Add a buffer callback to Renderer

src/backend.rs

@@ -321,6 +321,7 @@ impl Backend {
                         }
                     }
                 }
+                renderer_moved.guard().on_buffer(data);
             },
             move |err| {
                 // we got an error on stream, push it to the error queue

src/renderer.rs

@@ -9,13 +9,22 @@ pub trait Renderer: Clone + Send + 'static {
     ///
     /// Note: you can use a [`crate::Resampler`] to resample audio data.
     fn next_frame(&mut self, sample_rate: u32) -> Frame;
+
+    /// This gets called when an audio buffer is done processing.
+    fn on_buffer<T>(&mut self, _buffer: &mut [T])
+    where
+        T: cpal::SizedSample + cpal::FromSample<f32>,
+    {
+    }
 }
 
 /// Default audio renderer.
 #[derive(Debug, Clone, Default)]
 pub struct DefaultRenderer {
     /// All playing sounds.
     pub sounds: Vec<SoundHandle>,
+    /// The last buffer size given by the [cpal] backend.
+    pub last_buffer_size: usize,
 }
 
 impl DefaultRenderer {
@@ -50,6 +59,13 @@ impl Renderer for DefaultRenderer {
 
         out
     }
+
+    fn on_buffer<T>(&mut self, buffer: &mut [T])
+    where
+        T: cpal::SizedSample + cpal::FromSample<f32>,
+    {
+        self.last_buffer_size = buffer.len();
+    }
 }
 
 /// Wraps [`Renderer`] so it can be shared between threads.