audio: Create a new audio filter for ReplayGain and Background Mode

This links together our ReplayGain filter with a volume element that is
set to the user configured background volume when background listening
mode is enabled, and 100% when background listening mode is disabled.

Implements: #50 ("Background Music Mode")
Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>

emmental/audio/filter.py

@@ -0,0 +1,45 @@
+# Copyright 2023 (c) Anna Schumaker.
+"""A custom Gst.Bin with our audio filter effects."""
+from gi.repository import GObject
+from gi.repository import Gst
+from . import replaygain
+
+
+class Filter(Gst.Bin):
+    """The audio filter element."""
+
+    bg_enabled = GObject.Property(type=bool, default=False)
+    bg_volume = GObject.Property(type=float, default=0.5)
+    rg_mode = GObject.Property(type=str, default="disabled")
+
+    def __init__(self):
+        """Initialize the audio filter."""
+        super().__init__()
+        self._replaygain = replaygain.Filter()
+        self._volume = Gst.ElementFactory.make("volume")
+
+        self.add(self._replaygain)
+        self.add(self._volume)
+
+        rg_pad = self._replaygain.get_static_pad("src")
+        rg_pad.link(self._volume.get_static_pad("sink"))
+
+        self.__add_ghost_pad("sink", self._replaygain)
+        self.__add_ghost_pad("src", self._volume)
+
+        self.connect("notify", self.__notify)
+
+    def __add_ghost_pad(self, pad: str, elm: Gst.Element) -> None:
+        self.add_pad(Gst.GhostPad.new(pad, elm.get_static_pad(pad)))
+
+    def __notify(self, filter: Gst.Bin, param: GObject.ParamSpec) -> None:
+        match param.name:
+            case "bg-enabled" | "bg-volume":
+                vol = self.bg_volume if self.bg_enabled else 1.0
+                if vol != self._volume.get_property("volume"):
+                    vs = f"{round(vol * 100)}%" if self.bg_enabled else "off"
+                    print(f"audio: setting background listening to {vs}")
+                    self._volume.set_property("volume", vol)
+            case "rg-mode":
+                if self.rg_mode != self._replaygain.mode:
+                    self._replaygain.mode = self.rg_mode

tests/audio/test_filter.py

@@ -0,0 +1,72 @@
+# Copyright 2023 (c) Anna Schumaker.
+"""Tests our combined Filter element."""
+import io
+import unittest
+import emmental.audio.filter
+from gi.repository import Gst
+
+
+class TestFilter(unittest.TestCase):
+    """Tests our custom Filter element."""
+
+    def setUp(self):
+        """Set up common variables."""
+        self.filter = emmental.audio.filter.Filter()
+
+    def test_init(self):
+        """Test that the filter is set up correctly."""
+        self.assertIsInstance(self.filter, Gst.Bin)
+        self.assertIsInstance(self.filter.get_static_pad("src"), Gst.GhostPad)
+        self.assertIsInstance(self.filter.get_static_pad("sink"), Gst.GhostPad)
+
+    @unittest.mock.patch("sys.stdout", new_callable=io.StringIO)
+    def test_replaygain(self, mock_stdout: io.StringIO):
+        """Test the ReplayGain element in the filter."""
+        self.assertIsInstance(self.filter._replaygain,
+                              emmental.audio.replaygain.Filter)
+
+        self.assertEqual(self.filter.rg_mode, "disabled")
+        self.filter.rg_mode = "track"
+        self.assertEqual(self.filter._replaygain.mode, "track")
+        self.filter.rg_mode = "track"
+        self.assertEqual(mock_stdout.getvalue(),
+                         "audio: setting ReplayGain mode to 'track'\n")
+
+    @unittest.mock.patch("sys.stdout", new_callable=io.StringIO)
+    def test_volume(self, mock_stdout: io.StringIO):
+        """Test the Volume element in the filter."""
+        self.assertIsInstance(self.filter._volume, Gst.Element)
+        self.assertRegex(self.filter._volume.name, r"volume\d+")
+        self.assertEqual(self.filter._volume.get_property("volume"), 1.0)
+
+        self.assertFalse(self.filter.bg_enabled)
+        self.assertEqual(self.filter.bg_volume, 0.5)
+
+        self.filter.bg_enabled = True
+        self.assertEqual(self.filter._volume.get_property("volume"), 0.5)
+        self.assertEqual(mock_stdout.getvalue(),
+                         "audio: setting background listening to 50%\n")
+
+        self.filter.bg_volume = 0.50
+        self.assertEqual(mock_stdout.getvalue(),
+                         "audio: setting background listening to 50%\n")
+
+        self.filter.bg_volume = 0.75
+        self.assertEqual(self.filter._volume.get_property("volume"), 0.75)
+        self.assertEqual(mock_stdout.getvalue(),
+                         "audio: setting background listening to 50%\n"
+                         "audio: setting background listening to 75%\n")
+
+        self.filter.bg_enabled = False
+        self.assertEqual(self.filter._volume.get_property("volume"), 1.0)
+        self.assertEqual(mock_stdout.getvalue(),
+                         "audio: setting background listening to 50%\n"
+                         "audio: setting background listening to 75%\n"
+                         "audio: setting background listening to off\n")
+
+        self.filter.bg_volume = 0.5
+        self.assertEqual(self.filter._volume.get_property("volume"), 1.0)
+        self.assertEqual(mock_stdout.getvalue(),
+                         "audio: setting background listening to 50%\n"
+                         "audio: setting background listening to 75%\n"
+                         "audio: setting background listening to off\n")