async-await - 具有特征的 tokio-async-await

Question

我想在 trait 中编写异步函数，但是因为 async fn in traits 还不被支持，我试图找到等效的方法接口(interface)。这是我在 Rust nightly (2019-01-01) 中尝试过的:

playground

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

直接返回impl trait是不允许的，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

没有 deref，我得到的错误是没有 into_awaitable存在 Box<>.

我可以使用非尺寸 impl Future或 *Box<Future>与 await! ？什么是最适合 trait 中异步函数的接口(interface)？

Answer 1

async 函数和 impl Trait 都不允许出现在特征中。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
}

这样你就可以在prepare中复制inner并将其移动到Future中。