add SDL backend based on sdl2 crate

playback/src/audio_backend/sdl.rs

@@ -0,0 +1,56 @@
+use super::{Open, Sink};
+use sdl2::audio::{AudioQueue, AudioSpecDesired};
+use std::{io, thread, time};
+
+type Channel = i16;
+
+pub struct SdlSink {
+    queue: AudioQueue<Channel>,
+}
+
+impl Open for SdlSink {
+    fn open(device: Option<String>) -> SdlSink {
+        debug!("Using SDL sink");
+
+        if device.is_some() {
+            panic!("SDL sink does not support specifyng a device name");
+        }
+
+        let ctx = sdl2::init().expect("Could not init SDL");
+        let audio = ctx.audio().expect("Could not init SDL audio subsystem");
+
+        let desired_spec = AudioSpecDesired {
+            freq: Some(44_100),
+            channels: Some(2),
+            samples: None, // default
+        };
+        let queue = audio
+            .open_queue(None, &desired_spec)
+            .expect("Could not open SDL audio device");
+
+        SdlSink { queue: queue }
+    }
+}
+
+impl Sink for SdlSink {
+    fn start(&mut self) -> io::Result<()> {
+        self.queue.clear(); //
+        self.queue.resume();
+        Ok(())
+    }
+
+    fn stop(&mut self) -> io::Result<()> {
+        self.queue.pause();
+        self.queue.clear();
+        Ok(())
+    }
+
+    fn write(&mut self, data: &[i16]) -> io::Result<()> {
+        while self.queue.size() > (2 * 2 * 44_100) {
+            // sleep and wait for sdl thread to drain the queue a bit
+            thread::sleep(time::Duration::from_millis(10));
+        }
+        self.queue.queue(data);
+        Ok(())
+    }
+}