c# - 将 IObservable<Task<T>> 解包为 IObservable<T> 并保留顺序

Question

有没有办法解开IObservable<Task<T>>进入IObservable<T>保持相同的事件顺序，就像这样？

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，其中一些消息需要大量的后期处理:

IObservable<Message> streamOfMessages = ...;

IObservable<Task<Result>> streamOfTasks = streamOfMessages
    .Select(async msg => await PostprocessAsync(msg));

IObservable<Result> streamOfResults = ???; // unwrap streamOfTasks

我设想了两种处理方式。

首先，我可以订阅streamOfTasks使用异步事件处理程序:

streamOfTasks.Subscribe(async task =>
{
    var result = await task;
    Display(result);
});

其次，我可以转换streamOfTasks使用 Observable.Create ，像这样:

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，消息的顺序都不会保留:一些较晚的消息 不需要任何后期处理比早期的消息更快 需要后期处理。我的两个解决方案都处理传入的消息 并行处理，但我希望它们按顺序、一个接一个地处理。

我可以编写一个简单的任务队列来一次只处理一个任务， 但也许这是一个矫枉过正。在我看来，我遗漏了一些明显的东西。

---

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，所有解决方案都不会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

完整的源代码如下:

// "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;
    }
}

Answer 1

结合@Enigmativity 的 simple approach与@VMAtm 的想法 attaching the counter和来自 this SO question 的一些代码片段，我想出了这个解决方案:

// usage
var processedStream = timerEvents.SelectAsync(async t => await PostprocessAsync(t));

processedStream.Subscribe(x => Console.WriteLine($"Processed: {x}"));

// my sample console program prints the events ordered properly:
Timer: 0
Timer: 1
Timer: 2
Processed: 0
Processed: 1
Processed: 2
Timer: 3
Timer: 4
Timer: 5
Processed: 3
Processed: 4
Processed: 5
....

这是我的 SelectAsync转换扩展方法IObservable<Task<TSource>>进入IObservable<TResult>保持事件的原始顺序:

public static IObservable<TResult> SelectAsync<TSource, TResult>(
    this IObservable<TSource> src,
    Func<TSource, Task<TResult>> selectorAsync)
{
    // using local variable for counter is easier than src.Scan(...)
    var counter = 0;
    var streamOfTasks =
        from source in src
        from result in Observable.FromAsync(async () => new
        {
            Index = Interlocked.Increment(ref counter) - 1,
            Result = await selectorAsync(source)
        })
        select result;

    // buffer the results coming out of order
    return Observable.Create<TResult>(observer =>
    {
        var index = 0;
        var buffer = new Dictionary<int, TResult>();

        return streamOfTasks.Subscribe(item =>
        {
            buffer.Add(item.Index, item.Result);

            TResult result;
            while (buffer.TryGetValue(index, out result))
            {
                buffer.Remove(index);
                observer.OnNext(result);
                index++;
            }
        });
    });
}

我对我的解决方案不是特别满意，因为它对我来说太复杂了，但至少它不需要任何外部依赖项。我在这里使用一个简单的字典来缓冲和重新排序任务结果，因为订阅者 need not to be thread-safe (订阅不需要同时调用)。

欢迎提出任何意见或建议。我仍然希望找到无需自定义缓冲扩展方法的原生 RX 方式。