Computed Signals based on Computed Signals could not be updated

[CKussin]

Hey there,

first of all. Amazing work! As an angular dev i love this lib. it's just great.

But here is the reason for this Issue:

If i use something like the following:

`
public class Test {

public Signal BaseSignal { get; } = new("value");
public IReadOnlySignal FirstLevelComputed => Signal.Computed(() => {
if(BaseSignal.Value == "value")
return 1;
return 0;
});
public IReadOnlySignal SecondLevelComputed => Signal.Computed(() => {
return FirstLevelComputed.Value == 1;
});
}
`
In the first iteration everything works as expected. Base:value, First:1, Second:true
But if i change the Base to let's say "label", then somethings not working correctly

Expected:
Base:label, First:0, Second:false

Reality:
Base:label, First:0, Second:true

It seems like the SecondLevelComputed is not updated properly

Any hint?

Best regards

PS: the behavior is the same if i create and assign the computed properties in the constructor

[fedeAlterio]

Hey. Thank you for your interest in this library :)

I can't reproduce the issue with a unit test unfortunately.

This test passes

   [Fact]
   public void ShouldNotify_ComputedOfComputed()
   {
       var baseSignal = new Signal<string>("value");
       var firstLevelComputed = Signal.Computed(() => baseSignal.Value == "value" ? 1 : 0);
       var secondLevelComputed = Signal.Computed(() => firstLevelComputed.Value == 1);

       baseSignal.Value.Should().Be("value");
       firstLevelComputed.Value.Should().Be(1);
       secondLevelComputed.Value.Should().Be(true);

       baseSignal.Value = "label";
       firstLevelComputed.Value.Should().Be(0);
       secondLevelComputed.Value.Should().Be(false);
   }

Is your code a bit different maybe? Is it possible to share it here?

P.S. Note that R3 (And Rx) has single thread semantics. Since this library is built on top of R3, that means that its forbidden for 2 thread to set the same Signal in parallel, otherwise all the Rx operators are not guaranteed to work.

See the observable contract

Observables must issue notifications to observers serially (not in parallel). They may issue these notifications from different threads, but there must be a formal happens-before relationship between the notifications.

In angular, and Rxjs in general, this precondition is always satisfied since there is only one thread there. In C# the dev should be more careful when sending notifications (Also in .Net all UI frameworks are basically single threaded, but there are ways to escape from it).

The code below is not guaranteed to work for example. Not guaranteed to work means the signals could be not coherent in a similar way you described in the issue.

 public async Task ThisIsBad()
 {
     var baseSignal = new Signal<string>("value");
     var firstLevelComputed = Signal.Computed(() => baseSignal.Value == "value" ? 1 : 0);
     var secondLevelComputed = Signal.Computed(() => firstLevelComputed.Value == 1);

     await Task.WhenAll(Task.Run(() => baseSignal.Value = "A"),
                        Task.Run(() => baseSignal.Value = "B"));

     // The result of the signals here is undefined!

     baseSignal.Value.Should().Be("value");
     firstLevelComputed.Value.Should().Be(1);
     secondLevelComputed.Value.Should().Be(true);

     baseSignal.Value = "label";
     firstLevelComputed.Value.Should().Be(0);
     secondLevelComputed.Value.Should().Be(false);
 }

Can this be the issue?

Thanks