Add caching of OsaMixer volume

If volume is just managed through Mopidy it is always correct. If another
application changes the volume, Mopidy will be correct within 30 seconds.

mopidy/mixers/osa.py

@@ -1,15 +1,32 @@
 from subprocess import Popen, PIPE
+import time
 
 from mopidy.mixers import BaseMixer
 
+CACHE_TTL = 30
+
 class OsaMixer(BaseMixer):
+    _cache = None
+    _last_update = None
+
+    def _valid_cache(self):
+        return (self._cache is not None
+            and self._last_update is not None
+            and (int(time.time() - self._last_update) < CACHE_TTL))
+
     def _get_volume(self):
-        try:
-            return int(Popen(
-                ['osascript', '-e', 'output volume of (get volume settings)'],
-                stdout=PIPE).communicate()[0])
-        except ValueError:
-            return None
+	if not self._valid_cache():
+            try:
+                self._cache = int(Popen(
+                    ['osascript', '-e',
+                        'output volume of (get volume settings)'],
+                    stdout=PIPE).communicate()[0])
+            except ValueError:
+                self._cache = None
+            self._last_update = int(time.time())
+        return self._cache
 
     def _set_volume(self, volume):
         Popen(['osascript', '-e', 'set volume output volume %d' % volume])
+        self._cache = volume
+        self._last_update = int(time.time())