Fix #181

Added source_delay property to SourceMixin which means it'll now appear
everywhere. Default is 0.01 which is just enough to drop CPU usage while
remaining responsive enough for the majority of purposes.

gpiozero/devices.py

@@ -23,6 +23,7 @@ from .exc import (
     GPIODeviceClosed,
     GPIOBadQueueLen,
     GPIOBadSampleWait,
+    GPIOBadSourceDelay,
     )
 
 # Get a pin implementation to use as the default; we prefer RPi.GPIO's here
@@ -164,6 +165,7 @@ class SourceMixin(object):
     def __init__(self, *args, **kwargs):
         self._source = None
         self._source_thread = None
+        self._source_delay = 0.01
         super(SourceMixin, self).__init__(*args, **kwargs)
 
     def close(self):
@@ -176,13 +178,28 @@ class SourceMixin(object):
     def _copy_values(self, source):
         for v in source:
             self.value = v
-            if self._source_thread.stopping.wait(0):
+            if self._source_thread.stopping.wait(self._source_delay):
                 break
 
+    @property
+    def source_delay(self):
+        """
+        The delay (measured in seconds) in the loop used to read values from
+        :attr:`source`. Defaults to 0.01 seconds which is generally sufficient
+        to keep CPU usage to a minimum while providing adequate responsiveness.
+        """
+        return self._source_delay
+
+    @source_delay.setter
+    def source_delay(self, value):
+        if value < 0:
+            raise GPIOBadSourceDelay('source_delay must be 0 or greater')
+        self._source_delay = float(value)
+
     @property
     def source(self):
         """
-        The iterable to use as a source of values for `value`.
+        The iterable to use as a source of values for :attr:`value`.
         """
         return self._source
 

gpiozero/exc.py

@@ -31,6 +31,9 @@ class GPIOBadQueueLen(GPIODeviceError, ValueError):
 class GPIOBadSampleWait(GPIODeviceError, ValueError):
     "Error raised when a negative sampling wait period is specified"
 
+class GPIOBadSourceDelay(GPIODeviceError, ValueError):
+    "Error raised when a negative source delay is specified"
+
 class InputDeviceError(GPIODeviceError):
     "Base class for errors specific to the InputDevice hierarchy"