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< main >
< h1 id = "stream" > < a class = "header" href = "#stream" > Stream< / a > < / h1 >
< p > 大家有没有想过， Rust 中的迭代器在迭代时能否异步进行？若不可以，是不是有相应的解决方案？< / p >
< p > 以上的问题其实很重要，因为在实际场景中，迭代一个集合，然后异步的去执行是很常见的需求，好在 Tokio 为我们提供了 < code > stream< / code > ，我们可以在异步函数中对其进行迭代，甚至和迭代器 < code > Iterator< / code > 一样，< code > stream< / code > 还能使用适配器，例如 < code > map< / code > ! Tokio 在 < a href = "https://docs.rs/tokio-stream/0.1.8/tokio_stream/trait.StreamExt.html" > < code > StreamExt< / code > < / a > 特征上定义了常用的适配器。< / p >
< p > 要使用 < code > stream< / code > ，目前还需要手动引入对应的包：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > tokio-stream = "0.1"
< span class = "boring" > }< / span > < / code > < / pre >
< blockquote >
< p > stream 没有放在 < code > tokio< / code > 包的原因在于标准库中的 < code > Stream< / code > 特征还没有稳定，一旦稳定后，< code > stream< / code > 将移动到 < code > tokio< / code > 中来< / p >
< / blockquote >
< h2 id = "迭代" > < a class = "header" href = "#迭代" > 迭代< / a > < / h2 >
< p > 目前， Rust 语言还不支持异步的 < code > for< / code > 循环，因此我们需要 < code > while let< / code > 循环和 < a href = "https://docs.rs/tokio-stream/0.1.8/tokio_stream/trait.StreamExt.html#method.next" > < code > StreamExt::next()< / code > < / a > 一起使用来实现迭代的目的:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio_stream::StreamExt;
#[tokio::main]
async fn main() {
let mut stream = tokio_stream::iter(& [1, 2, 3]);
while let Some(v) = stream.next().await {
println!("GOT = {:?}", v);
}
}< / code > < / pre >
< p > 和迭代器 < code > Iterator< / code > 类似，< code > next()< / code > 方法返回一个 < code > Option< T> < / code > ，其中 < code > T< / code > 是从 < code > stream< / code > 中获取的值的类型。若收到 < code > None< / code > 则意味着 < code > stream< / code > 迭代已经结束。< / p >
< h4 id = "mini-redis-广播" > < a class = "header" href = "#mini-redis-广播" > mini-redis 广播< / a > < / h4 >
< p > 下面我们来实现一个复杂一些的 mini-redis 客户端，完整代码见< a href = "https://github.com/tokio-rs/website/blob/master/tutorial-code/streams/src/main.rs" > 这里< / a > 。< / p >
< p > 在开始之前，首先启动一下完整的 mini-redis 服务器端：< / p >
< pre > < code class = "language-console" > $ mini-redis-server
< / code > < / pre >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio_stream::StreamExt;
use mini_redis::client;
async fn publish() -> mini_redis::Result< ()> {
let mut client = client::connect("127.0.0.1:6379").await?;
// 发布一些数据
client.publish("numbers", "1".into()).await?;
client.publish("numbers", "two".into()).await?;
client.publish("numbers", "3".into()).await?;
client.publish("numbers", "four".into()).await?;
client.publish("numbers", "five".into()).await?;
client.publish("numbers", "6".into()).await?;
Ok(())
}
async fn subscribe() -> mini_redis::Result< ()> {
let client = client::connect("127.0.0.1:6379").await?;
let subscriber = client.subscribe(vec!["numbers".to_string()]).await?;
let messages = subscriber.into_stream();
tokio::pin!(messages);
while let Some(msg) = messages.next().await {
println!("got = {:?}", msg);
}
Ok(())
}
#[tokio::main]
async fn main() -> mini_redis::Result< ()> {
tokio::spawn(async {
publish().await
});
subscribe().await?;
println!("DONE");
Ok(())
}< / code > < / pre >
< p > 上面生成了一个异步任务专门用于发布消息到 min-redis 服务器端的 < code > numbers< / code > 消息通道中。然后，在 < code > main< / code > 中，我们订阅了 < code > numbers< / code > 消息通道，并且打印从中接收到的消息。< / p >
< p > 还有几点值得注意的:< / p >
< ul >
< li > < a href = "https://docs.rs/mini-redis/0.4.1/mini_redis/client/struct.Subscriber.html#method.into_stream" > < code > into_stream< / code > < / a > 会将 < code > Subscriber< / code > 变成一个 < code > stream< / code > < / li >
