Question

我想在特征中编写异步函数，但是由于尚未支持特征中的async fn，因此我试图找到等效的方法接口。这是我每晚（2019-01-01）在Rust中都尝试过的事情：

#![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();
  |     ^^^^^^^^^^^^^

我在某处看到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

没有解引用，我得到一个错误：into_awaitable不存在Box<>.

我可以将非大小的impl Future或*Box<Future>与await!一起使用吗？特质中最适合异步功能的接口是什么？

Answer 1

特征中不允许使用async函数和impl Trait。您可以使用关联的类型来接近。这里有一些想法：

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

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

实施此操作目前有些棘手，因为某些必需的工具尚不可用，稳定或有故障。

它可以实现为：

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.
            }
        ))
    }
}

存在类型的替代方法可能是：

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
        }
    }
}

这可能会或可能不会（因为该功能正在进行中）。为了在返回的将来正确借用诸如self或auth之类的参数，我们可能还需要首先使用通用关联类型。

为了解决self的借入问题，您可以定义

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

以便您可以将inner中的prepare复制到Future中。

Answer 2

如果您可以退还装箱的商品，可以使用async-trait条板箱：

#![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;
    }
}

具有特质的东京异步等待

2 个答案: