What is Rust future

A future is a representation of some operation which will complete in the future.

A future contains three state. The Poll, Wait, Wake states.

• Poll: A future is taking progress until a point which it can't make more progress. Or just be done.
• Wait: The future is waitng for a event, which will tell it that it is time to make more progress.
• Wake: Some event happens and the future is woken up by some function. Then this future will try to do progess until it is done or it met another point that it can't do more.

What is leaf futures

A resource, provided by runtime, that is non-blocking.

Non leaf futures

Treat it as a task. It is pause-able computation. It is for user which contains a set of await operation.

They are not performing a I/O process. They just run and get the leaf-future.

rust async await primer

https://rust-lang.github.io/async-book/01_getting_started/04_async_await_primer.html

async transforms a block of code into a state machine that implements a trait called Future. Blocked Futures will yield control of the thread, allowing other Futures to run.

The interface, or the outer async, or the first async function, should be run by executor.

use futures::executor::block_on; // from futures crate

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

fn main() {
  let future = hello();
  block_on(future);
}

And await is used inside the async block.

use futures::executor::block_on; // from futures crate

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

async fn hello() {
  println!("hello");
  world().await // <- await can only be called inside async block
}

fn main() {
  let future = hello();
  block_on(future);
}

.await doesn't block the current thread, but instead asynchronously waits for the future to complete, allowing other tasks to run if the future is currently unable to make progress.