Update mixer example to run sound effect in addition to music (#666)

* Add chunksize parameter documentation.
* Fix example name.

examples/mixer-demo.rs

@@ -1,22 +1,25 @@
+/// Demonstrates the simultaneous mixing of music and sound effects.
+
 extern crate sdl2;
 
 use std::env;
 use std::path::Path;
-use sdl2::mixer::{INIT_MP3, INIT_FLAC, INIT_MOD, INIT_FLUIDSYNTH, INIT_MODPLUG, INIT_OGG,
-                 AUDIO_S16LSB};
+use sdl2::mixer::{DEFAULT_CHANNELS, INIT_MP3, INIT_FLAC, INIT_MOD, INIT_FLUIDSYNTH, INIT_MODPLUG,
+                  INIT_OGG, AUDIO_S16LSB};
 
 fn main() {
 
     let args: Vec<_> = env::args().collect();
 
     if args.len() < 2 {
-        println!("Usage: ./demo audio.[mp3|wav|ogg]")
+        println!("Usage: ./demo music.[mp3|wav|ogg] [sound-effect.[mp3|wav|ogg]]")
     } else {
-        demo(Path::new(&args[1]));
+        let sound_file = args.get(2).map(|sound_file| Path::new(sound_file));
+        demo(Path::new(&args[1]), sound_file);
     }
 }
 
-fn demo(filename: &Path) {
+fn demo(music_file: &Path, sound_file: Option<&Path>) {
 
     println!("linked version: {}", sdl2::mixer::get_linked_version());
 
@@ -24,16 +27,18 @@ fn demo(filename: &Path) {
     let _audio = sdl.audio().unwrap();
     let mut timer = sdl.timer().unwrap();
     let _mixer_context = sdl2::mixer::init(INIT_MP3 | INIT_FLAC | INIT_MOD | INIT_FLUIDSYNTH |
-                                          INIT_MODPLUG |
-                                          INIT_OGG)
-                            .unwrap();
+                                           INIT_MODPLUG | INIT_OGG)
+            .unwrap();
 
     let frequency = 44100;
     let format = AUDIO_S16LSB; // signed 16 bit samples, in little-endian byte order
-    let channels = 2; // Stereo
+    let channels = DEFAULT_CHANNELS; // Stereo
     let chunk_size = 1024;
     sdl2::mixer::open_audio(frequency, format, channels, chunk_size).unwrap();
-    sdl2::mixer::allocate_channels(0);
+
+    // Number of mixing channels available for sound effect `Chunk`s to play
+    // simultaneously.
+    sdl2::mixer::allocate_channels(4);
 
     {
         let n = sdl2::mixer::get_chunk_decoders_number();
@@ -53,8 +58,7 @@ fn demo(filename: &Path) {
 
     println!("query spec => {:?}", sdl2::mixer::query_spec());
 
-
-    let music = sdl2::mixer::Music::from_file(filename).unwrap();
+    let music = sdl2::mixer::Music::from_file(music_file).unwrap();
 
     fn hook_finished() {
         println!("play ends! from rust cb");
@@ -67,6 +71,29 @@ fn demo(filename: &Path) {
     println!("music volume => {:?}", sdl2::mixer::Music::get_volume());
     println!("play => {:?}", music.play(1));
 
+    if let Some(sound_file_path) = sound_file {
+        let sound_chunk_res = sdl2::mixer::Chunk::from_file(sound_file_path);
+
+        match sound_chunk_res {
+            Ok(sound_chunk) => {
+                println!("chunk volume => {:?}", sound_chunk.get_volume());
+                println!("playing sound twice");
+                let play_res = sdl2::mixer::Channel::all().play(&sound_chunk, 1);
+
+                // This delay is needed because when the `Chunk` goes out of scope,
+                // the sound effect stops playing. Delay long enough to hear the
+                // sound.
+                timer.delay(5000);
+
+                match play_res {
+                    Ok(_) => println!("played sound"),
+                    Err(e) => println!("{:?}", e),
+                }
+            }
+            Err(e) => println!("Cannot load sound file: {:?}", e),
+        }
+    }
+
     timer.delay(10000);
 
     println!("fading out ... {:?}", sdl2::mixer::Music::fade_out(4000));
@@ -80,6 +107,5 @@ fn demo(filename: &Path) {
     sdl2::mixer::Music::halt();
     timer.delay(1000);
 
-
     println!("quitting sdl");
 }