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< h1 id = "asyncawait-和-stream-流处理" > < a class = "header" href = "#asyncawait-和-stream-流处理" > async/await 和 Stream 流处理< / a > < / h1 >
< p > 在入门章节中，我们简单学习了该如何使用 < code > async/.await< / code > ， < / p >
< p > < code > async/.await< / code > 是 Rust 语法的一部分，它在遇到阻塞操作时( 例如 IO )会让出当前线程的所有权而不是阻塞当前线程，这样就允许当前线程继续去执行其它代码，最终实现并发。< / p >
< p > 有两种方式可以使用 < code > async< / code > ： < code > async fn< / code > 用于声明函数，< code > async { ... }< / code > 用于声明语句块，它们会返回一个实现 < code > Future< / code > 特征的值:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > // `foo()`返回一个`Future< Output = u8> `,
// 当调用`foo().await`时，该`Future`将被运行，当调用结束后我们将获取到一个`u8`值
async fn foo() -> u8 { 5 }
fn bar() -> impl Future< Output = u8> {
// 下面的`async`语句块返回`Future< Output = u8> `
async {
let x: u8 = foo().await;
x + 5
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > < code > async< / code > 是懒惰的，直到被执行器 < code > poll< / code > 或者 < code > .await< / code > 后才会开始运行，其中后者是最常用的运行 < code > Future< / code > 的方法。 当 < code > .await< / code > 被调用时，它会尝试运行 < code > Future< / code > 直到完成，但是若该 < code > Future< / code > 进入阻塞，那就会让出当前线程的控制权。当 < code > Future< / code > 后面准备再一次被运行时(例如从 < code > socket< / code > 中读取到了数据)，执行器会得到通知，并再次运行该 < code > Future< / code > ，如此循环，直到完成。< / p >
< p > 以上过程只是一个简述，详细内容在< a href = "https://course.rs/advance/async/future-excuting.html" > 底层探秘< / a > 中已经被深入讲解过，因此这里不再赘述。< / p >
< h2 id = "async-的生命周期" > < a class = "header" href = "#async-的生命周期" > < code > async< / code > 的生命周期< / a > < / h2 >
< p > < code > async fn< / code > 函数如果拥有引用类型的参数，那它返回的 < code > Future< / code > 的生命周期就会被这些参数的生命周期所限制:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > async fn foo(x: & u8) -> u8 { *x }
// 上面的函数跟下面的函数是等价的:
fn foo_expanded< 'a> (x: & 'a u8) -> impl Future< Output = u8> + 'a {
async move { *x }
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 意味着 < code > async fn< / code > 函数返回的 < code > Future< / code > 必须满足以下条件: 当 < code > x< / code > 依然有效时， 该 < code > Future< / code > 就必须继续等待( < code > .await< / code > ), 也就是说 < code > x< / code > 必须比 < code > Future< / code > 活得更久。< / p >
< p > 在一般情况下，在函数调用后就立即 < code > .await< / code > 不会存在任何问题，例如< code > foo(& x).await< / code > 。但是，若 < code > Future< / code > 被先存起来或发送到另一个任务或者线程，就可能存在问题了:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::future::Future;
fn bad() -> impl Future< Output = u8> {
let x = 5;
borrow_x(& x) // ERROR: `x` does not live long enough
}
async fn borrow_x(x: & u8) -> u8 { *x }
< span class = "boring" > }< / span > < / code > < / pre >
< p > 以上代码会报错，因为 < code > x< / code > 的生命周期只到 < code > bad< / code > 函数的结尾。 但是 < code > Future< / code > 显然会活得更久：< / p >
< pre > < code class = "language-shell" > error[E0597]: `x` does not live long enough
--> src/main.rs:4:14
|
4 | borrow_x(& x) // ERROR: `x` does not live long enough
| ---------^^-
| | |
| | borrowed value does not live long enough
| argument requires that `x` is borrowed for `'static`
5 | }
| - `x` dropped here while still borrowed
< / code > < / pre >
< p > 其中一个常用的解决方法就是将具有引用参数的 < code > async fn< / code > 函数转变成一个具有 < code > 'static< / code > 生命周期的 < code > Future< / code > 。 以上解决方法可以通过将参数和对 < code > async fn< / code > 的调用放在同一个 < code > async< / code > 语句块来实现:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::future::Future;
async fn borrow_x(x: & u8) -> u8 { *x }
fn good() -> impl Future< Output = u8> {
async {
let x = 5;
borrow_x(& x).await
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 如上所示，通过将参数移动到 < code > async< / code > 语句块内， 我们将它的生命周期扩展到 < code > 'static< / code > ， < code > Future< / code > 保持了一致。< / p >
< h2 id = "async-move" > < a class = "header" href = "#async-move" > async move< / a > < / h2 >
< p > < code > async< / code > 允许我们使用 < code > move< / code > 关键字来将环境中变量的所有权转移到语句块内，就像闭包那样，好处是你不再发愁该如何解决借用生命周期的问题，坏处就是无法跟其它代码实现对变量的共享:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > // 多个不同的 `async` 语句块可以访问同一个本地变量，只要它们在该变量的作用域内执行
async fn blocks() {
let my_string = "foo".to_string();
let future_one = async {
// ...
println!("{my_string}");
};
let future_two = async {
// ...
println!("{my_string}");
};
// 运行两个 Future 直到完成
let ((), ()) = futures::join!(future_one, future_two);
}
// 由于 `async move` 会捕获环境中的变量，因此只有一个 `async move` 语句块可以访问该变量，
// 但是它也有非常明显的好处： 变量可以转移到返回的 Future 中，不再受借用生命周期的限制
fn move_block() -> impl Future< Output = ()> {
let my_string = "foo".to_string();
async move {
// ...
