examples: Add audio level meter python example

An example showcasing how to make a simple audio visualization for L/R volume levels

examples/clapper-gtk/audio/level-meter/python/example.py

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+#!/usr/bin/env python3
+
+import gi
+gi.require_version('Adw', '1')
+gi.require_version('Clapper', '0.0')
+gi.require_version('ClapperGtk', '0.0')
+gi.require_version('Gst', '1.0')
+gi.require_version('Gtk', '4.0')
+from gi.repository import Adw, Clapper, ClapperGtk, Gdk, Gst, Gtk
+
+Clapper.init(None)
+
+def on_level_message(player, msg, bars):
+    left_bar, right_bar = bars
+    if (peak := msg.get_structure().get_value("peak")):
+        # Convert dB to linear while taking player volume into account.
+        volume = player.props.volume;
+        left_bar.set_value(min(pow(10.0, peak[0] / 20.0) * volume, 1.0))
+        right_bar.set_value(min(pow(10.0, peak[1] / 20.0) * volume, 1.0))
+
+def on_state_changed(player, pspec, bars):
+    if (player.props.state < Clapper.PlayerState.PLAYING):
+        for bar in bars:
+            bar.set_value(0)
+
+def on_activate(app):
+    # Create our widgets.
+    win = Gtk.ApplicationWindow(application=app, title='Clapper Audio Level Meter', default_width=400, default_height=96)
+    audio = ClapperGtk.Audio()
+    hbox = Gtk.Box(valign=Gtk.Align.CENTER, margin_start=8, margin_end=8, spacing=4)
+    vbox = Gtk.Box(orientation=Gtk.Orientation.VERTICAL, valign=Gtk.Align.CENTER, hexpand=True, spacing=4)
+    play_btn = ClapperGtk.TogglePlayButton()
+    left_level = Gtk.LevelBar()
+    right_level = Gtk.LevelBar()
+
+    # Use GStreamer "level" element which measures volume peaks before audio output.
+    audio.props.player.props.audio_sink = Gst.parse_bin_from_description(
+        'level interval=25000000 post-messages=true ! autoaudiosink', True)
+
+    # Connect to player detailed "message" signal to receive "level" messages.
+    audio.props.player.connect('message::level', on_level_message, (left_level, right_level))
+
+    # Listen for player state changes to empty bars fill when not playing.
+    audio.props.player.connect('notify::state', on_state_changed, (left_level, right_level))
+
+    # Add media for playback. First media item in queue will be automatically selected.
+    item = Clapper.MediaItem(uri='https://www.soundhelix.com/examples/mp3/SoundHelix-Song-1.mp3')
+    audio.props.player.props.queue.add_item(item)
+
+    # Assemble window.
+    vbox.append(left_level)
+    vbox.append(right_level)
+    hbox.append(play_btn)
+    hbox.append(vbox)
+    audio.set_child(hbox)
+    win.set_child(audio)
+    win.present()
+
+    # Not too loud. Mind the ears.
+    audio.props.player.props.volume = 0.7
+
+    # Start playback.
+    audio.props.player.play()
+
+# Create a new application.
+app = Adw.Application(application_id='com.example.ClapperAudioLevelMeter')
+app.connect('activate', on_activate)
+
+# Run the application.
+app.run(None)