Receive in a thread to avoid blocking the window event handling

src/main.rs

@@ -1,7 +1,14 @@
+use core::mem::swap;
+use core::sync::atomic::AtomicBool;
+use core::sync::atomic::Ordering;
+
 use std::error;
 use std::io::Read;
 use std::path;
 use std::process;
+use std::sync::Arc;
+use std::sync::Mutex;
+use std::thread;
 
 use clap::Parser;
 
@@ -28,13 +35,29 @@ struct Cli {
     debug: u8,
 }
 
+struct Buffer {
+    frame: Vec<u8>,
+    count: u64,
+}
+impl Buffer {
+    fn new(size: usize) -> Buffer {
+        Buffer {
+            frame: vec![0; size],
+            count: 0,
+        }
+    }
+    fn is_newer(&self, other: &Buffer) -> bool {
+        self.count > other.count
+    }
+}
+
 fn main() -> Result<(), Box<dyn error::Error>> {
     let cli = Cli::parse();
     if cli.debug >= 2 {
         eprintln!("CLI: {:?}", cli);
     }
 
-    let mut child = process::Command::new(&cli.executable)
+    let mut app = process::Command::new(&cli.executable)
         .arg(cli.cols.to_string())
         .arg(cli.rows.to_string())
         .args(cli.args)
@@ -44,32 +67,60 @@ fn main() -> Result<(), Box<dyn error::Error>> {
         eprintln!(
             "Running {} as a child process with pid {}",
             cli.executable.display(),
-            child.id()
+            app.id()
         );
     }
 
-    let mut stdout = child
+    let mut stdout = app
         .stdout
         .take()
-        .ok_or("opening child process stdout failed")?;
+        .ok_or("opening app process stdout failed")?;
 
     // TODO support also RGBW
     let colors = 3;
     // TODO check for overflow and excessive size
     let buffer_size = cli.cols * cli.rows * colors;
-    let mut buffer = vec![0_u8; buffer_size];
     let mut frame_count = 0;
 
+    let x_size = cli.cols;
+    let y_size = cli.rows;
+
     let mut window: PistonWindow = WindowSettings::new(
         format!("pixelfoo-viewer {}", cli.executable.display()),
-        [1600, 800],
+        [20 * (x_size as u32), 20 * (y_size as u32)],
     )
     .exit_on_esc(true)
     .vsync(true)
     .build()?;
 
-    let x_size = cli.cols;
-    let y_size = cli.rows;
+    // Shared buffer for one frame of data from the app.
+    let buffer0 = Arc::new(Mutex::new(Buffer::new(buffer_size)));
+    let buffer1 = buffer0.clone();
+
+    let receiver_stop = AtomicBool::new(false);
+    thread::scope(|scope| {
+        let receiver_thread = scope.spawn(|| {
+            let mut buffer = Buffer::new(buffer_size);
+            while !receiver_stop.load(Ordering::Relaxed) {
+                // Read a frame from app process.
+                stdout
+                    .read_exact(&mut buffer.frame)
+                    .expect("pipe read failed");
+                if cli.debug >= 3 {
+                    eprintln!("Received frame {}", frame_count);
+                }
+                frame_count += 1;
+                buffer.count = frame_count;
+
+                // Put the frame in the shared buffer.
+                {
+                    let mut b = buffer0.lock().unwrap();
+                    swap(&mut *b, &mut buffer);
+                }
+            }
+        });
+
+        let mut buffer = Buffer::new(buffer_size);
         while let Some(event) = window.next() {
             window.draw_2d(&event, |context, graphics, _device| {
                 let [vsx, vsy] = context.get_view_size();
@@ -81,26 +132,33 @@ fn main() -> Result<(), Box<dyn error::Error>> {
 
                 pw::clear([0.5, 0.5, 0.5, 1.0], graphics);
 
-            stdout.read_exact(&mut buffer).expect("pipe read failed");
-            if cli.debug >= 3 {
-                eprintln!("Received frame {}", frame_count);
+                // Swap the shared buffer with our own buffer if it is newer.
+                {
+                    let mut b = buffer1.lock().unwrap();
+                    if b.is_newer(&buffer) {
+                        swap(&mut *b, &mut buffer);
                     }
+                };
 
+                // Display the buffer.
                 for y in 0..y_size {
                     for x in 0..x_size {
                         let i = colors * (y * x_size + x);
-                    let r = f32::from(buffer[i + 0]) / 255.0;
-                    let g = f32::from(buffer[i + 1]) / 255.0;
-                    let b = f32::from(buffer[i + 2]) / 255.0;
+                        let r = f32::from(buffer.frame[i + 0]) / 255.0;
+                        let g = f32::from(buffer.frame[i + 1]) / 255.0;
+                        let b = f32::from(buffer.frame[i + 2]) / 255.0;
                         let color = [r, g, b, 1.0];
                         let rectangle = [(x as f64) * cs, (y as f64) * cs, cs, cs];
                         pw::rectangle(color, rectangle, context.transform, graphics);
                     }
                 }
             });
-        frame_count += 1;
         }
-    child.kill()?;
+        receiver_stop.store(true, Ordering::Relaxed);
+        let _result = receiver_thread.join();
+    });
+
+    app.kill()?;
 
     Ok(())
 }