Reactive Extensions Subscribe calling await

Question

I want to perform an async call based for each event raised by a Reactive Extensions Observable. I'm also trying to keep everything synchronized as I want the async call to finish before the next event is handled.

How would one go about doing something similar to the following? I say similar as the code below does not compile.

settingsChangedInMemory
    .Subscribe(async _ => {
        var settings = Extract();
        await SaveSettings(settings);
    });

I'm not sure if it changes anything, but I would need to Subscribe to multiple Observables. For example another subscription like this.

settingsChangedOnDisk
    .Subscribe(async _ => {
        var settings = await ReadSettings(settings);
        Apply(settings);
    });

How would you use Reactive Extensions to do this?

score 21 · Accepted Answer · answered Jul 19 '14 at 21:01

21

How about:

settingsChangedInMemory
    .SelectMany(async _ => await SaveSettings(Extract()))
    .Subscribe(x => Apply(x));

Never put an async in a Subscribe, you always want to put it in a SelectMany instead.

answered Jul 19 '14 at 21:01

Ana Betts

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Is this still true as of RX 2.0 (which predates your answer)? The above code now seems to compile and there's an example from the RX team here http://blogs.msdn.com/b/rxteam/archive/2012/03/12/reactive-extensions-v2-0-beta-available-now.aspx of a Subscribe(async () => ...) – Polemarch Sep 14 '15 at 17:11
2

Absolutely still true – Ana Betts Sep 14 '15 at 20:30
9

Do you mind elaborating Paul? They explicitly state they support async in subscribe, and your response doesn't really explain why the developers of Reactive are wrong. – Austin Salgat Jun 23 '16 at 19:28
`async void` methods can be called anywhere `void` methods can be called. Therefore `Subscribe` supports async methods because the methods are void. You can't await a Disposable/Subscription anyways. If you need something like that, use `SelectMany`, etc, first, and then subscribe. – cwharris Dec 12 '17 at 17:10
2

Can the `async` safely go in a `Select`, as well as in a `SelectMany`? – Stuart Hallows Jan 31 '18 at 06:05
1

@StuartHallows no it can't, select doesn't have an overload that accepts a Task – Rob May 18 '18 at 10:00

Muhammad Rehan Saeed · Answer 2 · 2017-12-13T08:56:15.243

2

You can use the new ForEachAsync method released in Reactive Extensions (Rx) 2.0 like so:

await observable
    .ForEachAsync(async x =>
    {
        Console.WriteLine(x);
        await Task.Delay(1000);
    });

ForEachAsync returns a Task<T> which completes when the observable completes. More information in my blog post here or this blog post by the reactive extensions team.

edited Dec 13 '17 at 08:56

answered Dec 12 '17 at 15:32

Muhammad Rehan Saeed

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Reactive Extensions Subscribe calling await

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