带有特征的 tokio-async-await

Question

我想在 trait 中编写异步函数，但由于async fn尚不支持 in trait，我试图找到等效的方法接口。这是我每晚在 Rust 中尝试过的（2019-01-01）：

操场

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

impl trait不允许直接返回，所以我尝试了一个boxed trait：

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 deref，我得到不into_awaitable存在的错误Box<>.

我可以使用非尺码impl Future或*Box<Future>搭配await!吗？trait 中最适合异步函数的接口是什么？

Answer 1

特征中既不允许async使用功能，也impl Trait不允许使用。您可以使用关联类型来更接近。以下是一些想法：

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

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

目前实现这一点有点棘手，因为一些所需的工具要么尚不可用，要么不稳定，要么有缺陷。

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）。为了正确借用像selforauth在返回的 future 中的参数，我们可能还需要首先使用通用关联类型。

为了解决 的借用问题self，您也许可以定义

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

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

这样您就可以复制inner并将prepare其移动到Future.