how to WaitAll in Rx - asynchronous

Question

I'm Learning Reactive Extentions, these days I come across this situation, codes here:
class Program
{
private static void Main(string[] args)
{
var ls = new List<int> { 1, 2, 3, 4, 5, 6, 7, 8, 9 }.ToObservable();
ls.Select(m => new
{
t = Observable.Start(() =>
{
Thread.Sleep(100);
return new Random().Next(3, 20);
}),
i = m
}).Subscribe(item => item.t.Subscribe(Console.WriteLine));
Task.WaitAll();
Console.WriteLine("all done");
Console.ReadKey();
}
}
It shows that there is an IObservable in an Observable, and I want to print "all done" after all process done, but this didn't work. "all done" print very quickly as soon as the program start, not wait anymore, in my situation here, what should I do to get a REAL WaitAll?

Answer 1

That is not really how Rx works. There is no link here between Task.WaitAll() and your Rx code. You dont even pass any tasks to the WaitAll() method ;-)
So firstly, the Subscribe method is non-blocking. It just states that at this point I want to start consuming this sequence, and this is what to do when value/error/completion notifications are send to me.
Your nested Observable sequence is a fairly advanced topic to jump straight into, but that is ok we can work with that.
class Program
{
private static void Main(string[] args)
{
//Let go, we are not IEnumerable any more :-)
var ls = new List<int> { 1, 2, 3, 4, 5, 6, 7, 8, 9 }.ToObservable();
var subscription = ls.Select(_ =>
Observable.Start(() =>
{
Thread.Sleep(100);
return new Random().Next(3, 20);
})
})
.Merge() //Merge the IO<IO<T>> into Io<T> so we get a single completion.
.Subscribe(
item => item.Subscribe(Console.WriteLine),
()=>Console.WriteLine("all done"));
Console.ReadKey();
subscription.Dispose();
}
}
You could further improve the code by replacing the Observable.Start + Thread.Sleep with an Rx method likeObservable.Timer or a Scheduler.
The key thing to take away here is that Rx is Async. The point is NOT to block. The only things in this code that are blocking are the Thread.Sleep and the Console.ReadKey(). Ideally as noted above you would replace the Thread.Sleep anyway.

Answer 2

You seem to be doing a bit of "non-Rx" kind of coding here. The task you're trying to perform is actually quite straight forward.
First up, you have some code that produces values after doing some work. I've recoded it as this:
var rnd = new Random();
Func<int> produceValue = () =>
{
Thread.Sleep(100);
return rnd.Next(3, 20);
};
This keeps it nice a separate from the Rx code. As a side note, I've pulled the new Random() declaration outside of the function as it isn't correct to keep instantiating a new Random instance - you won't necessarily get random numbers that way. You should also instantiate once and use the same instance.
So now the code to produce the observable is straight forward:
var query =
from n in Observable.Range(1, 9)
from m in Observable.Start(produceValue)
select m;
And subscribing to it is also easy:
query.Subscribe(
Console.WriteLine,
() => Console.WriteLine("All Done."));
I think that does exactly what you were trying to code for without any nasty WaitFor code.