Summary - async/await
- Rust course
- Advanced
- Lesson 6/81
- 5 min
Combine async fn, await and block to await two tasks in sequence.
async/await summary
async fn and async { } produce lazy futures; .await waits for the result and is a suspension point. The Future trait consists of a single poll method: Ready(v) or Pending. A future returning Pending wakes the executor via the Waker when it's ready. Pin guarantees that self-referential state machines don't move. The standard library has no executor; real projects use tokio.
await in a loop
Async functions use normal control flow; loops, conditions and ? work the same way:
async fn adim(n: u32) -> u32 {
n * n
}
async fn hepsi() -> Vec<u32> {
let mut v = Vec::new();
for i in 1..=3 {
v.push(adim(i).await);
}
v
}Practice Wait in a loop
Write the async functions adim and hepsi above. Print the result of block_on(hepsi()) with {:?}.
Starter code
use std::future::Future;
use std::pin::pin;
use std::task::{Context, Poll, Waker};
fn block_on<F: Future>(f: F) -> F::Output {
let mut f = pin!(f);
let mut cx = Context::from_waker(Waker::noop());
loop {
if let Poll::Ready(v) = f.as_mut().poll(&mut cx) {
return v;
}
}
}
fn main() {
// buraya yaz
}Sample solution
use std::future::Future;
use std::pin::pin;
use std::task::{Context, Poll, Waker};
fn block_on<F: Future>(f: F) -> F::Output {
let mut f = pin!(f);
let mut cx = Context::from_waker(Waker::noop());
loop {
if let Poll::Ready(v) = f.as_mut().poll(&mut cx) {
return v;
}
}
}
async fn adim(n: u32) -> u32 {
n * n
}
async fn hepsi() -> Vec<u32> {
let mut v = Vec::new();
for i in 1..=3 {
v.push(adim(i).await);
}
v
}
fn main() {
println!("{:?}", block_on(hepsi()));
}Expected output
[1, 4, 9]
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 When is async code a better fit than threads?
- When there are many concurrent jobs mostly waiting on I/O Correct answer
- For heavy computation
- Always
- Never
Explanation: Tens of thousands of connections can be handled with few threads; for CPU-heavy work, threads or rayon fit better.
2 Ready result
if let Poll::____(v) = f.as_mut().poll(&mut cx) {
return v;
}if let Poll::Ready(v) = f.as_mut().poll(&mut cx) {
return v;
}Explanation: Poll::Ready carries the result.
3 Group summary
| Term | Meaning |
|---|---|
async fn |
Lazy future |
.await |
Suspension point |
Future::poll |
One step |
Pin |
Immovable |