Mixer to skip volume control

Implement a null mixer that does not control volume with software.

playback/src/mixer/mod.rs

@@ -39,12 +39,16 @@ impl Default for MixerConfig {
 pub mod softmixer;
 use self::softmixer::SoftMixer;
 
+pub mod nullmixer;
+use self::nullmixer::NullMixer;
+
 fn mk_sink<M: Mixer + 'static>(device: Option<MixerConfig>) -> Box<Mixer> {
     Box::new(M::open(device))
 }
 
 pub fn find<T: AsRef<str>>(name: Option<T>) -> Option<fn(Option<MixerConfig>) -> Box<Mixer>> {
     match name.as_ref().map(AsRef::as_ref) {
+        Some("null") => Some(mk_sink::<NullMixer>),
         None | Some("softvol") => Some(mk_sink::<SoftMixer>),
         #[cfg(feature = "alsa-backend")]
         Some("alsa") => Some(mk_sink::<AlsaMixer>),

playback/src/mixer/nullmixer.rs

@@ -0,0 +1,29 @@
+use std::sync::Arc;
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+use super::AudioFilter;
+use super::{Mixer, MixerConfig};
+
+#[derive(Clone)]
+pub struct NullMixer {
+    volume: Arc<AtomicUsize>,
+}
+
+impl Mixer for NullMixer {
+    fn open(_: Option<MixerConfig>) -> NullMixer {
+        NullMixer {
+            volume: Arc::new(AtomicUsize::new(0xFFFF)),
+        }
+    }
+    fn start(&self) {}
+    fn stop(&self) {}
+    fn volume(&self) -> u16 {
+        self.volume.load(Ordering::Relaxed) as u16
+    }
+    fn set_volume(&self, volume: u16) {
+        self.volume.store(volume as usize, Ordering::Relaxed);
+    }
+    fn get_audio_filter(&self) -> Option<Box<AudioFilter + Send>> {
+        None
+    }
+}