rust - 如何一次执行多个异步函数并得到结果？

Question

我尝试过 Tokio 任务，但没有可以同时执行多个任务的工作示例。这段代码有什么问题？

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

Answer 1

对于两个 future ，就像你一样，使用 future::join

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

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

有三、四和五输入 future 的变体: join3 , join4 , join5 .
还有 try_join (和 try_join3 、 try_join4 、 try_join5 )用于当您的 future 返回 Result 时.
宏 join 是另一种处理静态数量的 future 加入的方法。
如果需要支持动态数量 future ，可以使用 future::join_all (或 try_join_all )，但您必须拥有所有一种向量。这是最简单的方法 FutureExt::boxed (或 FutureExt::boxed_local ):

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 future ，但不会并行运行它们。对于并行执行，您需要引入某种任务。
也可以看看:

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?