The source/values toolkit

Me and my big mouth. No sooner do I declare the base classes "relatively
stable" than I go and mess around with it all again. Anyway, this is the
long promised set of utilities to make source/values more interesting.
It includes a few interesting little utility functions, a whole bunch of
examples and introduces the notion of "pseudo" devices with no (obvious)
hardware representation like a time-of-day device.

This necessitated making the event system a little more generic (it's
not exclusive the GPIO devices after all; no reason we can't use it on
composite devices in future) and by this point the mixins have gotten
large enough to justify their own module.

The pseudo-devices are a bit spartan and basic at the moment but I'm
sure there'll be plenty of future ideas...

gpiozero/threads.py

@@ -6,13 +6,7 @@ from __future__ import (
     )
 str = type('')
 
-import weakref
-from collections import deque
-from threading import Thread, Event, RLock
-try:
-    from statistics import median, mean
-except ImportError:
-    from .compat import median, mean
+from threading import Thread, Event
 
 from .exc import (
     GPIOBadQueueLen,
@@ -46,46 +40,3 @@ class GPIOThread(Thread):
         super(GPIOThread, self).join()
         _THREADS.discard(self)
 
-
-class GPIOQueue(GPIOThread):
-    def __init__(
-            self, parent, queue_len=5, sample_wait=0.0, partial=False,
-            average=median):
-        assert callable(average)
-        super(GPIOQueue, self).__init__(target=self.fill)
-        if queue_len < 1:
-            raise GPIOBadQueueLen('queue_len must be at least one')
-        if sample_wait < 0:
-            raise GPIOBadSampleWait('sample_wait must be 0 or greater')
-        self.queue = deque(maxlen=queue_len)
-        self.partial = partial
-        self.sample_wait = sample_wait
-        self.full = Event()
-        self.parent = weakref.proxy(parent)
-        self.average = average
-
-    @property
-    def value(self):
-        if not self.partial:
-            self.full.wait()
-        try:
-            return self.average(self.queue)
-        except ZeroDivisionError:
-            # No data == inactive value
-            return 0.0
-
-    def fill(self):
-        try:
-            while (not self.stopping.wait(self.sample_wait) and
-                    len(self.queue) < self.queue.maxlen):
-                self.queue.append(self.parent._read())
-                if self.partial:
-                    self.parent._fire_events()
-            self.full.set()
-            while not self.stopping.wait(self.sample_wait):
-                self.queue.append(self.parent._read())
-                self.parent._fire_events()
-        except ReferenceError:
-            # Parent is dead; time to die!
-            pass
-