New interface for implementing backends.

Backends that simply poll for changes and wait some time
between checks can just implement a `loop()` method and,
optionally, `__enter__` and `__exit__` methods, so they
can perform initialization/cleanup logic within a context
manager.

platypush/backend/__init__.py

@@ -5,8 +5,10 @@
 
 import logging
 import threading
+import time
 
 from threading import Thread
+from typing import Optional
 
 from platypush.bus import Bus
 from platypush.config import Config
@@ -35,13 +37,15 @@ class Backend(Thread, EventGenerator):
 
     _default_response_timeout = 5
 
-    def __init__(self, bus=None, **kwargs):
+    # Loop function, can be implemented by derived classes
+    loop = None
+
+    def __init__(self, bus: Optional[Bus] = None, poll_seconds: Optional[float] = None, **kwargs):
         """
         :param bus: Reference to the bus object to be used in the backend
-        :type bus: platypush.bus.Bus
-
+        :param poll_seconds: If the backend implements a ``loop`` method, this parameter expresses how often the
+            loop should run in seconds.
         :param kwargs: Key-value configuration for the backend
-        :type kwargs: dict
         """
 
         self._thread_name = self.__class__.__name__
@@ -51,6 +55,7 @@ class Backend(Thread, EventGenerator):
         # If no bus is specified, create an internal queue where
         # the received messages will be pushed
         self.bus = bus or Bus()
+        self.poll_seconds = float(poll_seconds) if poll_seconds else None
         self.device_id = Config.get('device_id')
         self.thread_id = None
         self._should_stop = False
@@ -218,6 +223,28 @@ class Backend(Thread, EventGenerator):
         """ Starts the backend thread. To be implemented in the derived classes """
         self.thread_id = threading.get_ident()
         set_thread_name(self._thread_name)
+        if not callable(self.loop):
+            return
+
+        with self:
+            while not self.should_stop():
+                try:
+                    self.loop()
+                except Exception as e:
+                    self.logger.error(str(e))
+                    self.logger.exception(e)
+                finally:
+                    if self.poll_seconds:
+                        time.sleep(self.poll_seconds)
+
+    def __enter__(self):
+        """ Invoked when the backend is initialized, if the main logic is within a ``loop()`` function """
+        self.logger.info('Initialized backend {}'.format(self.__class__.__name__))
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        """ Invoked when the backend is terminated, if the main logic is within a ``loop()`` function """
+        self.on_stop()
+        self.logger.info('Terminated backend {}'.format(self.__class__.__name__))
 
     def on_stop(self):
         """ Callback invoked when the process stops """

platypush/backend/foursquare.py

@@ -1,4 +1,4 @@
-import time
+from typing import Optional
 
 from platypush.backend import Backend
 from platypush.context import get_plugin
@@ -22,34 +22,31 @@ class FoursquareBackend(Backend):
 
     _last_created_at_varname = '_foursquare_checkin_last_created_at'
 
-    def __init__(self, poll_seconds: float = 60.0, *args, **kwargs):
+    def __init__(self, poll_seconds: Optional[float] = 60.0, *args, **kwargs):
         """
         :param poll_seconds: How often the backend should check for new check-ins (default: one minute).
         """
-        super().__init__(*args, **kwargs)
-        self.poll_seconds = poll_seconds
+        super().__init__(*args, poll_seconds=poll_seconds, **kwargs)
         self._last_created_at = None
 
-    def run(self):
-        super().run()
+    def __enter__(self):
         self._last_created_at = int(get_plugin('variable').get(self._last_created_at_varname).
                                     output.get(self._last_created_at_varname) or 0)
         self.logger.info('Started Foursquare backend')
 
-        while not self.should_stop():
-            try:
-                checkins = get_plugin('foursquare').get_checkins().output
-                if checkins:
-                    last_checkin = checkins[0]
-                    if not self._last_created_at or last_checkin.get('createdAt', 0) > self._last_created_at:
-                        self.bus.post(FoursquareCheckinEvent(checkin=last_checkin))
-                        self._last_created_at = last_checkin.get('createdAt', 0)
-                        get_plugin('variable').set(**{self._last_created_at_varname: self._last_created_at})
-            except Exception as e:
-                self.logger.error('Error while retrieving the list of checkins: {}'.format(str(e)))
-                self.logger.exception(e)
-            finally:
-                time.sleep(self.poll_seconds)
+    def loop(self):
+        checkins = get_plugin('foursquare').get_checkins().output
+        if not checkins:
+            return
+
+        last_checkin = checkins[0]
+        last_checkin_created_at = last_checkin.get('createdAt', 0)
+        if self._last_created_at and last_checkin_created_at <= self._last_created_at:
+            return
+
+        self.bus.post(FoursquareCheckinEvent(checkin=last_checkin))
+        self._last_created_at = last_checkin_created_at
+        get_plugin('variable').set(**{self._last_created_at_varname: self._last_created_at})
 
 
 # vim:sw=4:ts=4:et: