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< h1 id = "select" > < a class = "header" href = "#select" > select!< / a > < / h1 >
< p > 在实际使用时，一个重要的场景就是同时等待多个异步操作的结果，并且对其结果进行进一步处理，在本章节，我们来看看，强大的 < code > select!< / code > 是如何帮助咱们更好的控制多个异步操作并发执行的。< / p >
< h2 id = "tokioselect" > < a class = "header" href = "#tokioselect" > tokio::select!< / a > < / h2 >
< p > < code > select!< / code > 允许同时等待多个计算操作，然后当其中一个操作完成时就退出等待:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio::sync::oneshot;
#[tokio::main]
async fn main() {
let (tx1, rx1) = oneshot::channel();
let (tx2, rx2) = oneshot::channel();
tokio::spawn(async {
let _ = tx1.send("one");
});
tokio::spawn(async {
let _ = tx2.send("two");
});
tokio::select! {
val = rx1 => {
println!("rx1 completed first with {:?}", val);
}
val = rx2 => {
println!("rx2 completed first with {:?}", val);
}
}
// 任何一个 select 分支结束后，都会继续执行接下来的代码
}< / code > < / pre >
< p > 这里用到了两个 < code > oneshot< / code > 消息通道，虽然两个操作的创建在代码上有先后顺序，但在实际执行时却不这样。因此， < code > select< / code > 在从两个通道< strong > 阻塞等待< / strong > 接收消息时，< code > rx1< / code > 和 < code > rx2< / code > 都有可能被先打印出来。< / p >
< p > 需要注意，任何一个 < code > select< / code > 分支完成后，都会继续执行后面的代码，没被执行的分支会被丢弃( < code > dropped< / code > )。< / p >
< h4 id = "取消" > < a class = "header" href = "#取消" > 取消< / a > < / h4 >
< p > 对于 < code > Async Rust< / code > 来说，释放( drop )掉一个 < code > Future< / code > 就意味着取消任务。从上一章节可以得知， < code > async< / code > 操作会返回一个 < code > Future< / code > ，而后者是惰性的，直到被 < code > poll< / code > 调用时，才会被执行。一旦 < code > Future< / code > 被释放，那操作将无法继续，因为所有相关的状态都被释放。< / p >
< p > 对于 Tokio 的 < code > oneshot< / code > 的接收端来说，它在被释放时会发送一个关闭通知到发送端，因此发送端可以通过释放任务的方式来终止正在执行的任务。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio::sync::oneshot;
async fn some_operation() -> String {
// 在这里执行一些操作...
}
#[tokio::main]
async fn main() {
let (mut tx1, rx1) = oneshot::channel();
let (tx2, rx2) = oneshot::channel();
tokio::spawn(async {
// 等待 `some_operation` 的完成
// 或者处理 `oneshot` 的关闭通知
tokio::select! {
val = some_operation() => {
let _ = tx1.send(val);
}
_ = tx1.closed() => {
// 收到了发送端发来的关闭信号
// `select` 即将结束，此时，正在进行的 `some_operation()` 任务会被取消，任务自动完成，
// tx1 被释放
}
}
});
tokio::spawn(async {
let _ = tx2.send("two");
});
tokio::select! {
val = rx1 => {
println!("rx1 completed first with {:?}", val);
}
val = rx2 => {
println!("rx2 completed first with {:?}", val);
}
}
}< / code > < / pre >
< p > 上面代码的重点就在于 < code > tx1.closed< / code > 所在的分支，一旦发送端被关闭，那该分支就会被执行，然后 < code > select< / code > 会退出，并清理掉还没执行的第一个分支 < code > val = some_operation()< / code > ，这其中 < code > some_operation< / code > 返回的 < code > Future< / code > 也会被清理，根据之前的内容，< code > Future< / code > 被清理那相应的任务会立即取消，因此 < code > some_operation< / code > 会被取消，不再执行。< / p >
< h4 id = "future-的实现" > < a class = "header" href = "#future-的实现" > Future 的实现< / a > < / h4 >
< p > 为了更好的理解 < code > select< / code > 的工作原理，我们来看看如果使用 < code > Future< / code > 该如何实现。当然，这里是一个简化版本，在实际中，< code > select!< / code > 会包含一些额外的功能，例如一开始会随机选择一个分支进行 < code > poll< / code > 。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio::sync::oneshot;
