Add a supports_play_method

This allows us to discover if a given Play method works for a given
Source via the features in desc.xml. It's done with some kind of
complex xpath, broken into parts, to make it a bit easier to read.

Tests include.

rxv/rxv.py

@@ -279,6 +279,24 @@ def supports_method(self, source, *args):
                     return True
         return False
 
+    def supports_play_method(self, source, method):
+        # if there was a complete xpath implementation we could do
+        # this all with xpath, but without it it's lots of
+        # iteration. This is probably not worth optimizing, these
+        # loops are cheep in the long run.
+        source_xml = self._desc_xml.find('.//*[@YNC_Tag="%s"]' % source)
+        if source_xml is None:
+            return False
+
+        play_control = source_xml.find('.//*[@Func="Play_Control"]')
+        if play_control is None:
+            return False
+
+        supports = play_control.find('.//Put_1[@Func="%s"]' % method)
+        if supports is None:
+            return False
+        return True
+
     def _src_name(self, cur_input):
         if cur_input not in self.inputs():
             return None

tests/test_feature_support.py

@@ -44,3 +44,19 @@ def test_supports_method(self):
         self.assertTrue(rec.supports_method("Tuner", "Config"))
         self.assertFalse(
             rec.supports_method("Tuner", "Play_Control", "Playback"))
+
+    def test_supports_play_method(self):
+        rec = self.rec
+        self.assertFalse(rec.supports_play_method("NET_RADIO", "Pause"))
+        self.assertTrue(rec.supports_play_method("NET_RADIO", "Play"))
+        self.assertTrue(rec.supports_play_method("NET_RADIO", "Stop"))
+        # Plus_1 / Minus_1 are the FF and RW methods in the xml api
+        self.assertFalse(rec.supports_play_method("NET_RADIO", "Plus_1"))
+        self.assertFalse(rec.supports_play_method("NET_RADIO", "Minus_1"))
+
+        self.assertTrue(rec.supports_play_method("SERVER", "Pause"))
+        self.assertTrue(rec.supports_play_method("SERVER", "Play"))
+        self.assertTrue(rec.supports_play_method("SERVER", "Stop"))
+        # Plus_1 / Minus_1 are the FF and RW methods in the xml api
+        self.assertTrue(rec.supports_play_method("SERVER", "Plus_1"))
+        self.assertTrue(rec.supports_play_method("SERVER", "Minus_1"))