Add a dummy jack example

Signed-off-by: Nikolai Rodionov <allanger@badhouseplants.net>

examples/jack-sine/src/main.rs

@@ -0,0 +1,94 @@
+//! Sine wave generator with frequency configuration exposed through standard
+//! input.
+
+use crossbeam_channel::bounded;
+use std::io;
+use std::str::FromStr;
+
+fn main() {
+    // 1. open a client
+    let (client, _status) =
+        jack::Client::new("rust_jack_sine", jack::ClientOptions::default()).unwrap();
+
+    // 2. register port
+    let out_port = client
+        .register_port("sine_out", jack::AudioOut::default())
+        .unwrap();
+
+    // 3. define process callback handler
+    let (tx, rx) = bounded(1_000_000);
+    struct State {
+        out_port: jack::Port<jack::AudioOut>,
+        rx: crossbeam_channel::Receiver<f64>,
+        frequency: f64,
+        frame_t: f64,
+        time: f64,
+    }
+    let process = jack::contrib::ClosureProcessHandler::with_state(
+        State {
+            out_port,
+            rx,
+            frequency: 220.0,
+            frame_t: 1.0 / client.sample_rate() as f64,
+            time: 0.0,
+        },
+        |state, _, ps| -> jack::Control {
+            // Get output buffer
+            let out = state.out_port.as_mut_slice(ps);
+
+            // Check frequency requests
+            while let Ok(f) = state.rx.try_recv() {
+                state.time = 0.0;
+                state.frequency = f;
+            }
+
+            // Write output
+            for v in out.iter_mut() {
+                let x = state.frequency * state.time * 2.0 * std::f64::consts::PI;
+                let y = x.sin();
+                *v = y as f32;
+                state.time += state.frame_t;
+            }
+
+            // Continue as normal
+            jack::Control::Continue
+        },
+        move |_, _, _| jack::Control::Continue,
+    );
+
+    // 4. Activate the client. Also connect the ports to the system audio.
+    let active_client = client.activate_async((), process).unwrap();
+    active_client
+        .as_client()
+        .connect_ports_by_name("rust_jack_sine:sine_out", "system:playback_1")
+        .unwrap();
+    active_client
+        .as_client()
+        .connect_ports_by_name("rust_jack_sine:sine_out", "system:playback_2")
+        .unwrap();
+    // processing starts here
+
+    // 5. wait or do some processing while your handler is running in real time.
+    println!("Enter an integer value to change the frequency of the sine wave.");
+    while let Some(f) = read_freq() {
+        tx.send(f).unwrap();
+    }
+
+    // 6. Optional deactivate. Not required since active_client will deactivate on
+    // drop, though explicit deactivate may help you identify errors in
+    // deactivate.
+    if let Err(err) = active_client.deactivate() {
+        eprintln!("JACK exited with error: {err}");
+    };
+}
+
+/// Attempt to read a frequency from standard in. Will block until there is
+/// user input. `None` is returned if there was an error reading from standard
+/// in, or the retrieved string wasn't a compatible u16 integer.
+fn read_freq() -> Option<f64> {
+    let mut user_input = String::new();
+    match io::stdin().read_line(&mut user_input) {
+        Ok(_) => u16::from_str(user_input.trim()).ok().map(|n| n as f64),
+        Err(_) => None,
+    }
+}