Rust:方法“轮询"在 `impl std::future::Future` 中找不到

问题描述

我正在尝试学习异步编程，但这个非常基本的示例不起作用:

I'm trying to learn async programming, but this very basic example doesn't work:

use std::future::Future;

fn main() {
    let t = async {
        println!("Hello, world!");
    };
    t.poll();
}

我从规范中读到的所有内容都说这应该可行，但是货物抱怨在impl std::future::Future"中找不到方法poll".我做错了什么?

Everything I've read from the specs says this should work, but cargo complains that method "poll" can't be found in "impl std::future::Future". What am I doing wrong?

推荐答案

poll 有这个签名:

poll has this signature:

fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;

以您的方式调用它有两个问题:

There are two problems with calling this in the way you do:

1. poll 不是在未来的 Fut 上实现的，而是在 Pin 上实现的，所以你需要首先获得固定参考.pin_mut! 通常很有用，并且如果未来实现 Unpin，你可以使用Pin::new 也是.

1. poll is not implement on a future Fut, but on Pin<&mut Fut>, so you need to get a pinned reference first. pin_mut! is often useful, and if the future implements Unpin, you can use Pin::new as well.

然而，更大的问题是 poll 需要一个 &mut Context 参数.上下文由异步运行时创建并传递给最外层 future 的 poll 函数.这意味着您不能像那样轮询未来，您需要在异步运行时中才能做到这一点.

The bigger problem however is that poll takes a &mut Context<'_> argument. The context is created by the asynchronous runtime and passed in to the poll function of the outermost future. This means that you can't just poll a future like that, you need to be in an asynchronous runtime to do it.

相反，您可以使用像 tokio 这样的板条箱或 async-std 以运行未来同步上下文:

Instead, you can use a crate like tokio or async-std to run a future in a synchronous context:

// tokio
use tokio::runtime::Runtime;
let runtime = Runtime::new().unwrap();
let result = runtime.block_on(async {
  // ...
});

// async-std
let result = async_std::task::block_on(async {
  // ...
})

或者更好的是，您可以使用 #[tokio::main] 或 #[async_std::main] 将您的 main 函数转换为异步函数:

Or even better, you can use #[tokio::main] or #[async_std::main] to convert your main function into an asynchronous function:

// tokio
#[tokio::main]
async fn main() {
  // ...
}

// async-std
#[async_std::main]
async fn main() {
  // ...
}

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