Task sequencing and re-entracy

Question

I\'ve got the following scenario, which I think might be quite common:

1. There is a task (a UI command handler) which can complete either synchronously or asy

Answer 1

Microsoft's Rx does provide an easy way to do this kind of thing. Here's a simple (perhaps overly simple) way of doing it:

var subject = new BehaviorSubject(0);

IDisposable subscription =
    subject
        .Scan((x0, x1) =>
        {
            Console.WriteLine($"previous value {x0}");
            return x1;
        })
        .Skip(1)
        .Subscribe(x => Console.WriteLine($"current value {x}\r\n"));

subject.OnNext(1000);
subject.OnNext(900);
subject.OnNext(800);

Console.WriteLine("\r\nPress any key to continue to test #2...\r\n");
Console.ReadLine();

subject.OnNext(200);
subject.OnNext(100);

Console.WriteLine("\r\nPress any key to exit...");
Console.ReadLine();

The output I get is this:

previous value 0
current value 1000

previous value 1000
current value 900

previous value 900
current value 800


Press any key to continue to test #2...

previous value 800
current value 200

previous value 200
current value 100


Press any key to exit...

It's easy to cancel at any time by calling subscription.Dispose().

---

Error handling in Rx is generally a little more bespoke than normal. It's not just a matter of throwing a try/catch around things. You also can repeat steps that error with a Retry operator in the case of things like IO errors.

In this circumstance, because I've used a BehaviorSubject (which repeats its last value whenever it is subscribed to) you can easily just resubscribe using a Catch operator.

var subject = new BehaviorSubject(0);
var random = new Random();

IDisposable subscription =
    subject
        .Select(x =>
        {
            if (random.Next(10) == 0)
                throw new Exception();
            return x;
        })
        .Catch(ex => subject.Select(x => -x))
        .Scan((x0, x1) =>
        {
            Console.WriteLine($"previous value {x0}");
            return x1;
        })
        .Skip(1)
        .Subscribe(x => Console.WriteLine($"current value {x}\r\n"));

Now with the .Catch(ex => subject.Select(x => -x)) it inverts the value of the query should an exception be raised.

A typical output may be like this:

previous value 0
current value 1000

previous value 1000
current value 900

previous value 900
current value 800


Press any key to continue to test #2...

previous value 800
current value -200

previous value -200
current value -100


Press any key to exit...

Note the -ve numbers in the second half. An exception was handled and the query was able to continue.