use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};
struct MySelect {
rx1: oneshot::Receiver< & 'static str> ,
rx2: oneshot::Receiver< & 'static str> ,
}
impl Future for MySelect {
type Output = ();
fn poll(mut self: Pin< & mut Self> , cx: & mut Context< '_> ) -> Poll< ()> {
if let Poll::Ready(val) = Pin::new(& mut self.rx1).poll(cx) {
println!("rx1 completed first with {:?}", val);
return Poll::Ready(());
}
if let Poll::Ready(val) = Pin::new(& mut self.rx2).poll(cx) {
println!("rx2 completed first with {:?}", val);
return Poll::Ready(());
}
Poll::Pending
}
}
#[tokio::main]
async fn main() {
let (tx1, rx1) = oneshot::channel();
let (tx2, rx2) = oneshot::channel();
// 使用 tx1 和 tx2
MySelect {
rx1,
rx2,
}.await;
}< / code > < / pre >
< p > < code > MySelect< / code > 包含了两个分支中的 < code > Future< / code > ，当它被 < code > poll< / code > 时，第一个分支会先执行。如果执行完成，那取出的值会被使用，然后 < code > MySelect< / code > 也随之结束。而另一个分支对应的 < code > Future< / code > 会被释放掉，对应的操作也会被取消。< / p >
< p > 还记得上一章节中很重要的一段话吗？< / p >
< blockquote >
< p > 当一个 < code > Future< / code > 返回 < code > Poll::Pending< / code > 时，它必须确保会在某一个时刻通过 < code > Waker< / code > 来唤醒，不然该 < code > Future< / code > 将永远地被挂起< / p >
< / blockquote >
< p > 但是仔细观察我们之前的代码，里面并没有任何的 < code > wake< / code > 调用！事实上，这是因为参数 < code > cx< / code > 被传入了内层的 < code > poll< / code > 调用。 只要内部的 < code > Future< / code > 实现了唤醒并且返回了 < code > Poll::Pending< / code > ，那 < code > MySelect< / code > 也等于实现了唤醒！< / p >
< h2 id = "语法" > < a class = "header" href = "#语法" > 语法< / a > < / h2 >
< p > 目前来说，< code > select!< / code > 最多可以支持 64 个分支，每个分支形式如下：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > < 模式> = < async 表达式> => < 结果处理> ,
< span class = "boring" > }< / span > < / code > < / pre >
< p > 当 < code > select< / code > 宏开始执行后，所有的分支会开始并发的执行。当任何一个< strong > 表达式< / strong > 完成时，会将结果跟< strong > 模式< / strong > 进行匹配。若匹配成功，则剩下的表达式会被释放。< / p >
< p > 最常用的< strong > 模式< / strong > 就是用变量名去匹配表达式返回的值，然后该变量就可以在< strong > 结果处理< / strong > 环节使用。< / p >
< p > 如果当前的模式不能匹配，剩余的 < code > async< / code > 表达式将继续并发的执行，直到下一个完成。< / p >
< p > 由于 < code > select!< / code > 使用的是一个 < code > async< / code > 表达式，因此我们可以定义一些更复杂的计算。< / p >
< p > 例如从在分支中进行 TCP 连接：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio::net::TcpStream;
use tokio::sync::oneshot;
#[tokio::main]
async fn main() {
let (tx, rx) = oneshot::channel();
// 生成一个任务，用于向 oneshot 发送一条消息
tokio::spawn(async move {
tx.send("done").unwrap();
});
tokio::select! {
socket = TcpStream::connect("localhost:3465") => {
println!("Socket connected {:?}", socket);
}
msg = rx => {
println!("received message first {:?}", msg);
}
}
}< / code > < / pre >
< p > 再比如，在分支中进行 TCP 监听:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio::net::TcpListener;
use tokio::sync::oneshot;
use std::io;
#[tokio::main]
async fn main() -> io::Result< ()> {
let (tx, rx) = oneshot::channel();
tokio::spawn(async move {
tx.send(()).unwrap();
