Faster highlighting via single loop and callback

Thanks yet again to @tshirtman for suggesting this.

Instead of defining a `on_mouse_pos()` on every widget simply
register and track each widget and loop through them all once (or as much
as is necessary) in a single callback. The assumption here is that we
get a performance boost by looping widgets instead of having `kivy` loop
and call back each widget thus avoiding costly python function calls.

piker/ui/kivy/hoverable.py

@@ -5,7 +5,6 @@ __author__ = 'Olivier Poyen'
 
 
 from kivy.properties import BooleanProperty, ObjectProperty
-from kivy.factory import Factory
 from kivy.core.window import Window
 
 
@@ -23,38 +22,50 @@ class HoverBehavior(object):
     # be used in `on_enter` or `on_leave` in order to know where was dispatched
     # the event.
     border_point = ObjectProperty(None)
+    _widgets = []
+    _last_hovered = None
 
     def __init__(self, **kwargs):
         self.register_event_type('on_enter')
         self.register_event_type('on_leave')
-        Window.bind(mouse_pos=self.on_mouse_pos)
+        HoverBehavior._widgets.append(self)
         super(HoverBehavior, self).__init__(**kwargs)
+        Window.bind(mouse_pos=self.on_mouse_pos)
 
-    def on_mouse_pos(self, *args):
+    @classmethod
+    def on_mouse_pos(cls, *args):
+        # XXX: how to still do this at the class level?
         # don't proceed if I'm not displayed <=> If have no parent
-        if not self.get_root_window():
-            return
+        # if not self.get_root_window():
+        #     return
+
         pos = args[1]
         # Next line to_widget allow to compensate for relative layout
-        inside = self.collide_point(*self.to_widget(*pos))
-        if self.hovered == inside:
-            # We have already done what was needed
-            return
-        self.border_point = pos
-        self.hovered = inside
-        if inside:
-            self.dispatch('on_enter')
-        else:
-            self.dispatch('on_leave')
+        for widget in cls._widgets:
+            w_coords = widget.to_widget(*pos)
+            inside = widget.collide_point(*w_coords)
+            if inside and widget.hovered:
+                return
+            elif inside:
+                # un-highlight the last highlighted
+                last_hovered = cls._last_hovered
+                if last_hovered:
+                    last_hovered.dispatch('on_leave')
+                    last_hovered.hovered = False
+
+                # highlight new widget
+                widget.border_point = pos
+                widget.hovered = True
+                widget.dispatch('on_enter')
+                cls._last_hovered = widget
+                return
 
     # implement these in the widget impl
 
-    def on_enter(self):
+    @classmethod
+    def on_enter(cls):
         pass
 
-    def on_leave(self):
+    @classmethod
+    def on_leave(cls):
         pass
-
-
-# register for global use via kivy.factory.Factory
-Factory.register('HoverBehavior', HoverBehavior)