Implement Future

Contract

Future is a regular trait which has a method poll
poll should return Poll::Ready(...) if you think the Future complets.
poll should return Poll::Pending if you think the Future is in progress
poll is initially invoked underneath by the async runtime.
You need to tell the async runtime to invoke poll again if your Future is in progress. Otherwise, it never complete.

We will use tokio as the async runtime.

Poll::Ready

The program completes immediately.

Poll::Pending

The program never stop.

How do we tell the async runtime invoke poll again?

Use Waker

busy polling

The program keeps polling.

cx.waker().wake_by_ref();

Waker is implemented by tokio

final complete

Finally, the program needs return Poll::Ready.

interval polling

The program keeps interval polling using a thread.

Use OS mechanism

Thread spawning is not costless. We could use OS timer somehow, but it’s a bit of complex to setup OS layer. Luckily, tokio Sleep has already been implemented for us.

this is unsafe code

We declare a heap-allocated pointer first, then convert that to a Pin pointer.
this is safe code

We declare a Pin pointer directly using Box::pin which is type Pin<Box<T>>. Pin<Box<T>> could be converted to Pin<&mut T> using its Box::as_mut method.

For how to use Pin, see pin

Explorer

Table of Contents