});
let mut listener = TcpListener::bind("localhost:3465").await?;
tokio::select! {
_ = async {
loop {
let (socket, _) = listener.accept().await?;
tokio::spawn(async move { process(socket) });
}
// 给予 Rust 类型暗示
Ok::< _, io::Error> (())
} => {}
_ = rx => {
println!("terminating accept loop");
}
}
Ok(())
}< / code > < / pre >
< p > 分支中接收连接的循环会一直运行，直到遇到错误才停止，或者当 < code > rx< / code > 中有值时，也会停止。 < code > _< / code > 表示我们并不关心这个值，这样使用唯一的目的就是为了结束第一分支中的循环。< / p >
< h2 id = "返回值" > < a class = "header" href = "#返回值" > 返回值< / a > < / h2 >
< p > < code > select!< / code > 还能返回一个值:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > async fn computation1() -> String {
// .. 计算
}
async fn computation2() -> String {
// .. 计算
}
#[tokio::main]
async fn main() {
let out = tokio::select! {
res1 = computation1() => res1,
res2 = computation2() => res2,
};
println!("Got = {}", out);
}< / code > < / pre >
< p > 需要注意的是，此时 < code > select!< / code > 的所有分支必须返回一样的类型，否则编译器会报错！< / p >
< h2 id = "错误传播" > < a class = "header" href = "#错误传播" > 错误传播< / a > < / h2 >
< p > 在 Rust 中使用 < code > ?< / code > 可以对错误进行传播，但是在 < code > select!< / code > 中，< code > ?< / code > 如何工作取决于它是在分支中的 < code > async< / code > 表达式使用还是在结果处理的代码中使用:< / p >
< ul >
< li > 在分支中 < code > async< / code > 表达式使用会将该表达式的结果变成一个 < code > Result< / code > < / li >
< li > 在结果处理中使用，会将错误直接传播到 < code > select!< / code > 之外< / li >
< / ul >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio::net::TcpListener;
use tokio::sync::oneshot;
use std::io;
#[tokio::main]
async fn main() -> io::Result< ()> {
// [设置 `rx` oneshot 消息通道]
let listener = TcpListener::bind("localhost:3465").await?;
tokio::select! {
res = async {
loop {
let (socket, _) = listener.accept().await?;
tokio::spawn(async move { process(socket) });
}
Ok::< _, io::Error> (())
} => {
res?;
}
_ = rx => {
println!("terminating accept loop");
}
}
Ok(())
}< / code > < / pre >
< p > < code > listener.accept().await?< / code > 是分支表达式中的 < code > ?< / code > ，因此它会将表达式的返回值变成 < code > Result< / code > 类型，然后赋予给 < code > res< / code > 变量。< / p >
< p > 与之不同的是，结果处理中的 < code > res?;< / code > 会让 < code > main< / code > 函数直接结束并返回一个 < code > Result< / code > ，可以看出，这里 < code > ?< / code > 的用法跟我们平时的用法并无区别。< / p >
< h2 id = "模式匹配" > < a class = "header" href = "#模式匹配" > 模式匹配< / a > < / h2 >
< p > 既然是模式匹配，我们需要再来回忆下 < code > select!< / code > 的分支语法形式:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > < 模式> = < async 表达式> => < 结果处理> ,
< span class = "boring" > }< / span > < / code > < / pre >
< p > 迄今为止，我们只用了变量绑定的模式，事实上，< a href = "https://course.rs/basic/match-pattern/all-patterns.html" > 任何 Rust 模式< / a > 都可以在此处使用。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio::sync::mpsc;
#[tokio::main]
async fn main() {
let (mut tx1, mut rx1) = mpsc::channel(128);
let (mut tx2, mut rx2) = mpsc::channel(128);
tokio::spawn(async move {
// 用 tx1 和 tx2 干一些不为人知的事
});
tokio::select! {
Some(v) = rx1.recv() => {
println!("Got {:?} from rx1", v);
}
Some(v) = rx2.recv() => {