< li > 在 < code > stream< / code > 上调用 < code > next< / code > 方法要求该 < code > stream< / code > 被固定住(< a href = "https://doc.rust-lang.org/std/pin/index.html" > < code > pinned< / code > < / a > )，因此需要调用 < code > tokio::pin!< / code > < / li >
< / ul >
< blockquote >
< p > 关于 Pin 的详细解读，可以阅读< a href = "https://course.rs/async/pin-unpin.html" > 这篇文章< / a > < / p >
< / blockquote >
< p > 大家可以去掉 < code > pin!< / code > 的调用，然后观察下报错，若以后你遇到这种错误，可以尝试使用下 < code > pin!< / code > 。< / p >
< p > 此时，可以运行下我们的客户端代码看看效果(别忘了先启动前面提到的 mini-redis 服务端):< / p >
< pre > < code class = "language-console" > got = Ok(Message { channel: "numbers", content: b"1" })
got = Ok(Message { channel: "numbers", content: b"two" })
got = Ok(Message { channel: "numbers", content: b"3" })
got = Ok(Message { channel: "numbers", content: b"four" })
got = Ok(Message { channel: "numbers", content: b"five" })
got = Ok(Message { channel: "numbers", content: b"6" })
< / code > < / pre >
< p > 在了解了 < code > stream< / code > 的基本用法后，我们再来看看如何使用适配器来扩展它。< / p >
< h2 id = "适配器" > < a class = "header" href = "#适配器" > 适配器< / a > < / h2 >
< p > 在前面章节中，我们了解了迭代器有< a href = "https://course.rs/advance/functional-programing/iterator.html#消费者与适配器" > 两种适配器< / a > ： < / p >
< ul >
< li > 迭代器适配器，会将一个迭代器转变成另一个迭代器，例如 < code > map< / code > ， < code > filter< / code > 等< / li >
< li > 消费者适配器，会消费掉一个迭代器，最终生成一个值，例如 < code > collect< / code > 可以将迭代器收集成一个集合< / li >
< / ul >
< p > 与迭代器类似，< code > stream< / code > 也有适配器，例如一个 < code > stream< / code > 适配器可以将一个 < code > stream< / code > 转变成另一个 < code > stream< / code > ，例如 < code > map< / code > 、< code > take< / code > 和 < code > filter< / code > 。< / p >
< p > 在之前的客户端中，< code > subscribe< / code > 订阅一直持续下去，直到程序被关闭。现在，让我们来升级下，让它在收到三条消息后就停止迭代，最终结束。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let messages = subscriber
.into_stream()
.take(3);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这里关键就在于 < code > take< / code > 适配器，它会限制 < code > stream< / code > 只能生成最多 < code > n< / code > 条消息。运行下看看结果：< / p >
< pre > < code class = "language-console" > got = Ok(Message { channel: "numbers", content: b"1" })
got = Ok(Message { channel: "numbers", content: b"two" })
got = Ok(Message { channel: "numbers", content: b"3" })
< / code > < / pre >
< p > 程序终于可以正常结束了。现在，让我们过滤 < code > stream< / code > 中的消息，只保留数字类型的值:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let messages = subscriber
.into_stream()
.filter(|msg| match msg {
Ok(msg) if msg.content.len() == 1 => true,
_ => false,
})
.take(3);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 运行后输出：< / p >
< pre > < code class = "language-console" > got = Ok(Message { channel: "numbers", content: b"1" })
got = Ok(Message { channel: "numbers", content: b"3" })
got = Ok(Message { channel: "numbers", content: b"6" })
< / code > < / pre >
< p > 需要注意的是，适配器的顺序非常重要，< code > .filter(...).take(3)< / code > 和 < code > .take(3).filter(...)< / code > 的结果可能大相径庭，大家可以自己尝试下。< / p >
< p > 现在，还有一件事要做，咱们的消息被不太好看的 < code > Ok(...)< / code > 所包裹，现在通过 < code > map< / code > 适配器来简化下:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let messages = subscriber
.into_stream()
.filter(|msg| match msg {
Ok(msg) if msg.content.len() == 1 => true,
_ => false,
})
.map(|msg| msg.unwrap().content)
.take(3);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 注意到 < code > msg.unwrap< / code > 了吗？大家可能会以为我们是出于示例的目的才这么用，实际上并不是，由于 < code > filter< / code > 的先执行， < code > map< / code > 中的 < code > msg< / code > 只能是 < code > Ok(...)< / code > ，因此 < code > unwrap< / code > 非常安全。< / p >
< pre > < code class = "language-console" > got = b"1"
got = b"3"
got = b"6"
< / code > < / pre >
