如何一次执行多个异步功能并获得结果？

问题描述

我已经尝试了Tokio任务，但是没有可以同时执行多个任务的示例。此代码有什么问题？

fn main() {
    block_on(speak());
}

async fn speak() {
    let hold = vec![say(),greet()];
    let results = join_all(hold).await;
}

async fn say() {
    println!("hello");
}

async fn greet() {
    println!("world");
}

这是编译器的输出

error[E0308]: mismatched types
  --> sync\src\main.rs:14:27
   |
14 |     let hold = vec![say(),greet()];
   |                           ^^^^^^^ expected opaque type,found a different opaque type
...
23 | async fn greet(){
   |                 - the `Output` of this `async fn`'s found opaque type
   |
   = note:     expected type `impl core::future::future::Future` (opaque type at <sync\src\main.rs:19:15>)
           found opaque type `impl core::future::future::Future` (opaque type at <sync\src\main.rs:23:17>)
   = note: distinct uses of `impl Trait` result in different opaque types

解决方法

对于两个期货，如您所愿，请使用future::join

use futures::{executor,future}; // 0.3.5

async fn speak() {
    let (_s,_g) = future::join(say(),greet()).await;
}

有三种，四种和五种输入期货的变体：join3，join4，join5。

当您的未来返回Result时，还有try_join（和try_join3，try_join4，try_join5）。

join宏是处理静态数量的期货加入的另一种方式。

如果需要支持动态数量的期货，则可以使用future::join_all（或try_join_all），但是必须具有所有类型的向量。通过FutureExt::boxed（或FutureExt::boxed_local）最简单：

use futures::{executor,future,FutureExt}; // 0.3.5

async fn speak() {
    let futures = vec![say().boxed(),greet().boxed()];
    let _results = future::join_all(futures).await;
}

请注意，此代码可以同时运行期货，但不能并行运行。对于并行执行，您需要引入一些任务。

另请参阅：

• How can I join all the futures in a vector without cancelling on failure like join_all does?
• Join futures with limited concurrency
• How can I perform parallel asynchronous HTTP GET requests with reqwest?
• How do I create a heterogeneous collection of objects?
• What is the purpose of async/await in Rust?
• What is the difference between concurrency and parallelism?