println!("Got {:?} from rx2", v);
}
else => {
println!("Both channels closed");
}
}
}< / code > < / pre >
< p > 上面代码中，< code > rx< / code > 通道关闭后，< code > recv()< / code > 方法会返回一个 < code > None< / code > ，可以看到没有任何模式能够匹配这个 < code > None< / code > ，那为何不会报错？秘密就在于 < code > else< / code > 上：当使用模式去匹配分支时，若之前的所有分支都无法被匹配，那 < code > else< / code > 分支将被执行。< / p >
< h2 id = "借用" > < a class = "header" href = "#借用" > 借用< / a > < / h2 >
< p > 当在 Tokio 中生成( spawn )任务时，其 async 语句块必须拥有其中数据的所有权。而 < code > select!< / code > 并没有这个限制，它的每个分支表达式可以直接借用数据，然后进行并发操作。只要遵循 Rust 的借用规则，多个分支表达式可以不可变的借用同一个数据，或者在一个表达式可变的借用某个数据。< / p >
< p > 来看个例子，在这里我们同时向两个 TCP 目标发送同样的数据:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use tokio::io::AsyncWriteExt;
use tokio::net::TcpStream;
use std::io;
use std::net::SocketAddr;
async fn race(
data: & [u8],
addr1: SocketAddr,
addr2: SocketAddr
) -> io::Result< ()> {
tokio::select! {
Ok(_) = async {
let mut socket = TcpStream::connect(addr1).await?;
socket.write_all(data).await?;
Ok::< _, io::Error> (())
} => {}
Ok(_) = async {
let mut socket = TcpStream::connect(addr2).await?;
socket.write_all(data).await?;
Ok::< _, io::Error> (())
} => {}
else => {}
};
Ok(())
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 有趣的题外话：由于 TCP 的连接过程是在模式中发生，因此当某一个连接过程失败后，它通过 < code > ?< / code > 返回的 < code > Err< / code > 类型无法匹配 < code > Ok< / code > ，导致另一个分支会继续被执行、继续连接。< / p >
< p > 如果你把连接过程放在了结果处理中，那连接失败会直接从 < code > race< / code > 函数中返回，而不是继续执行另一个分支中的连接！< / p >
< p > 还有一个非常重要的点，< strong > 借用规则在分支表达式和结果处理中存在很大的不同< / strong > 。例如上面代码中，我们在两个分支表达式中分别对 < code > data< / code > 做了不可变借用，这当然 ok， ， ： ， ， ， ： ？ < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio::sync::oneshot;
#[tokio::main]
async fn main() {
let (tx1, rx1) = oneshot::channel();
let (tx2, rx2) = oneshot::channel();
let mut out = String::new();
tokio::spawn(async move {
});
tokio::select! {
_ = rx1 => {
out.push_str("rx1 completed");
}
_ = rx2 => {
out.push_str("rx2 completed");
}
}
println!("{}", out);
}< / code > < / pre >
< p > 例如以上代码，就在两个分支的结果处理中分别进行了可变借用，并不会报错。原因就在于：< code > select!< / code > 会保证只有一个分支的结果处理会被运行，然后在运行结束后，另一个分支会被直接丢弃。< / p >
< h2 id = "循环" > < a class = "header" href = "#循环" > 循环< / a > < / h2 >
< p > 来看看该如何在循环中使用 < code > select!< / code > ，顺便说一句，跟循环一起使用是最常见的使用方式。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio::sync::mpsc;
#[tokio::main]
async fn main() {
let (tx1, mut rx1) = mpsc::channel(128);
let (tx2, mut rx2) = mpsc::channel(128);
let (tx3, mut rx3) = mpsc::channel(128);
loop {
let msg = tokio::select! {
Some(msg) = rx1.recv() => msg,
Some(msg) = rx2.recv() => msg,
Some(msg) = rx3.recv() => msg,
else => { break }
};
println!("Got {}", msg);
}
println!("All channels have been closed.");
}< / code > < / pre >
< p > 在循环中使用 < code > select!< / code > 最大的不同就是，当某一个分支执行完成后，< code > select!< / code > 会继续循环等待并执行下一个分支，直到所有分支最终都完成，最终匹配到 < code > else< / code > 分支，然后通过 < code > break< / code > 跳出循环。< / p >
