异步特征

Async methods in traits are were stabilized only recently, in the 1.75 release. This required support for using return-position impl Trait (RPIT) in traits, as the desugaring for async fn includes -> impl Future<Output = ...>.

However, even with the native support today there are some pitfalls around async fn and RPIT in traits:

  • Return-position impl Trait captures all in-scope lifetimes (so some patterns of borrowing cannot be expressed)

  • Traits whose methods use return-position impl trait or async are not dyn compatible.

If we do need dyn support, the crate async_trait provides a workaround through a macro, with some caveats:

  1. use async_trait::async_trait;
  2. use std::time::Instant;
  3. use tokio::time::{sleep, Duration};
  5. #[async_trait]
  6. trait Sleeper {
  7.     async fn sleep(&self);
  8. }
  10. struct FixedSleeper {
  11.     sleep_ms: u64,
  12. }
  14. #[async_trait]
  15. impl Sleeper for FixedSleeper {
  16.     async fn sleep(&self) {
  17.         sleep(Duration::from_millis(self.sleep_ms)).await;
  18.     }
  19. }
  21. async fn run_all_sleepers_multiple_times(
  22.     sleepers: Vec<Box<dyn Sleeper>>,
  23.     n_times: usize,
  24. ) {
  25.     for _ in 0..n_times {
  26.         println!("running all sleepers..");
  27.         for sleeper in &sleepers {
  28.             let start = Instant::now();
  29.             sleeper.sleep().await;
  30.             println!("slept for {}ms", start.elapsed().as_millis());
  31.         }
  32.     }
  33. }
  35. #[tokio::main]
  36. async fn main() {
  37.     let sleepers: Vec<Box<dyn Sleeper>> = vec![
  38.         Box::new(FixedSleeper { sleep_ms: 50 }),
  39.         Box::new(FixedSleeper { sleep_ms: 100 }),
  40.     ];
  41.     run_all_sleepers_multiple_times(sleepers, 5).await;
  42. }

  • async_trait 易于使用，但请注意，它通过堆分配来实现这一点。这种堆分配会产生性能开销。

  • 对于 async trait 的语言支持中的挑战是深入 Rust的，并且可能不值得深入描述。如果您对深入了解感兴趣，Niko Matsakis 在这篇文章中对它们做了很好的解释。

  • 尝试创建一个新的 sleeper 结构，使其随机休眠一段时间，并将其添加到 Vec 中。