Question

I have three (or more) observable sequences of bool:

IObservable<bool> a0 = ...;
IObservable<bool> a1 = ...;
IObservable<bool> a2 = ...;

These sequences generate mostly false values but once in a while there is a true (the sequences can also be empty). I’m not interested in the values themselves but rather in the order of the true values. So if a0 is true, then a2 is true and finally a1, I’d like to perform some action:

(a0, a2, a1) => ...do something...

But if the true values appear in another order, I’d like to do something else:

(a2, a1, a0) => ...do something else...
(a0, a1, a2) => ...do something entirely else...

So I’m only interested in the first true value of each sequence and depending on the order in which they occur, I’d like to perform some action.

I could record the timestamp of each first occurrence of true and then sort in a CombineLatest call, but I have a feeling there is a better way. One problem with this approach is that I am now dependent on granularity of recorded timestamps.

Answer 1

Firstly, you only need the true values so ignore the rest. Second, you can encode each observable into a more semantic meaning (perhaps a string?). Third, you can then merge these streams, wait for 3 values to arrive, and act based on them.

Set up:

var a0 = new Subject<bool>();
var a1 = new Subject<bool>();
var a2 = new Subject<bool>();

Encoding:

var a0t = a0.Where(x => x).Select(x => "a0").Take(1); // thanks Matthew Finlay
var a1t = a1.Where(x => x).Select(x => "a1").Take(1);
var a2t = a2.Where(x => x).Select(x => "a2").Take(1);

Merge and get after three values:

var merged = a0t.Merge(a1t).Merge(a2t);
var message = merged.Buffer(3).Subscribe(x => ProcessMessage(String.Join("", x)));

The processing section:

public void ProcessMessage(string message)
{
    switch(message) {
        case "a0a1a2": Console.WriteLine("1"); break;
        case "a1a2a0": Console.WriteLine("2"); break;
        case "a2a1a0": Console.WriteLine("3"); break;
    }
}

And test:

a1.OnNext(true);
a2.OnNext(true);
a0.OnNext(true);