feat: add EventStream::select to combine multiple event streams

[mattsse]

Motivation

Combine multiple EventStream with different items

Solution

PR Checklist

• Added Tests
• Added Documentation
• Updated the changelog

[gakonst]

Do we want the filter_map here? Won't this silently eat all errors if any are observed?

[mattsse]

yes this skips all erroneous values, but is probably useful if you're only interested in the ok values.

[gakonst]

Wonder if there's a world where we do this with a macro instead of a function (like the futures select!), so that we are not limited to only selecting 2 events?

[mattsse]

driving multiple streams to completion with select! is a bit verbose and the core problem is that we need to return a type that represents all the different events.
you can do this with EventStream::select

let ev3 = contract.event::<OtherEvent>();

let stream = ev3.stream().await().unwrap().select(
   ev1.stream().await.unwrap().select(ev2.stream().await.unwrap())
);

which will return an Either<OtherEvent, Either<Approval, Transfer>>

But there'd be also the possibility to create your own event type and use the Contract::event_with_filter function

 #[derive(Debug, Clone, PartialEq, Eq)]
    pub enum Events {
        Approval(Approval),
        Transfer(Transfer),
        Other(Other),
    }
  impl EthLogDecode for Events {...}

[gakonst]

Ah, right! OK if that pattern works then should be fine.