feat: add select2

__concurrent/async_study/examples/select2.rs

@@ -0,0 +1,44 @@
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+use std::time::Duration;
+use tokio::sync::oneshot::{channel, Receiver};
+
+#[tokio::main]
+async fn main() {
+    let (tx1, rx1) = channel::<i32>();
+    let (_tx2, rx2) = channel::<i32>();
+
+    tokio::spawn(async move {
+        tokio::time::sleep(Duration::from_millis(200)).await;
+        let _ = tx1.send(1);
+    });
+
+    println!("start select");
+    MySelect { rx1, rx2 }.await;
+    println!("end select");
+}
+
+
+struct MySelect {
+    rx1: Receiver<i32>,
+    rx2: Receiver<i32>,
+}
+
+impl Future for MySelect {
+    type Output = ();
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
+        if let Poll::Ready(val) = Pin::new(&mut self.rx1).poll(cx) {
+            println!("rx1 completed first with {:?}", val);
+            return Poll::Ready(());
+        }
+
+        if let Poll::Ready(val) = Pin::new(&mut self.rx2).poll(cx) {
+            println!("rx2 completed first with {:?}", val);
+            return Poll::Ready(());
+        }
+
+        Poll::Pending
+    }
+}