< p > 老生常谈的一句话：< code > select!< / code > 中哪个分支先被执行是无法确定的，因此不要依赖于分支执行的顺序！想象一下，在异步编程场景，若 < code > select!< / code > 按照分支的顺序来执行会如何：若 < code > rx1< / code > 中总是有数据，那每次循环都只会去处理第一个分支，后面两个分支永远不会被执行。< / p >
< h4 id = "恢复之前的异步操作" > < a class = "header" href = "#恢复之前的异步操作" > 恢复之前的异步操作< / a > < / h4 >
< pre class = "playground" > < code class = "language-rust edition2021" > async fn action() {
// 一些异步逻辑
}
#[tokio::main]
async fn main() {
let (mut tx, mut rx) = tokio::sync::mpsc::channel(128);
let operation = action();
tokio::pin!(operation);
loop {
tokio::select! {
_ = & mut operation => break,
Some(v) = rx.recv() => {
if v % 2 == 0 {
break;
}
}
}
}
}< / code > < / pre >
< p > 在上面代码中，我们没有直接在 < code > select!< / code > 分支中调用 < code > action()< / code > ，而是在 < code > loop< / code > 循环外面先将 < code > action()< / code > 赋值给 < code > operation< / code > ，因此 < code > operation< / code > 是一个 < code > Future< / code > 。< / p >
< p > < strong > 重点来了< / strong > ，在 < code > select!< / code > 循环中，我们使用了一个奇怪的语法 < code > & mut operation< / code > ，大家想象一下，如果不加 < code > & mut< / code > 会如何？答案是，每一次循环调用的都是一次全新的 < code > action()< / code > 调用，但是当加了 < code > & mut operatoion< / code > 后，每一次循环调用就变成了对同一次 < code > action()< / code > 的调用。也就是我们实现了在每次循环中恢复了之前的异步操作！< / p >
< p > < code > select!< / code > 的另一个分支从消息通道收取消息，一旦收到值是偶数，就跳出循环，否则就继续循环。< / p >
< p > 还有一个就是我们使用了 < code > tokio::pin!< / code > ，具体的细节这里先不介绍，值得注意的点是：如果要在一个引用上使用 < code > .await< / code > ，那么引用的值就必须是不能移动的或者实现了 < code > Unpin< / code > ，关于 < code > Pin< / code > 和 < code > Unpin< / code > 可以参见< a href = "https://course.rs/async/pin-unpin.html" > 这里< / a > 。< / p >
< p > 一旦移除 < code > tokio::pin!< / code > 所在行的代码，然后试图编译，就会获得以下错误:< / p >
< pre > < code class = "language-console" > error[E0599]: no method named `poll` found for struct
`std::pin::Pin< & mut & mut impl std::future::Future> `
in the current scope
--> src/main.rs:16:9
|
16 | / tokio::select! {
17 | | _ = & mut operation => break,
18 | | Some(v) = rx.recv() => {
19 | | if v % 2 == 0 {
... |
22 | | }
23 | | }
| |_________^ method not found in
| `std::pin::Pin< & mut & mut impl std::future::Future> `
|
= note: the method `poll` exists but the following trait bounds
were not satisfied:
`impl std::future::Future: std::marker::Unpin`
which is required by
`& mut impl std::future::Future: std::future::Future`
< / code > < / pre >
< p > 虽然我们已经学了很多关于 < code > Future< / code > 的知识，但是这个错误依然不太好理解。但是它不难解决：当你试图在< strong > 一个引用上调用 < code > .await< / code > 然后遇到了 < code > Future 未实现< / code > 这种错误时< / strong > ，往往只需要将对应的 < code > Future< / code > 进行固定即可: < code > tokio::pin!(operation);< / code > 。< / p >
< h4 id = "修改一个分支" > < a class = "header" href = "#修改一个分支" > 修改一个分支< / a > < / h4 >
< p > 下面一起来看一个稍微复杂一些的 < code > loop< / code > 循环，首先，我们拥有：< / p >
< ul >
< li > 一个消息通道可以传递 < code > i32< / code > 类型的值< / li >
< li > 定义在 < code > i32< / code > 值上的一个异步操作< / li >
< / ul >
< p > 想要实现的逻辑是：< / p >
< ul >
< li > 在消息通道中等待一个偶数出现< / li >
< li > 使用该偶数作为输入来启动一个异步操作< / li >
< li > 等待异步操作完成，与此同时监听消息通道以获取更多的偶数< / li >
< li > 若在异步操作完成前一个新的偶数到来了，终止当前的异步操作，然后接着使用新的偶数开始异步操作< / li >
