有没有一种方法可以将IObservable<Task<T>>解包成IObservable<T>，同时保持相同的事件顺序，像这样？
Tasks: ----a-------b--c----------d------e---f---->
Values: -------A-----------B--C------D-----E---F-->
假设我有一个桌面应用程序，它消费消息流，其中一些消息需要进行重度后处理：
IObservable<Message> streamOfMessages = ...;
IObservable<Task<Result>> streamOfTasks = streamOfMessages
.Select(async msg => await PostprocessAsync(msg));
IObservable<Result> streamOfResults = ???; // unwrap streamOfTasks
我可以想象两种处理方式。
首先，我可以使用异步事件处理程序订阅
streamOfTasks:streamOfTasks.Subscribe(async task =>
{
var result = await task;
Display(result);
});
其次，我可以使用 Observable.Create 将 streamOfTasks 转换成如下形式:
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，消息的顺序都不会被保留：一些不需要任何后处理的较晚的消息比需要后处理的早期消息更快地出现。我的两个解决方案都可以并行处理传入的消息，但我希望它们按顺序逐个处理。
我可以编写一个简单的任务队列，一次只处理一个任务，但这可能有点过度杀伤力。对我来说，似乎我错过了一些显而易见的东西。
更新。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，所有解决方案都没有保留原始事件的顺序。以下是程序的输出：
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
以下是完整的源代码：
// "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;
}
}