Make use of polling functionality in 16-ev3dev kernel (#241)

* Make use of polling functionality in 16-ev3dev kernel

Introduces the following functions:

* Motor.wait(condition, timeout=None),
* Motor.wait_while(state, timeout=None)
* Motor.wait_until(state, timeout=None)

Makes the following possible:

m.wait_while('running')
m.wait_until('stalled')

Closes #230

Author: ddemidov

ev3dev/core.py

@@ -46,6 +46,8 @@
 import ctypes
 import re
 import stat
+import select
+import time
 from os.path import abspath
 from struct import pack, unpack
 from subprocess import Popen
@@ -314,6 +316,8 @@ def __init__(self, address=None, name_pattern=SYSTEM_DEVICE_NAME_CONVENTION, nam
 
 # ~autogen
 
+        self._poll = None
+
 # ~autogen generic-get-set classes.motor>currentClass
 
     @property
@@ -806,6 +810,64 @@ def reset(self, **kwargs):
 
 # ~autogen
 
+    def wait(self, cond, timeout=None):
+        """
+        Blocks until ``cond(self.state)`` is ``True``.  The condition is
+        checked when there is an I/O event related to the ``state`` attribute.
+        Exits early when ``timeout`` (in milliseconds) is reached.
+
+        Returns ``True`` if the condition is met, and ``False`` if the timeout
+        is reached.
+        """
+
+        tic = time.time()
+
+        if self._poll is None:
+            if self._state is None:
+                self._state = self._attribute_file_open('state')
+            self._poll = select.poll()
+            self._poll.register(self._state, select.POLLPRI)
+
+        while True:
+            self._poll.poll(None if timeout is None else timeout)
+
+            if timeout is not None and time.time() >= tic + timeout / 1000:
+                return False
+
+            if cond(self.state):
+                return True
+
+
+    def wait_until(self, s, timeout=None):
+        """
+        Blocks until ``s`` is in ``self.state``.  The condition is checked when
+        there is an I/O event related to the ``state`` attribute.  Exits early
+        when ``timeout`` (in milliseconds) is reached.
+
+        Returns ``True`` if the condition is met, and ``False`` if the timeout
+        is reached.
+
+        Example::
+
+            m.wait_until('stalled')
+        """
+        return self.wait(lambda state: s in state, timeout)
+
+    def wait_while(self, s, timeout=None):
+        """
+        Blocks until ``s`` is not in ``self.state``.  The condition is checked
+        when there is an I/O event related to the ``state`` attribute.  Exits
+        early when ``timeout`` (in milliseconds) is reached.
+
+        Returns ``True`` if the condition is met, and ``False`` if the timeout
+        is reached.
+
+        Example::
+
+            m.wait_while('running')
+        """
+        return self.wait(lambda state: s not in state, timeout)
+
 def list_motors(name_pattern=Motor.SYSTEM_DEVICE_NAME_CONVENTION, **kwargs):
     """
     This is a generator function that enumerates all tacho motors that match