Iteration and Concurrency

类似于同步  方式的Iterator，这里有很多不同的方法可以迭代和处理一个Stream中的值。有组合器样式的方法，例如map，filter和fold和他们的有错误就早退的表弟try_map，try_filter和try_fold。

不幸，for循环不适用于Streams，但对于命令式代码，while let和next/try_next函数可以这样用：

#![allow(unused_variables)]
fn main() {
async fn sum_with_next(mut stream: Pin<&mut dyn Stream<Item = i32>>) -> i32 {
    use futures::stream::StreamExt; // for `next`
    let mut sum = 0;
    while let Some(item) = stream.next().await {
        sum += item;
    }
    sum
}

async fn sum_with_try_next(
    mut stream: Pin<&mut dyn Stream<Item = Result<i32, io::Error>>>,
) -> Result<i32, io::Error> {
    use futures::stream::TryStreamExt; // for `try_next`
    let mut sum = 0;
    while let Some(item) = stream.try_next().await? {
        sum += item;
    }
    Ok(sum)
}
}

但是，如果我们一次只处理一个元素，则可能会失去了并发的机会，这毕竟这是我们要编写异步代码的首要原因。要同时处理一个 stream 中的多个 items，请使用for_each_concurrent和try_for_each_concurrent方法：

#![allow(unused_variables)]
fn main() {
async fn jump_around(
    mut stream: Pin<&mut dyn Stream<Item = Result<u8, io::Error>>>,
) -> Result<(), io::Error> {
    use futures::stream::TryStreamExt; // for `try_for_each_concurrent`
    const MAX_CONCURRENT_JUMPERS: usize = 100;

    stream.try_for_each_concurrent(MAX_CONCURRENT_JUMPERS, |num| async move {
        jump_n_times(num).await?;
        report_n_jumps(num).await?;
        Ok(())
    }).await?;

    Ok(())
}
}