[WIP] Add GStreamerMixer

mopidy/mixers/gstreamer.py

@@ -0,0 +1,14 @@
+from mopidy.mixers import BaseMixer
+
+class GStreamerMixer(BaseMixer):
+    """Mixer which uses GStreamer to control volume."""
+
+    def __init__(self, *args, **kwargs):
+        super(GStreamerMixer, self).__init__(*args, **kwargs)
+
+    def _get_volume(self):
+        # TODO Get volume from GStreamerProcess
+
+    def _set_volume(self, volume):
+        # TODO Send volume to GStreamerProcess
+