具有特征的 tokio-async-await

Question

我想在特征中编写异步函数，但由于特征中的 async fn 尚不支持，我正在尝试寻找等效的方法接口。这是我在 Rust nightly (2019-01-01) 中尝试过的：

#![feature(await_macro, async_await, futures_api)]
#[macro_use]
extern crate tokio;
use tokio::prelude::*;

trait T {
    async fn f();
}

fn main() {
}

error[E0706]: trait fns cannot be declared `async`
 --> src/main.rs:7:5
  |
7 |     async fn f();
  |     ^^^^^^^^^^^^^

我在某处读到 async 只是 impl Future。

trait T {
    fn f() -> impl futures::Future<Item = (), Error = ()>;
}

error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
 --> src/lib.rs:2:15
  |
2 |     fn f() -> impl futures::Future<Item = (), Error = ()>;
  |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

不允许直接返回impl trait，所以我尝试了一个盒装特征：

trait Resource {
    fn loaded(&self) -> bool;
    fn init(&mut self, auth: &str) -> Box<dyn std::future::Future<Output=()>>;
    fn prepare(&mut self, auth: &str) -> Box<dyn std::future::Future<Output=()>> {
        Box::new(async {
            if !self.loaded() {
                await!(*(self.init(auth)));
            }
        })
    }
}

[rustc] the size for values of type `dyn std::future::Future<Output=()>` cannot be known at compilation time

如果没有 deref，我会得到这样的错误，即 Box<>.

不存在 into_awaitable

我可以将非 Sized impl Future 或 *Box<Future> 与 await! 一起使用吗？ trait中异步函数最适合的接口是什么？

Answer 1

特征中不允许使用 async 函数和 impl Trait。您可以使用关联类型来拉近距离。以下是一些想法：

pub trait ResourceTrait {
    type FutType: Future<Output = ()>;

    fn prepare(&mut self, auth: &str) -> Self::Next;
}

目前实现这个有点棘手，因为一些必需的工具尚不可用、稳定或有问题。

可以实现为：

impl ResourceTrait for Resource {
    type FutType = FutureObj<'static, ()>;

    fn prepare(&mut self, auth: &str) -> FutureObj<'static, ()> {
        FutureObj::new(Box::new(
            async move {
                // Do async things
                // You might get a lifetime issue here if trying to access auth,
                // since it's borrowed.
            }
        ))
    }
}

存在类型的替代方案可能是：

impl ResourceTrait for Resource {
    // this is required since the real type of the async function
    // is unnameable
    existential type FutType = Future<Output = ()>;

    fn prepare(&mut self, auth: &str) -> Self::FutType {
        async move {
            // Do async things. Might still encounter the same borrowing issues,
            // since the lifetime of the returned Future isn't coupled to the
            // lifetime of self.
            // The workaround is to make copies of all required fields and move
            // them into the Future
        }
    }
}

这可能有效也可能无效（因为该功能正在开发中）。为了在返回的 future 中正确借用 self 或 auth 等参数，我们可能还需要首先提供通用关联类型。

为了解决 self 的借用问题，您可以定义

struct Resource {
    inner: Arc<ResourceInner>, // carries all actual state
}

以便您可以在 prepare 中复制 inner 并将其移动到 Future。

Answer 2

如果您不介意返回盒装的未来，您可以使用 async-trait 板条箱：

#![feature(async_await)]

use async_trait::async_trait;

#[async_trait]
trait Advertisement {
    async fn run(&self);
}

struct Modal;

#[async_trait]
impl Advertisement for Modal {
    async fn run(&self) {
        self.render_fullscreen().await;
        for _ in 0..4u16 {
            remind_user_to_join_mailing_list().await;
        }
        self.hide_for_now().await;
    }
}

struct AutoplayingVideo {
    media_url: String,
}

#[async_trait]
impl Advertisement for AutoplayingVideo {
    async fn run(&self) {
        let stream = connect(&self.media_url).await;
        stream.play().await;

        // Video probably persuaded user to join our mailing list!
        Modal.run().await;
    }
}

具有特征的 tokio-async-await

tokio-async-await with trait

async-await

rust

rust-tokio