println!("{my_string}");
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< h2 id = "当await-遇见多线程执行器" > < a class = "header" href = "#当await-遇见多线程执行器" > 当.await 遇见多线程执行器< / a > < / h2 >
< p > 需要注意的是，当使用多线程 < code > Future< / code > 执行器( < code > executor< / code > )时， < code > Future< / code > 可能会在线程间被移动，因此 < code > async< / code > 语句块中的变量必须要能在线程间传递。 至于 < code > Future< / code > 会在线程间移动的原因是：它内部的任何< code > .await< / code > 都可能导致它被切换到一个新线程上去执行。< / p >
< p > 由于需要在多线程环境使用，意味着 < code > Rc< / code > 、 < code > RefCell< / code > 、没有实现 < code > Send< / code > 的所有权类型、没有实现 < code > Sync< / code > 的引用类型，它们都是不安全的，因此无法被使用< / p >
< blockquote >
< p > 需要注意！实际上它们还是有可能被使用的，只要在 < code > .await< / code > 调用期间，它们没有在作用域范围内。< / p >
< / blockquote >
< p > 类似的原因，在 < code > .await< / code > 时使用普通的锁也不安全，例如 < code > Mutex< / code > 。原因是，它可能会导致线程池被锁：当一个任务获取锁 < code > A< / code > 后，若它将线程的控制权还给执行器，然后执行器又调度运行另一个任务，该任务也去尝试获取了锁 < code > A< / code > ，结果当前线程会直接卡死，最终陷入死锁中。< / p >
< p > 因此，为了避免这种情况的发生，我们需要使用 < code > futures< / code > 包下的锁 < code > futures::lock< / code > 来替代 < code > Mutex< / code > 完成任务。< / p >
< h2 id = "stream-流处理" > < a class = "header" href = "#stream-流处理" > Stream 流处理< / a > < / h2 >
< p > < code > Stream< / code > 特征类似于 < code > Future< / code > 特征，但是前者在完成前可以生成多个值，这种行为跟标准库中的 < code > Iterator< / code > 特征倒是颇为相似。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > trait Stream {
// Stream生成的值的类型
type Item;
// 尝试去解析Stream中的下一个值,
// 若无数据，返回`Poll::Pending`, 若有数据，返回 `Poll::Ready(Some(x))`, `Stream`完成则返回 `Poll::Ready(None)`
fn poll_next(self: Pin< & mut Self> , cx: & mut Context< '_> )
-> Poll< Option< Self::Item> > ;
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 关于 < code > Stream< / code > 的一个常见例子是消息通道（ < code > futures< / code > 包中的）的消费者 < code > Receiver< / code > 。每次有消息从 < code > Send< / code > 端发送后，它都可以接收到一个 < code > Some(val)< / code > 值， 一旦 < code > Send< / code > 端关闭( < code > drop< / code > )，且消息通道中没有消息后，它会接收到一个 < code > None< / code > 值。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > async fn send_recv() {
const BUFFER_SIZE: usize = 10;
let (mut tx, mut rx) = mpsc::channel::< i32> (BUFFER_SIZE);
tx.send(1).await.unwrap();
tx.send(2).await.unwrap();
drop(tx);
// `StreamExt::next` 类似于 `Iterator::next`, 但是前者返回的不是值，而是一个 `Future< Output = Option< T> > `，
// 因此还需要使用`.await`来获取具体的值
assert_eq!(Some(1), rx.next().await);
assert_eq!(Some(2), rx.next().await);
assert_eq!(None, rx.next().await);
}
< span class = "boring" > }< / span > < / code > < / pre >
< h4 id = "迭代和并发" > < a class = "header" href = "#迭代和并发" > 迭代和并发< / a > < / h4 >
< p > 跟迭代器类似，我们也可以迭代一个 < code > Stream< / code > 。 例如使用 < code > map< / code > ， < code > filter< / code > ， < code > fold< / code > 方法，以及它们的< em > 遇到错误提前返回< / em > 的版本： < code > try_map< / code > ， < code > try_filter< / code > ， < code > try_fold< / code > 。< / p >
< p > 但是跟迭代器又有所不同，< code > for< / code > 循环无法在这里使用，但是命令式风格的循环< code > while let< / code > 是可以用的，同时还可以使用< code > next< / code > 和 < code > try_next< / code > 方法:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > async fn sum_with_next(mut stream: Pin< & mut dyn Stream< Item = i32> > ) -> i32 {
use futures::stream::StreamExt; // 引入 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; // 引入 try_next
let mut sum = 0;
while let Some(item) = stream.try_next().await? {
sum += item;
}
Ok(sum)
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 上面代码是一次处理一个值的模式，但是需要注意的是：< strong > 如果你选择一次处理一个值的模式，可能会造成无法并发，这就失去了异步编程的意义< / strong > 。 因此，如果可以的话我们还是要选择从一个 < code > Stream< / code > 并发处理多个值的方式，通过 < code > for_each_concurrent< / code > 或 < code > try_for_each_concurrent< / code > 方法来实现:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > async fn jump_around(
mut stream: Pin< & mut dyn Stream< Item = Result< u8, io::Error> > > ,
) -> Result< (), io::Error> {
use futures::stream::TryStreamExt; // 引入 `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(())
}
< span class = "boring" > }< / span > < / code > < / pre >
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