Fix #121

Change parent of PWMOutputDevice to OutputDevice and implement blink to
maintain compatibility. The version of blink implemented here is
slightly extended to include functionality like Explorer HAT's "pulse".
The parameter defaults behave identically to OutputDevice's blink but
can be adjusted to have the device smoothly fade in and out.

gpiozero/output_devices.py

@@ -8,7 +8,7 @@ from __future__ import (
 import warnings
 from time import sleep
 from threading import Lock
-from itertools import repeat
+from itertools import repeat, cycle, chain
 
 from RPi import GPIO
 
@@ -136,19 +136,19 @@ class DigitalOutputDevice(OutputDevice):
         Make the device turn on and off repeatedly.
 
         :param float on_time:
-            Number of seconds on
+            Number of seconds on. Defaults to 1 second.
 
         :param float off_time:
-            Number of seconds off
+            Number of seconds off. Defaults to 1 second.
 
         :param int n:
-            Number of times to blink; ``None`` means forever
+            Number of times to blink; ``None`` (the default) means forever.
 
         :param bool background:
-            If ``True``, start a background thread to continue blinking and
-            return immediately. If ``False``, only return when the blink is
-            finished (warning: the default value of *n* will result in this
-            method never returning).
+            If ``True`` (the default), start a background thread to continue
+            blinking and return immediately. If ``False``, only return when the
+            blink is finished (warning: the default value of *n* will result in
+            this method never returning).
         """
         self._stop_blink()
         self._blink_thread = GPIOThread(
@@ -233,7 +233,7 @@ class Buzzer(DigitalOutputDevice):
 Buzzer.beep = Buzzer.blink
 
 
-class PWMOutputDevice(DigitalOutputDevice):
+class PWMOutputDevice(OutputDevice):
     """
     Generic output device configured for pulse-width modulation (PWM).
 
@@ -247,6 +247,7 @@ class PWMOutputDevice(DigitalOutputDevice):
     """
     def __init__(self, pin=None, frequency=100):
         self._pwm = None
+        self._blink_thread = None
         super(PWMOutputDevice, self).__init__(pin)
         try:
             self._pwm = GPIO.PWM(self.pin, frequency)
@@ -295,6 +296,14 @@ class PWMOutputDevice(DigitalOutputDevice):
         self._stop_blink()
         self._write(value)
 
+    def on(self):
+        self._stop_blink()
+        self._write(self._active_state)
+
+    def off(self):
+        self._stop_blink()
+        self._write(self._inactive_state)
+
     def toggle(self):
         """
         Toggle the state of the device. If the device is currently off
@@ -302,6 +311,7 @@ class PWMOutputDevice(DigitalOutputDevice):
         1.0).  If the device has a duty cycle (:attr:`value`) of 0.1, this will
         toggle it to 0.9, and so on.
         """
+        self._stop_blink()
         self.value = 1.0 - self.value
 
     @property
@@ -325,6 +335,72 @@ class PWMOutputDevice(DigitalOutputDevice):
         self._pwm.ChangeFrequency(value)
         self._frequency = value
 
+    def blink(
+            self, on_time=1, off_time=1, fade_in_time=0, fade_out_time=0,
+            n=None, background=True):
+        """
+        Make the device turn on and off repeatedly.
+
+        :param float on_time:
+            Number of seconds on. Defaults to 1 second.
+
+        :param float off_time:
+            Number of seconds off. Defaults to 1 second.
+
+        :param float fade_in_time:
+            Number of seconds to spend fading in. Defaults to 0.
+
+        :param float fade_out_time:
+            Number of seconds to spend fading out. Defaults to 0.
+
+        :param int n:
+            Number of times to blink; ``None`` (the default) means forever.
+
+        :param bool background:
+            If ``True`` (the default), start a background thread to continue
+            blinking and return immediately. If ``False``, only return when the
+            blink is finished (warning: the default value of *n* will result in
+            this method never returning).
+        """
+        self._stop_blink()
+        self._blink_thread = GPIOThread(
+            target=self._blink_led,
+            args=(on_time, off_time, fade_in_time, fade_out_time, n)
+        )
+        self._blink_thread.start()
+        if not background:
+            self._blink_thread.join()
+            self._blink_thread = None
+
+    def _stop_blink(self):
+        if self._blink_thread:
+            self._blink_thread.stop()
+            self._blink_thread = None
+
+    def _blink_led(
+            self, on_time, off_time, fade_in_time, fade_out_time, n, fps=50):
+        sequence = []
+        if fade_in_time > 0.0:
+            sequence += [
+                (i * (1 / fps) / fade_in_time, 1 / fps)
+                for i in range(int(50 * fade_in_time))
+                ]
+        sequence.append((1.0, on_time))
+        if fade_out_time > 0.0:
+            sequence += [
+                (1 - (i * (1 / fps) / fade_out_time), 1 / fps)
+                for i in range(int(50 * fade_out_time))
+                ]
+        sequence.append((0.0, off_time))
+        sequence = (
+                cycle(sequence) if n is None else
+                chain.from_iterable(repeat(sequence, n))
+                )
+        for value, delay in sequence:
+            self._write(value)
+            if self._blink_thread.stopping.wait(delay):
+                break
+
 
 class PWMLED(PWMOutputDevice):
     """