add EmptyCallback source (#430)

Co-authored-by: est31 <est31@users.noreply.github.com>

src/source/empty_callback.rs

@@ -0,0 +1,55 @@
+use std::marker::PhantomData;
+use std::time::Duration;
+
+use crate::{Sample, Source};
+
+/// An empty source which executes a callback function
+pub struct EmptyCallback<S> {
+    pub phantom_data: PhantomData<S>,
+    pub callback: Box<dyn Send + Fn()>,
+}
+
+impl<S> EmptyCallback<S> {
+    #[inline]
+    pub fn new(callback: Box<dyn Send + Fn()>) -> EmptyCallback<S> {
+        EmptyCallback {
+            phantom_data: PhantomData,
+            callback,
+        }
+    }
+}
+
+impl<S> Iterator for EmptyCallback<S> {
+    type Item = S;
+
+    #[inline]
+    fn next(&mut self) -> Option<S> {
+        (self.callback)();
+        None
+    }
+}
+
+impl<S> Source for EmptyCallback<S>
+where
+    S: Sample,
+{
+    #[inline]
+    fn current_frame_len(&self) -> Option<usize> {
+        None
+    }
+
+    #[inline]
+    fn channels(&self) -> u16 {
+        1
+    }
+
+    #[inline]
+    fn sample_rate(&self) -> u32 {
+        48000
+    }
+
+    #[inline]
+    fn total_duration(&self) -> Option<Duration> {
+        Some(Duration::new(0, 0))
+    }
+}

src/source/mod.rs

@@ -9,6 +9,7 @@ use crate::Sample;
 pub use self::amplify::Amplify;
 pub use self::blt::BltFilter;
 pub use self::buffered::Buffered;
+pub use self::empty_callback::EmptyCallback;
 pub use self::channel_volume::ChannelVolume;
 pub use self::crossfade::Crossfade;
 pub use self::delay::Delay;
@@ -35,6 +36,7 @@ pub use self::zero::Zero;
 mod amplify;
 mod blt;
 mod buffered;
+mod empty_callback;
 mod channel_volume;
 mod crossfade;
 mod delay;