有没有办法像这样展开IObservable<Task<T>>保持IObservable<T>相同的事件顺序？
Tasks: ----a-------b--c----------d------e---f---->
Values: -------A-----------B--C------D-----E---F-->
假设我有一个使用消息流的桌面应用程序，其中一些需要大量的后处理：
IObservable<Message> streamOfMessages = ...;
IObservable<Task<Result>> streamOfTasks = streamOfMessages
.Select(async msg => await PostprocessAsync(msg));
IObservable<Result> streamOfResults = ???; // unwrap streamOfTasks
我想有两种处理方法。
首先，我可以streamOfTasks使用异步事件处理程序订阅：
streamOfTasks.Subscribe(async task =>
{
var result = await task;
Display(result);
});
其次，我可以转换streamOfTasksusing Observable.Create，如下所示：
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));
无论哪种方式，都不会保留消息的顺序：一些不需要任何后期处理的后期消息比需要后期处理的早期消息出来得更快。我的两个解决方案都并行处理传入的消息，但我希望它们按顺序处理，一个接一个。
我可以编写一个简单的任务队列来一次只处理一个任务，但这可能是一种矫枉过正。在我看来，我错过了一些明显的东西。
UPD。我编写了一个示例控制台程序来演示我的方法。到目前为止，所有解决方案都不保留事件的原始顺序。这是程序的输出：
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
这是完整的源代码：
// "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;
}
}