Reactive.Subject make onNext wait for previous onNext action

Question

I was trying to implement observable waiting for onNext action to complete before proceeding with next operation. Only way I found working is using SemaphoreSlim. Does Reactive have any way to do that without use of SemaphoreSlim? I couldn't find any.

 SemaphoreSlim _semaphoreSlim = new SemaphoreSlim(1,1);
  subject.Window(() => subject.Throttle(TimeSpan.FromMilliseconds(500)))
            .SelectMany(c => c.ToList())
            .Subscribe(async x =>
                {
                    await _semaphoreSlim.WaitAsync();
                    try
                    {
                        //await Async code here
                    }
                    finally
                    {
                        _semaphoreSlim.Release();
                    }
                });

Answer 1

To start with I created a sample Media_Load_Async that simulates what you're doing.

static int max = 5;
static Random random = new Random();
static TimeSpan[] delays = Enumerable.Range(0, max).Select(x => TimeSpan.FromSeconds(random.Next(5) + 2)).ToArray();

async Task<Unit> Media_Load_Async(int index, TimeSpan delay)
{
    Console.WriteLine($"Start {index} - {delay}");
    await Task.Delay(delay);
    Console.WriteLine($"End {index}");
    return Unit.Default;
}

If I run this simple query then I get the results that you're trying to avoid:

Observable
    .Range(0, max)
    .Subscribe(async x => await Media_Load_Async(x, delays[x]));

Start 0 - 00:00:05
Start 1 - 00:00:06
Start 2 - 00:00:02
Start 3 - 00:00:06
Start 4 - 00:00:04
End 2
End 4
End 0
End 3
End 1

If I put in your SemaporeSlim code I get what I think you want:

SemaphoreSlim _semaphoreSlim = new SemaphoreSlim(1, 1);
Observable
    .Range(0, max)
    .Subscribe(async x =>
    {
        await _semaphoreSlim.WaitAsync();
        try
        {
            await Media_Load_Async(x, delays[x]);
        }
        finally
        {
            _semaphoreSlim.Release();
        }
    });

Start 0 - 00:00:04
End 0
Start 1 - 00:00:03
End 1
Start 2 - 00:00:06
End 2
Start 3 - 00:00:04
End 3
Start 4 - 00:00:03
End 4

Next, if I move the call to Media_Load_Async to inside the query itself then I'm back to the original issue:

Observable
    .Range(0, max)
    .SelectMany(x => Observable.FromAsync(() => Media_Load_Async(x, delays[x])))
    .Subscribe();

Start 0 - 00:00:06
Start 1 - 00:00:02
Start 2 - 00:00:06
Start 3 - 00:00:05
Start 4 - 00:00:02
End 4
End 1
End 3
End 0
End 2

But if I change out the SelectMany for a Select/Concat pair then I get what you want without a SemaphoreSlim:

Observable
    .Range(0, max)
    .Select(x => Observable.FromAsync(() => Media_Load_Async(x, delays[x])))
    .Concat()
    .Subscribe();

Start 0 - 00:00:04
End 0
Start 1 - 00:00:05
End 1
Start 2 - 00:00:02
End 2
Start 3 - 00:00:06
End 3
Start 4 - 00:00:06
End 4