audio: Add a custom ReplayGainSink

I use output-switcher and funnel elements so we can disable the
replaygain plugins by routing the stream around them if necessary.

Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>

audio/replaygain.py

@@ -0,0 +1,52 @@
+# Copyright 2021 (c) Anna Schumaker.
+from gi.repository import GObject
+from gi.repository import Gst
+
+
+class ReplayGainSink(Gst.Bin):
+    def __init__(self):
+        Gst.Bin.__init__(self)
+        self.selector  = Gst.ElementFactory.make("output-selector")
+        self.funnel    = Gst.ElementFactory.make("funnel")
+        self.audiosink = Gst.ElementFactory.make("autoaudiosink")
+        self.rgvolume  = Gst.ElementFactory.make("rgvolume")
+        self.rglimiter = Gst.ElementFactory.make("rglimiter")
+
+        for element in [ self.selector, self.funnel, self.audiosink,
+                         self.rgvolume, self.rglimiter ]:
+            self.add(element)
+
+        # No ReplayGain: selector -> funnel -> audiosink
+        self.shortcut = self.selector.get_request_pad("src_%u")
+        self.shortcut.link(self.funnel.get_request_pad("sink_%u"))
+        self.funnel.link(self.audiosink)
+
+        # Replaygain: selector -> rgvolume -> rglimiter -> funnel -> audiosink
+        self.replaygain = self.selector.get_request_pad("src_%u")
+        self.replaygain.link(self.rgvolume.get_static_pad("sink"))
+        self.rgvolume.link(self.rglimiter)
+        self.rglimiter.get_static_pad("src").link(
+                                    self.funnel.get_request_pad("sink_%u"))
+
+        self.selector.set_property("pad-negotiation-mode", 2)
+        self.selector.set_property("active-pad", self.shortcut)
+
+        pad = self.selector.get_static_pad("sink")
+        ghost = Gst.GhostPad.new("sink", pad)
+        ghost.set_active(True)
+        self.add_pad(ghost)
+
+    @GObject.Property
+    def mode(self):
+        if self.selector.get_property("active-pad") == self.shortcut:
+            return "disabled"
+        album_mode = self.rgvolume.get_property("album-mode")
+        return "album" if album_mode else "track"
+
+    @mode.setter
+    def mode(self, mode):
+        if mode == "disabled":
+            self.selector.set_property("active-pad", self.shortcut)
+        else:
+            self.rgvolume.set_property("album-mode", mode == "album")
+            self.selector.set_property("active-pad", self.replaygain)

audio/test_replaygain.py

@@ -0,0 +1,38 @@
+# Copyright 2021 (c) Anna Schumaker.
+import unittest
+from gi.repository import Gst
+from . import replaygain
+
+class TestReplayGainSink(unittest.TestCase):
+    def test_replay_gain_sink_init(self):
+        rgsink = replaygain.ReplayGainSink()
+
+        self.assertIsInstance(rgsink, Gst.Bin)
+        self.assertIsInstance(rgsink.selector, Gst.Element)
+        self.assertIsInstance(rgsink.funnel, Gst.Element)
+        self.assertIsInstance(rgsink.audiosink, Gst.Element)
+        self.assertIsInstance(rgsink.rgvolume, Gst.Element)
+        self.assertIsInstance(rgsink.rglimiter, Gst.Element)
+
+        self.assertIsInstance(rgsink.shortcut, Gst.Pad)
+        self.assertIsInstance(rgsink.replaygain, Gst.Pad)
+        self.assertIsInstance(rgsink.get_static_pad("sink"), Gst.GhostPad)
+
+    def test_replay_gain_sink_mode(self):
+        rgsink = replaygain.ReplayGainSink()
+        self.assertEqual(rgsink.get_property("mode"), "disabled")
+
+        rgsink.set_property("mode", "album")
+        self.assertEqual(rgsink.get_property("mode"), "album")
+        self.assertEqual(rgsink.selector.get_property("active-pad"),
+                                                      rgsink.replaygain)
+
+        rgsink.set_property("mode", "track")
+        self.assertEqual(rgsink.get_property("mode"), "track")
+        self.assertEqual(rgsink.selector.get_property("active-pad"),
+                                                      rgsink.replaygain)
+
+        rgsink.set_property("mode", "disabled")
+        self.assertEqual(rgsink.get_property("mode"), "disabled")
+        self.assertEqual(rgsink.selector.get_property("active-pad"),
+                                                      rgsink.shortcut)