< p > 还有一点可以改进的地方：当 < code > filter< / code > 和 < code > map< / code > 一起使用时，你往往可以用一个统一的方法来实现 < a href = "https://docs.rs/tokio-stream/0.1.8/tokio_stream/trait.StreamExt.html#method.filter_map" > < code > filter_map< / code > < / a > 。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let messages = subscriber
.into_stream()
.filter_map(|msg| match msg {
Ok(msg) if msg.content.len() == 1 => Some(msg.content),
_ => None,
})
.take(3);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 想要学习更多的适配器，可以看看 < a href = "https://docs.rs/tokio-stream/0.1.8/tokio_stream/trait.StreamExt.html" > < code > StreamExt< / code > < / a > 特征。< / p >
< h2 id = "实现-stream-特征" > < a class = "header" href = "#实现-stream-特征" > 实现 Stream 特征< / a > < / h2 >
< p > 如果大家还没忘记 < code > Future< / code > 特征，那 < code > Stream< / code > 特征相信你也会很快记住，因为它们非常类似：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::pin::Pin;
use std::task::{Context, Poll};
pub trait Stream {
type Item;
fn poll_next(
self: Pin< & mut Self> ,
cx: & mut Context< '_>
) -> Poll< Option< Self::Item> > ;
fn size_hint(& self) -> (usize, Option< usize> ) {
(0, None)
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > < code > Stream::poll_next()< / code > 函数跟 < code > Future::poll< / code > 很相似，区别就是前者为了从 < code > stream< / code > 收到多个值需要重复的进行调用。 就像在 < a href = "https://course.rs/tokio/async.html" > < code > 深入async< / code > < / a > 章节提到的那样，当一个 < code > stream< / code > 没有做好返回一个值的准备时，它将返回一个 < code > Poll::Pending< / code > ，同时将任务的 < code > waker< / code > 进行注册。一旦 < code > stream< / code > 准备好后， < code > waker< / code > 将被调用。< / p >
< p > 通常来说，如果想要手动实现一个 < code > Stream< / code > ，需要组合 < code > Future< / code > 和其它 < code > Stream< / code > 。下面，还记得在< a href = "https://course.rs/tokio/async.html" > < code > 深入async< / code > < / a > 中构建的 < code > Delay Future< / code > 吗？现在让我们来更进一步，将它转换成一个 < code > stream< / code > ，每 10 毫秒生成一个值，总共生成 3 次:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use tokio_stream::Stream;
use std::pin::Pin;
use std::task::{Context, Poll};
use std::time::Duration;
struct Interval {
rem: usize,
delay: Delay,
}
impl Stream for Interval {
type Item = ();
fn poll_next(mut self: Pin< & mut Self> , cx: & mut Context< '_> )
-> Poll< Option< ()> >
{
if self.rem == 0 {
// 去除计时器实现
return Poll::Ready(None);
}
match Pin::new(& mut self.delay).poll(cx) {
Poll::Ready(_) => {
let when = self.delay.when + Duration::from_millis(10);
self.delay = Delay { when };
self.rem -= 1;
Poll::Ready(Some(()))
}
Poll::Pending => Poll::Pending,
}
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< h4 id = "async-stream" > < a class = "header" href = "#async-stream" > async-stream< / a > < / h4 >
< p > 手动实现 < code > Stream< / code > 特征实际上是相当麻烦的事， ， < code > async/await< / code > 语法目前还不能用于定义 < code > stream< / code > ，虽然相关的工作已经在进行中。< / p >
< p > 作为替代方案，< a href = "https://docs.rs/async-stream/latest/async_stream/" > < code > async-stream< / code > < / a > 包提供了一个 < code > stream!< / code > 宏，它可以将一个输入转换成 < code > stream< / code > ，使用这个包，上面的代码可以这样实现：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use async_stream::stream;
use std::time::{Duration, Instant};
stream! {
let mut when = Instant::now();
for _ in 0..3 {
let delay = Delay { when };
delay.await;
yield ();
when += Duration::from_millis(10);
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 嗯，看上去还是相当不错的，代码可读性大幅提升！< / p >
<!-- todo generators -->
< p > 是不是发现了一个关键字 < code > yield< / code > ，他是用来配合生成器使用的。详见< a href = "https://doc.rust-lang.org/beta/unstable-book/language-features/generators.html" > 原文< / a > < / p >
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