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CSharp · Console

/Program.cs

using System;
using System.Threading.Tasks;
using System.Reactive.Linq;

public static class Program
{
    public static void Main()
    {
        Observable
            .Interval(TimeSpan.FromMilliseconds(200))
            .Select(x => (int)x + 1)
            .Take(10)
            .Do(x => Console.WriteLine($"Produced #{x}"))
            .ExhaustMap(async x => { await Task.Delay(x % 3 == 0 ? 500 : 100); return x; })
            .Do(x => Console.WriteLine($"--Result: {x}"))
            .Wait();
        Console.WriteLine($"Finished");
    }

    /// <summary>Invokes an asynchronous function for each element of an observable
    /// sequence, ignoring elements that are emitted before the completion of an
    /// asynchronous function of a preceding element.</summary>
    public static IObservable<TResult> ExhaustMap<TSource, TResult>(
        this IObservable<TSource> source,
        Func<TSource, Task<TResult>> function)
    {
        return source
            .Scan((Task: Task.FromResult<TResult>(default), Id: 0), (previous, item) =>
                !previous.Task.IsCompleted ? previous : (function(item), unchecked(previous.Id + 1)))
            .DistinctUntilChanged()
            .Select(e => e.Task)
            .Concat();
    }
}