async-std/docs/src/tutorial/implementing_a_client.md

Implementing a client

Let's now implement the client for the chat. Because the protocol is line-based, the implementation is pretty straightforward:

• Lines read from stdin should be sent over the socket.
• Lines read from the socket should be echoed to stdout.

Unlike the server, the client needs only limited concurrency, as it interacts with only a single user. For this reason, async doesn't bring a lot of performance benefits in this case.

However, async is still useful for managing concurrency! Specifically, the client should simultaneously read from stdin and from the socket. Programming this with threads is cumbersome, especially when implementing clean shutdown. With async, we can just use the select! macro.

# extern crate async_std;
# extern crate futures;
use async_std::{
    io::{stdin, BufRead, BufReader, Write},
    net::{TcpStream, ToSocketAddrs},
    task,
};
use futures::{select, FutureExt, StreamExt};

type Result<T> = std::result::Result<T, Box<dyn std::error::Error + Send + Sync>>;

// main
fn run() -> Result<()> {
    task::block_on(try_run("127.0.0.1:8080"))
}

async fn try_run(addr: impl ToSocketAddrs) -> Result<()> {
    let stream = TcpStream::connect(addr).await?;
    let (reader, mut writer) = (&stream, &stream); // 1
    let mut lines_from_server = BufReader::new(reader).lines().fuse(); // 2
    let mut lines_from_stdin = BufReader::new(stdin()).lines().fuse(); // 2
    loop {
        select! { // 3
            line = lines_from_server.next() => match line {
                Some(line) => {
                    let line = line?;
                    println!("{}", line);
                },
                None => break,
            },
            line = lines_from_stdin.next() => match line {
                Some(line) => {
                    let line = line?;
                    writer.write_all(line.as_bytes()).await?;
                    writer.write_all(b"\n").await?;
                }
                None => break,
            }
        }
    }
    Ok(())
}

1. Here we split TcpStream into read and write halves: there's impl AsyncRead for &'_ TcpStream, just like the one in std.
2. We create a stream of lines for both the socket and stdin.
3. In the main select loop, we print the lines we receive from the server and send the lines we read from the console.