使用 tokio_timer 重复 Rust 任务

Question

我正在使用 Tokio 框架在 Rust 中创建重复任务。以下代码基于 completed change request 将此功能添加到 tokio-timer crate。

尝试编译时，我收到错误消息：

error[E0281]: type mismatch: the type `fn() {my_cron_func}` implements the trait `std::ops::FnMut<()>`, but the trait `std::ops::FnMut<((),)>` is required (expected tuple, found ())
  --> src/main.rs:19:36
   |
19 |     let background_tasks = wakeups.for_each(my_cron_func);
   |                                    ^^^^^^^^

error[E0281]: type mismatch: the type `fn() {my_cron_func}` implements the trait `std::ops::FnOnce<()>`, but the trait `std::ops::FnOnce<((),)>` is required (expected tuple, found ())
  --> src/main.rs:19:36
   |
19 |     let background_tasks = wakeups.for_each(my_cron_func);
   |                                    ^^^^^^^^

error[E0281]: type mismatch: the type `fn() {my_cron_func}` implements the trait `std::ops::FnMut<()>`, but the trait `std::ops::FnMut<((),)>` is required (expected tuple, found ())
  --> src/main.rs:20:10
   |
20 |     core.run(background_tasks).unwrap();
   |          ^^^
   |
   = note: required because of the requirements on the impl of `futures::Future` for `futures::stream::ForEach<tokio_timer::Interval, fn() {my_cron_func}, _>`

error[E0281]: type mismatch: the type `fn() {my_cron_func}` implements the trait `std::ops::FnOnce<()>`, but the trait `std::ops::FnOnce<((),)>` is required (expected tuple, found ())
  --> src/main.rs:20:10
   |
20 |     core.run(background_tasks).unwrap();
   |          ^^^
   |
   = note: required because of the requirements on the impl of `futures::Future` for `futures::stream::ForEach<tokio_timer::Interval, fn() {my_cron_func}, _>`

错误指出 my_cron_func 函数的 return 签名不正确。我需要什么 change/add 才能使签名正确才能编译？

extern crate futures;
extern crate tokio_core;
extern crate tokio_timer;

use std::time::*;
use futures::*;
use tokio_core::reactor::Core;
use tokio_timer::*;

pub fn main() {

    println!("The start");
    let mut core = Core::new().unwrap();
    let timer = Timer::default();
    let duration = Duration::new(2, 0); // 2 seconds
    let wakeups = timer.interval(duration);

    // issues here
    let background_tasks = wakeups.for_each(my_cron_func);
    core.run(background_tasks).unwrap();

    println!("The end???");

}

fn my_cron_func() {
    println!("Repeating");
    Ok(());
}

Answer 1

我不确定错误消息的哪一部分给您带来了麻烦，但是...

type mismatch

您提供的类型有误

the type fn() {my_cron_func} implements the trait std::ops::FnMut<()>

当使用 my_cron_func 时，这是一个不带参数的函数

but the trait std::ops::FnMut<((),)> is required

但是需要一个接受单个参数的函数，即空元组。

(expected tuple, found ())

并且编译器试图缩小问题范围。

如果查看您正在使用的库的文档，特别是 tokio_timer::Interval，您可以看到它实现了 futures::Stream 和关联类型 Item = ()。

这会更改错误消息：

error[E0277]: the trait bound `(): futures::Future` is not satisfied
  --> src/main.rs:19:36
   |
19 |     let background_tasks = wakeups.for_each(my_cron_func);
   |                                    ^^^^^^^^ the trait `futures::Future` is not implemented for `()`
   |
   = note: required because of the requirements on the impl of `futures::IntoFuture` for `()`

error[E0277]: the trait bound `(): futures::Future` is not satisfied
  --> src/main.rs:20:10
   |
20 |     core.run(background_tasks).unwrap();
   |          ^^^ the trait `futures::Future` is not implemented for `()`
   |
   = note: required because of the requirements on the impl of `futures::IntoFuture` for `()`
   = note: required because of the requirements on the impl of `futures::Future` for `futures::stream::ForEach<tokio_timer::Interval, fn(()) {my_cron_func}, ()>`

查看 documentation for futures::Stream，我们可以看到传递给 for_each 的闭包需要 return 一个可以转化为未来的值，该值将产生 () :

fn for_each<F, U>(self, f: F) -> ForEach<Self, F, U> 
    where F: FnMut(Self::Item) -> U,
          U: IntoFuture<Item=(), Error=Self::Error>,
          Self: Sized

您的函数尝试 return 某些东西，除了没有 return 类型并且您使用了 ; 来结束函数：

fn my_cron_func(a: ()) {
    println!("Repeating");
    Ok(());
}

futures::future::ok 成功了：

fn my_cron_func(_: ()) -> futures::future::FutureResult<(), tokio_timer::TimerError> {
    println!("Repeating");
    futures::future::ok(())
}

使用 tokio_timer 重复 Rust 任务

Repeating a Rust task with tokio_timer

rust

rust-tokio