select宏

select宏也允许并发的执行Future，但是和join、try_join不同的是，select宏只要有一个Future返回，就会返回。

示例

• 源码

use futures::{select, future::FutureExt, pin_mut};
use tokio::runtime::Runtime;
use std::io::Result;

async fn function1() -> Result<()> {
    tokio::time::delay_for(tokio::time::Duration::from_secs(10)).await;
    println!("function1 ++++ ");
    Ok(())
}

async fn function2() -> Result<()> {
    println!("function2 ++++ ");
    Ok(())
}

async fn async_main() {
    let f1 = function1().fuse();
    let f2 = function2().fuse();

    pin_mut!(f1, f2);

    select! {
        _ = f1 => println!("task one completed first"),
        _ = f2 => println!("task two completed first"),
    }
}

fn main() {
    let mut runtime = Runtime::new().unwrap();
    runtime.block_on(async_main());
    println!("Hello, world!");
}

• 配置文件

[dependencies]
futures = "0.3.5"
tokio = { version = "0.2", features = ["full"] }

• 运行结果

function2 ++++ 
task two completed first
Hello, world!