mixer/null: new mixer plugin

src/mixer/MixerList.hxx

@@ -27,6 +27,7 @@
 
 struct MixerPlugin;
 
+extern const MixerPlugin null_mixer_plugin;
 extern const MixerPlugin software_mixer_plugin;
 extern const MixerPlugin alsa_mixer_plugin;
 extern const MixerPlugin oss_mixer_plugin;

src/mixer/MixerType.cxx

@@ -34,6 +34,8 @@ mixer_type_parse(const char *input)
 		return MixerType::HARDWARE;
 	else if (strcmp(input, "software") == 0)
 		return MixerType::SOFTWARE;
+	else if (strcmp(input, "null") == 0)
+		return MixerType::NULL_;
 	else
 		return MixerType::UNKNOWN;
 }

src/mixer/MixerType.hxx

@@ -27,6 +27,9 @@ enum class MixerType {
 	/** mixer disabled */
 	NONE,
 
+	/** "null" mixer (virtual fake) */
+	NULL_,
+
 	/** software mixer with pcm_volume() */
 	SOFTWARE,
 

src/mixer/plugins/NullMixerPlugin.cxx

@@ -0,0 +1,67 @@
+/*
+ * Copyright (C) 2003-2014 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "config.h"
+#include "mixer/MixerInternal.hxx"
+
+class NullMixer final : public Mixer {
+	/**
+	 * The current volume in percent (0..100).
+	 */
+	unsigned volume;
+
+public:
+	NullMixer(MixerListener &_listener)
+		:Mixer(null_mixer_plugin, _listener),
+		 volume(100)
+	{
+	}
+
+	/* virtual methods from class Mixer */
+	bool Open(gcc_unused Error &error) override {
+		return true;
+	}
+
+	void Close() override {
+	}
+
+	int GetVolume(gcc_unused Error &error) override {
+		return volume;
+	}
+
+	bool SetVolume(unsigned _volume, gcc_unused Error &error) override {
+		volume = _volume;
+		return true;
+	}
+};
+
+static Mixer *
+null_mixer_init(gcc_unused EventLoop &event_loop,
+		gcc_unused AudioOutput &ao,
+		MixerListener &listener,
+		gcc_unused const config_param &param,
+		gcc_unused Error &error)
+{
+	return new NullMixer(listener);
+}
+
+const MixerPlugin null_mixer_plugin = {
+	null_mixer_init,
+	true,
+};

src/output/Init.cxx

@@ -126,6 +126,10 @@ audio_output_load_mixer(EventLoop &event_loop, AudioOutput &ao,
 	case MixerType::UNKNOWN:
 		return nullptr;
 
+	case MixerType::NULL_:
+		return mixer_new(event_loop, null_mixer_plugin, ao, listener,
+				 param, error);
+
 	case MixerType::HARDWARE:
 		if (plugin == nullptr)
 			return nullptr;