what should I pay attention to when implmenting poll?

[shaitao]

It's confusing to implement poll, when and where to use NotReady

[carllerche]

Thanks for the feedback.

Do you have specific examples where you got tripped up? What were you trying to implement? What did your first attempt look like?

[jonhoo]

I completely agree that a write-up of "how you likely want to implement and think about poll" is likely a good addition.

[carllerche]

@jonhoo What would you say should go in that section?

[shaitao]

Thanks for the feedback.

Do you have specific examples where you got tripped up? What were you trying to implement? What did your first attempt look like? It looks like that if the poll of a source future block and all the futures will block. if I implement

fn poll(&self)->Poll<T,E>{
let x = loading()? //this is not a future and would block
Ok(Async::Ready(x))  //how to return NotReady
}

[jonhoo]

@carllerche

• Keep doing work as long as work is available.
• Make sure to check all "internal" futures, not just the first one that is ready. You can't give up the moment one of them returns NotReady.
• You probably want to wrap the contents of your poll in a loop to ensure the first point.
• try_ready! is a great tool, but keep in mind that it will prevent you from polling a second internal future if the first you check returns NotReady, which is not okay!
• Maybe mention UnorderedFutures and UnorderedStream, show what they replace (Vec<Future> and Vec<Stream> respectively) and why the naive implementation isn't great.