main.rs

use futures::{StreamExt};
use std::time::Duration;
use async_std::task;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error + 'static>> {
    let data:Vec<u32> = (0..50).collect();

    println!("Waiting...");

    let fetches = futures::stream::iter(
        data.into_iter().map(|id| {
            let x = id.clone();
            async move {
                let request = async { x }; 
                task::sleep(Duration::from_secs(1)).await;
                request.await;
            }
    })
    ).buffer_unordered(12).collect::<Vec<()>>();
    fetches.await;
    println!("Done...");

    /*
    stream::iter(data).for_each(|id| async move {
        let request = async { id }; // async io request
        let res = request.await;
        println!("res: {:?}", res);
        ()
    }).await;
    */
    
    Ok(())
}