using System;

using System.Threading.Tasks;

using System.Reactive;

using System.Reactive.Linq;

using System.Reactive.Concurrency;

​

public class Program

{

    public static IObservable<TResult> Generate<TResult>(

        Func<Task<TResult>> initialState,

        Func<TResult, bool> condition,

        Func<TResult, Task<TResult>> iterate,

        Func<TResult, TResult> resultSelector,

        IScheduler scheduler = null)

    {

        scheduler ??= Scheduler.Immediate;

        return Observable.Create<TResult>(observer =>

        {

            return scheduler.ScheduleAsync(async (self, cancellationToken) =>

            {

                var state = await initialState();

                while (condition(state))

                {

                    await self.Yield();

                    observer.OnNext(resultSelector(state));

                    state = await iterate(state);

                }

                observer.OnCompleted();

            });

        });

    }

}

​