.0);
+
+  new_value = priv->value + delta * scroll_unit;
+  st_adjustment_set_value (adjustment, new_value);
+}
diff --git a/src/st/st-adjustment.h b/src/st/st-adjustment.h
index 7660a08..fe041cf 100644
--- a/src/st/st-adjustment.h
+++ b/src/st/st-adjustment.h
@@ -99,6 +99,10 @@ void          st_adjustment_get_values  (StAdjustment *adjustment,
                                          gdouble      *step_increment,
                                          gdouble      *page_increment,
                                          gdouble      *page_size);
+
+void          st_adjustment_adjust_for_scroll_event (StAdjustment *adjustment,
+                                                     gdouble       delta);
+
 G_END_DECLS
 
 #endif /* __ST_ADJUSTMENT_H__ */
diff --git a/src/st/st-scroll-view.c b/src/st/st-scroll-view.c
index c8f89ce..285ba2d 100644
--- a/src/st/st-scroll-view.c
+++ b/src/st/st-scroll-view.c
@@ -666,6 +666,30 @@ st_scroll_view_allocate (ClutterActor          *actor,
 }
 
 static void
+adjust_with_direction (StAdjustment           *adj,
+                       ClutterScrollDirection  direction)
+{
+  gdouble delta;
+
+  switch (direction)
+    {
+    case CLUTTER_SCROLL_UP:
+    case CLUTTER_SCROLL_LEFT:
+      delta = -1.0;
+      break;
+    case CLUTTER_SCROLL_RIGHT:
+    case CLUTTER_SCROLL_DOWN:
+      delta = 1.0;
+      break;
+    case CLUTTER_SCROLL_SMOOTH:
+      g_assert_not_reached ();
+      break;
+    }
+
+  st_adjustment_adjust_for_scroll_event (adj, delta);
+}
+
+static void
 st_scroll_view_style_changed (StWidget *widget)
 {
   StScrollView *self = ST_SCROLL_VIEW (widget);
@@ -687,57 +711,31 @@ st_scroll_view_scroll_event (ClutterActor       *self,
                              ClutterScrollEvent *event)
 {
   StScrollViewPrivate *priv = ST_SCROLL_VIEW (self)->priv;
-  gdouble value, step, hvalue, vvalue, delta_x, delta_y;
 
   /* don't handle scroll events if requested not to */
   if (!priv->mouse_scroll)
     return FALSE;
 
-  switch (event->direction)
-    {
-    case CLUTTER_SCROLL_SMOOTH:
-      clutter_event_get_scroll_delta ((ClutterEvent *)event,
-                                      &delta_x, &delta_y);
-      g_object_get (priv->hadjustment,
-                    "value", &hvalue,
-                    NULL);
-      g_object_get (priv->vadjustment,
-                    "value", &vvalue,
-                    NULL);
-      break;
-    case CLUTTER_SCROLL_UP:
-    case CLUTTER_SCROLL_DOWN:
-      g_object_get (priv->vadjustment,
-                    "step-increment", &step,
-                    "value", &value,
-                    NULL);
-      break;
-    case CLUTTER_SCROLL_LEFT:
-    case CLUTTER_SCROLL_RIGHT:
-      g_object_get (priv->hadjustment,
-                    "step-increment", &step,
-                    "value", &value,
-                    NULL);
-      break;
-    }
+  if (clutter_event_is_pointer_emulated ((ClutterEvent *) event))
+    return TRUE;
 
   switch (event->direction)
     {
     case CLUTTER_SCROLL_SMOOTH:
-      st_adjustment_set_value (priv->hadjustment, hvalue + delta_x);
-      st_adjustment_set_value (priv->vadjustment, vvalue + delta_y);
+      {
+        gdouble delta_x, delta_y;
+        clutter_event_get_scroll_delta ((ClutterEvent *)event, &delta_x, &delta_y);
+        st_adjustment_adjust_for_scroll_event (priv->hadjustment, delta_x);
+        st_adjustment_adjust_for_scroll_event (priv->vadjustment, delta_y);
+      }
       break;
     case CLUTTER_SCROLL_UP:
-      st_adjustment_set_value (priv->vadjustment, value - step);
-      break;
     case CLUTTER_SCROLL_DOWN:
-      st_adjustment_set_value (priv->vadjustment, value + step);
+      adjust_with_direction (priv->vadjustment, event->direction);
       break;
     case CLUTTER_SCROLL_LEFT:
-      st_adjustment_set_value (priv->hadjustment, value - step);
-      break;
     case CLUTTER_SCROLL_RIGHT:
-      st_adjustment_set_value (priv->hadjustment, value + step);
+      adjust_with_direction (priv->hadjustment, event->direction);
       break;
     }