Improve CPU usage using a clock trigger and deque

Copy out `kivy.clock.triggered` from version 1.10.1 since it isn't yet
available in the `trio`/async branch and use it to throttle the callback
rate. Use a `collections.deque` to LIFO iterate widgets each call
using the heuristic that it's more likely the mouse is still within the
currently highlighted (or it's adjacent neighbors) widget as opposed
to some far away widget (the case when the mouse is moved very
drastically across the window).

piker/ui/kivy/mouse_over.py

@@ -3,10 +3,69 @@
 Based on initial LGPL work by O. Poyen. here:
 https://gist.github.com/opqopq/15c707dc4cffc2b6455f
 """
-
+import time
+from functools import wraps
+from collections import deque
 
 from kivy.properties import BooleanProperty, ObjectProperty
 from kivy.core.window import Window
+from kivy.clock import Clock
+
+
+from ...log import get_logger
+
+
+log = get_logger('kivy')
+
+
+# XXX: copied from 1.10.1 since the async branch isn't ported yet.
+def triggered(timeout=0, interval=False):
+    '''Decorator that will trigger the call of the function at the specified
+    timeout, through the method :meth:`CyClockBase.create_trigger`. Subsequent
+    calls to the decorated function (while the timeout is active) are ignored.
+    It can be helpful when an expensive funcion (i.e. call to a server) can be
+    triggered by different methods. Setting a proper timeout will delay the
+    calling and only one of them wil be triggered.
+        @triggered(timeout, interval=False)
+        def callback(id):
+            print('The callback has been called with id=%d' % id)
+        >> callback(id=1)
+        >> callback(id=2)
+        The callback has been called with id=2
+    The decorated callback can also be unscheduled using:
+        >> callback.cancel()
+    .. versionadded:: 1.10.1
+    '''
+
+    def wrapper_triggered(func):
+
+        _args = []
+        _kwargs = {}
+
+        def cb_function(dt):
+            func(*tuple(_args), **_kwargs)
+
+        cb_trigger = Clock.create_trigger(
+            cb_function,
+            timeout=timeout,
+            interval=interval)
+
+        @wraps(func)
+        def trigger_function(*args, **kwargs):
+            _args[:] = []
+            _args.extend(list(args))
+            _kwargs.clear()
+            _kwargs.update(kwargs)
+            cb_trigger()
+
+        def trigger_cancel():
+            cb_trigger.cancel()
+
+        setattr(trigger_function, 'cancel', trigger_cancel)
+
+        return trigger_function
+
+    return wrapper_triggered
 
 
 class MouseOverBehavior(object):
@@ -23,18 +82,24 @@ class MouseOverBehavior(object):
     # be used in `on_enter` or `on_leave` in order to know where was dispatched
     # the event.
     border_point = ObjectProperty(None)
-    _widgets = []
+    _widgets = deque()
     _last_hovered = None
+    _last_time = time.time()
 
     def __init__(self, **kwargs):
         self.register_event_type('on_enter')
         self.register_event_type('on_leave')
         MouseOverBehavior._widgets.append(self)
-        super(MouseOverBehavior, self).__init__(**kwargs)
-        Window.bind(mouse_pos=self.on_mouse_pos)
+        super().__init__(**kwargs)
+        Window.bind(mouse_pos=self._on_mouse_pos)
 
     @classmethod
-    def on_mouse_pos(cls, *args):
+    # try throttling to 1ms latency (doesn't seem to work
+    # best I can get is 0.01...)
+    @triggered(timeout=0.001, interval=False)
+    def _on_mouse_pos(cls, *args):
+        log.debug(f"{cls} time since last call: {time.time() - cls._last_time}")
+        cls._last_time = time.time()
         # 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():
@@ -42,7 +107,7 @@ class MouseOverBehavior(object):
 
         pos = args[1]
         # Next line to_widget allow to compensate for relative layout
-        for widget in cls._widgets:
+        for widget in cls._widgets.copy():
             w_coords = widget.to_widget(*pos)
             inside = widget.collide_point(*w_coords)
             if inside and widget.hovered:
@@ -54,11 +119,16 @@ class MouseOverBehavior(object):
                     last_hovered.dispatch('on_leave')
                     last_hovered.hovered = False
 
+                # stick last highlighted at the front of the stack
+                # resulting in LIFO iteration for efficiency
+                cls._widgets.remove(widget)
+                cls._widgets.appendleft(widget)
+                cls._last_hovered = widget
+
                 # 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