使用tokio_timer重復Rust任務

Question

我正在使用Tokio框架在Rust中創建一個重復任務。 以下代碼基於已完成的更改請求，以將此功能添加到tokio-timer包。

在嘗試編譯時，我收到錯誤消息：

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}, _>`

該錯誤指出my_cron_func函數的返回簽名不正確。 我需要更改/添加什么才能使簽名正確以便編譯？

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

我不確定錯誤信息的哪一部分會給您帶來麻煩，但......

類型不匹配

你提供了錯誤的類型

類型fn() {my_cron_func}實現了特征std::ops::FnMut<()>

使用my_cron_func ，這是一個不帶參數的函數

但特征std::ops::FnMut<((),)>是必需的

但是需要一個帶有單個參數的函數，即空元組。

（預期的元組，發現（））

編譯器試圖縮小問題的范圍。

如果您查看您正在使用的庫的文檔，特別是tokio_timer::Interval ，您可以看到它實現了具有關聯類型Item = () futures::Stream 。

這會更改錯誤消息：

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future futures::Stream的文檔 ，我們可以看到傳遞給for_each的閉包需要返回一個值，該值可以轉換為yield ()的未來：

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

你的函數試圖返回一些東西，除了沒有返回類型你用過了; 結束功能：

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

futures::future::ok訣竅：

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