Is there a way to unwrap the IObservable<Task<T>> into IObservable<T> keeping the same order of events, like this?
Tasks: ----a-------b--c----------d------e---f---->
Values: -------A-----------B--C------D-----E---F-->
Let's say I have a desktop application that consumes a stream of messages, some of which require heavy post-processing:
IObservable<Message> streamOfMessages = ...;
IObservable<Task<Result>> streamOfTasks = streamOfMessages
.Select(async msg => await PostprocessAsync(msg));
IObservable<Result> streamOfResults = ???; // unwrap streamOfTasks
I imagine two ways of dealing with that.
First, I can subscribe to streamOfTasks using the asynchronous event handler:
streamOfTasks.Subscribe(async task =>
{
var result = await task;
Display(result);
});
Second, I can convert streamOfTasks using Observable.Create, like this:
var streamOfResults =
from task in streamOfTasks
from value in Observable.Create<T>(async (obs, cancel) =>
{
var v = await task;
obs.OnNext(v);
// TODO: don't know when to call obs.OnComplete()
})
select value;
streamOfResults.Subscribe(result => Display(result));
Either way, the order of messages is not preserved: some later messages that don't need any post-processing come out faster than earlier messages that require post-processing. Both my solutions handle the incoming messages in parallel, but I'd like them to be processed sequentially, one by one.
I can write a simple task queue to process just one task at a time, but perhaps it's an overkill. Seems to me that I'm missing something obvious.
UPD. I wrote a sample console program to demonstrate my approaches. All solutions by far don't preserve the original order of events. Here is the output of the program:
Timer: 0
Timer: 1
Async handler: 1
Observable.Create: 1
Observable.FromAsync: 1
Timer: 2
Async handler: 2
Observable.Create: 2
Observable.FromAsync: 2
Observable.Create: 0
Async handler: 0
Observable.FromAsync: 0
Here is the complete source code:
// "C:\Program Files (x86)\MSBuild\14.0\Bin\csc.exe" test.cs /r:System.Reactive.Core.dll /r:System.Reactive.Linq.dll /r:System.Reactive.Interfaces.dll
using System;
using System.Reactive;
using System.Reactive.Concurrency;
using System.Reactive.Linq;
using System.Threading.Tasks;
class Program
{
static void Main()
{
Console.WriteLine("Press ENTER to exit.");
// the source stream
var timerEvents = Observable.Timer(TimeSpan.Zero, TimeSpan.FromSeconds(1));
timerEvents.Subscribe(x => Console.WriteLine($"Timer: {x}"));
// solution #1: using async event handler
timerEvents.Subscribe(async x =>
{
var result = await PostprocessAsync(x);
Console.WriteLine($"Async handler: {x}");
});
// solution #2: using Observable.Create
var processedEventsV2 =
from task in timerEvents.Select(async x => await PostprocessAsync(x))
from value in Observable.Create<long>(async (obs, cancel) =>
{
var v = await task;
obs.OnNext(v);
})
select value;
processedEventsV2.Subscribe(x => Console.WriteLine($"Observable.Create: {x}"));
// solution #3: using FromAsync, as answered by @Enigmativity
var processedEventsV3 =
from msg in timerEvents
from result in Observable.FromAsync(() => PostprocessAsync(msg))
select result;
processedEventsV3.Subscribe(x => Console.WriteLine($"Observable.FromAsync: {x}"));
Console.ReadLine();
}
static async Task<long> PostprocessAsync(long x)
{
// some messages require long post-processing
if (x % 3 == 0)
{
await Task.Delay(TimeSpan.FromSeconds(2.5));
}
// and some don't
return x;
}
}
