[WIP] Asynchronous Rx implementation

[bartdesmet]

This pull request contains an initial implementation of "Async Rx" with the following essential interfaces:

• IAsyncObservable<T>, the dual to IAsyncEnumerable<T> and the async variant of IObservable<T>
• IAsyncObserver<T>, the dual to IAsyncEnumerator<T> and the async variant of IObserver<T>
• IAsyncSubject<T>, the async variant of ISubject<T>
• IAsyncDisposable, the async variant of IDisposable
• IAsyncScheduler, the async variant of IScheduler

Note that this PR is work in progress. While it has most operators implemented, there's some outstanding design work and a ton of testing to be done. The main goals of this initial PR are:

• research the duality with the latest "async streams" proposal in C# 8.0;
• get a feel for the APIs and the overload bloat for async variants;
• noodle around with different implementation techniques, e.g. observer operators;
• give the resulting prototype a spin in practice.

Interface variants would be possible with regards to supporting CancellationToken parameters, but it brings similar pains as the ones observed (pun intended) by the LDM for the C# 8.0 design of async streams. As such, this initial implementation uses a "cancellation agnostic" approach, though further study is warranted.

In the context of Rx, the use of CancellationToken in various places clearly shows the pain these induce on top of the algebra of sequences, e.g. any cancellation of an On*Async call leaves observers in indeterminate states, so termination of the subscription is warranted, which indicates that the IAsyncDisposable handle to the subscription and any CancellationTokens for event flow aren't orthogonal. Similar concerns occur when attempt to cancel the creation of a subscription.

While more fine-grained control over cancellation can be useful for service abstractions of Rx (as done in the "Reactor" project in Bing) that cross I/O boundaries, such distributed abstractions could be regarded as being layered on top (i.e. we don't need a "one size fits all" interface necessarily). In fact, other orthogonal requirements are imposed in the distributed variants, such as the ability to pass in explicit identifiers for resources. More on that another time.

[BigBabay]

Hi, great job, want to use it in our project. Can you answer, how long time acceptance of PR can take?

[pekkah]

Any news on this?

[bartdesmet]

Merging this initial prototype as-is, with future development planned after IAsyncEnumerable<T> is set in stone and the interface duality can be re-evaluated.

[ramonsmits]

@bartdesmet Is the develop branch part of a nightly? It seems https://github.com/Reactive-Extensions/Rx.NET/tree/develop/AsyncRx.NET will be a new package?

[petroemil]

@bartdesmet would you be able to provide any news on the development of these set of features and/or potentially the progress of opening up Reactor to the public?

[17-09]

Any news about Asynchronous Rx implementation in 2019?

[pihai]

Any news about Asynchronous Rx implementation in 2020?

[casperOne]

2022 perhaps?