< / ul >
< pre class = "playground" > < code class = "language-rust edition2021" > async fn action(input: Option< i32> ) -> Option< String> {
// 若 input（ ） ，
// 事实上也可以这么写: `let i = input?;`
let i = match input {
Some(input) => input,
None => return None,
};
// 这里定义一些逻辑
}
#[tokio::main]
async fn main() {
let (mut tx, mut rx) = tokio::sync::mpsc::channel(128);
let mut done = false;
let operation = action(None);
tokio::pin!(operation);
tokio::spawn(async move {
let _ = tx.send(1).await;
let _ = tx.send(3).await;
let _ = tx.send(2).await;
});
loop {
tokio::select! {
res = & mut operation, if !done => {
done = true;
if let Some(v) = res {
println!("GOT = {}", v);
return;
}
}
Some(v) = rx.recv() => {
if v % 2 == 0 {
// `.set` 是 `Pin` 上定义的方法
operation.set(action(Some(v)));
done = false;
}
}
}
}
}< / code > < / pre >
< p > 当第一次循环开始时， 第一个分支会立即完成，因为 < code > operation< / code > 的参数是 < code > None< / code > 。当第一个分支执行完成时，< code > done< / code > 会变成 < code > true< / code > ，此时第一个分支的条件将无法被满足，开始执行第二个分支。< / p >
< p > 当第二个分支收到一个偶数时，< code > done< / code > 会被修改为 < code > false< / code > ，且 < code > operation< / code > 被设置了值。 此后再一次循环时，第一个分支会被执行，且 < code > operation< / code > 返回一个 < code > Some(2)< / code > ，因此会触发 < code > return< / code > ，最终结束循环并返回。< / p >
< p > 这段代码引入了一个新的语法: < code > if !done< / code > ，在解释之前，先看看去掉后会如何：< / p >
< pre > < code class = "language-console" > thread 'main' panicked at '`async fn` resumed after completion', src/main.rs:1:55
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
< / code > < / pre >
< p > < code > '`async fn` resumed after completion'< / code > 错误的含义是：< code > async fn< / code > 异步函数在完成后，依然被恢复了(继续使用)。< / p >
< p > 回到例子中来，这个错误是由于 < code > operation< / code > 在它已经调用完成后依然被使用。通常来说，当使用 < code > .await< / code > 后，调用 < code > .await< / code > 的值会被消耗掉，因此并不存在这个问题。但是在这例子中，我们在引用上调用 < code > .await< / code > ，因此之后该引用依然可以被使用。< / p >
< p > 为了避免这个问题，需要在第一个分支的 < code > operation< / code > 完成后禁止再使用该分支。这里的 < code > done< / code > 的引入就很好的解决了问题。对于 < code > select!< / code > 来说 < code > if !done< / code > 的语法被称为预条件( < strong > precondition< / strong > )，该条件会在分支被 < code > .await< / code > 执行前进行检查。< / p >
< p > 那大家肯定有疑问了，既然 < code > operation< / code > 不能再被调用了，我们该如何在有偶数值时，再回到第一个分支对其进行调用呢？答案就是 < code > operation.set(action(Some(v)));< / code > ，该操作会重新使用新的参数设置 < code > operation< / code > 。< / p >
< h2 id = "spawn-和-select-的一些不同" > < a class = "header" href = "#spawn-和-select-的一些不同" > spawn 和 select! 的一些不同< / a > < / h2 >
< p > 学到现在，相信大家对于 < code > tokio::spawn< / code > 和 < code > select!< / code > 已经非常熟悉，它们的共同点就是都可以并发的运行异步操作。
然而它们使用的策略大相径庭。< / p >
< p > < code > tokio::spawn< / code > 函数会启动新的任务来运行一个异步操作，每个任务都是一个独立的对象可以单独被 Tokio 调度运行，因此两个不同的任务的调度都是独立进行的，甚至于它们可能会运行在两个不同的操作系统线程上。鉴于此，生成的任务和生成的线程有一个相同的限制：不允许对外部环境中的值进行借用。< / p >
< p > 而 < code > select!< / code > 宏就不一样了，它在同一个任务中并发运行所有的分支。正是因为这样，在同一个任务中，这些分支无法被同时运行。 < code > select!< / code > 宏在单个任务中实现了多路复用的功能。< / p >
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