Merge pull request #1439 from adamcik/fix/1304-postpone-tags

Postpone tags between about-to-finish and stream-start

docs/changelog.rst

@@ -176,6 +176,10 @@ Audio
   time of the GStreamer queue. If you experience buffering before track
   changes, it may help to increase this. Workaround for :issue:`1409`.
 
+- ``tags_changed`` events are only emitted for fields that have changed.
+  Previous behavior was to emit this for all fields received from GStreamer.
+  (PR: :issue:`1439`)
+
 Gapless
 -------
 

mopidy/audio/actor.py

@@ -326,9 +326,24 @@ class _Handler(object):
     def on_tag(self, taglist):
         tags = tags_lib.convert_taglist(taglist)
         gst_logger.debug('Got TAG bus message: tags=%r', dict(tags))
-        self._audio._tags.update(tags)
-        logger.debug('Audio event: tags_changed(tags=%r)', tags.keys())
-        AudioListener.send('tags_changed', tags=tags.keys())
+
+        # Postpone emitting tags until stream start.
+        if self._audio._pending_tags is not None:
+            self._audio._pending_tags.update(tags)
+            return
+
+        # TODO: Add proper tests for only emitting changed tags.
+        unique = object()
+        changed = []
+        for key, value in tags.items():
+            # Update any tags that changed, and store changed keys.
+            if self._audio._tags.get(key, unique) != value:
+                self._audio._tags[key] = value
+                changed.append(key)
+
+        if changed:
+            logger.debug('Audio event: tags_changed(tags=%r)', changed)
+            AudioListener.send('tags_changed', tags=changed)
 
     def on_missing_plugin(self, msg):
         desc = GstPbutils.missing_plugin_message_get_description(msg)
@@ -349,6 +364,14 @@ class _Handler(object):
         logger.debug('Audio event: stream_changed(uri=%r)', uri)
         AudioListener.send('stream_changed', uri=uri)
 
+        # Emit any postponed tags that we got after about-to-finish.
+        tags, self._audio._pending_tags = self._audio._pending_tags, None
+        self._audio._tags = tags
+
+        if tags:
+            logger.debug('Audio event: tags_changed(tags=%r)', tags.keys())
+            AudioListener.send('tags_changed', tags=tags.keys())
+
     def on_segment(self, segment):
         gst_logger.debug(
             'Got SEGMENT pad event: '
@@ -386,6 +409,7 @@ class Audio(pykka.ThreadingActor):
         self._buffering = False
         self._tags = {}
         self._pending_uri = None
+        self._pending_tags = None
 
         self._playbin = None
         self._outputs = None
@@ -536,8 +560,8 @@ class Audio(pykka.ThreadingActor):
         else:
             current_volume = None
 
-        self._tags = {}  # TODO: add test for this somehow
         self._pending_uri = uri
+        self._pending_tags = {}
         self._playbin.set_property('uri', uri)
 
         if self.mixer is not None and current_volume is not None:

mopidy/audio/tags.py

@@ -58,6 +58,7 @@ gstreamer-GstTagList.html
                     log.TRACE_LOG_LEVEL,
                     'Ignoring unknown tag data: %r = %r', tag, value)
 
+    # TODO: dict(result) to not leak the defaultdict, or just use setdefault?
     return result