Detect scrolling in treviews and optimize drawing

Detecting if the treeview is scrolling we can improve the performance of
the CellRendererIcon by avoiding unneeded calculations.

The example scrollingdetector.py shows how this can be used.  The
changes are backward compatible.

Signed-off-by: Manuel Quiñones <manuel.por.aca@gmail.com>
Signed-off-by: Gonzalo Odiard <godiard@sugarlabs.org>

src/sugar3/graphics/scrollingdetector.py

@@ -0,0 +1,72 @@
+# Copyright (C) 2014, Sugarlabs
+#
+# This library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 2 of the License, or (at your option) any later version.
+#
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this library; if not, write to the
+# Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+# Boston, MA 02111-1307, USA.
+
+from gi.repository import GObject
+from gi.repository import GLib
+
+from sugar3.graphics.icon import CellRendererIcon
+
+
+class ScrollingDetector(GObject.GObject):
+    """
+    """
+
+    scroll_start_signal = GObject.Signal('scroll-start')
+    scroll_end_signal = GObject.Signal('scroll-end')
+
+    def __init__(self, scrolled_window, timeout=100):
+        self._scrolled_window = scrolled_window
+        self._timeout = timeout
+        self.is_scrolling = False
+        self._prev_value = 0
+
+        self.connect_scrolled_window()
+        GObject.GObject.__init__(self)
+
+    def connect_scrolled_window(self):
+        adj = self._scrolled_window.get_vadjustment()
+        adj.connect('value-changed', self._value_changed_cb)
+
+    def _check_scroll_cb(self, adj):
+        if (adj.props.value == self._prev_value):
+            self.is_scrolling = False
+            self.scroll_end_signal.emit()
+            return False
+
+        self._prev_value = adj.props.value
+        return True
+
+    def _value_changed_cb(self, adj):
+        if (self.is_scrolling):
+            return
+
+        self.is_scrolling = True
+        self.scroll_start_signal.emit()
+        self._prev_value = adj.props.value
+        GLib.timeout_add(self._timeout, self._check_scroll_cb, adj)
+
+    def connect_treeview(self, treeview):
+        """
+        treeview -- Gtk.TreeView: a TreeView with CellRendererIcons already
+                    added.
+        This method need be executed after all the columns and renderers
+        were added to the treeview.
+        """
+        for column in treeview.get_columns():
+            for cell_renderer in column.get_cells():
+                if isinstance(cell_renderer, CellRendererIcon):
+                    cell_renderer.connect_to_scroller(self)