使用tokio_timer重复执行Rust任务

Question

我正在使用Tokio框架在Rust中创建重复任务。下面的代码基于完成的更改请求，以将该功能添加到Tokio-Timer板条箱中。

尝试编译时，出现错误消息：

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}, _>`

该错误指出my_cron_func函数的返回签名不正确。我需要更改/添加什么才能使签名正确以进行编译？

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......

类型不匹配

您提供了错误的类型

类型fn() {my_cron_func}实现了特征std::ops::FnMut<()>

当使用 时my_cron_func，这是一个不带参数的函数

但特质std::ops::FnMut<((),)>是必需的

但是需要一个采用单个参数（空元组）的函数。

（预期元组，找到 ()）

编译器会尝试缩小问题范围。

如果您查看正在使用的库的文档，特别是tokio_timer::Interval，您可以看到它是futures::Stream使用关联的类型实现的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 的文档futures::Stream，我们可以看到传递给 的闭包for_each需要返回一个可以转换为将产生的 future 的值()：

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

您的函数尝试返回某些内容，但没有返回类型并且您使用了 a;来结束该函数：

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

futures::future::ok诀窍是：

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