[feature request] BehaviorSubject to Signals migration

[ShacharHarshuv]

In the spirit of writing migration scripts to modernize Angular code, I want to propose the following script. Let me know if you think this is something that could fit the library.

The assumption here that everything that a BehaviorSubject is used for, a signal can be used instead, and I think it is possible to write a (relatively) robust script to do that migration. The script will:

• Replace member instantiation of new BehaviorSubject with signal, while keeping any used generics / inferred types.
• Replace .next with .set
• Rename the member if it ends with $
• Replace myValue$.value usages to myValue()
• Replace any other myValue$ with `toObservable(myValue)
  • caveat: it might not be in an injection context. A possible solution of it will be duplicating the member in instantiation so that we'll have both myValue as a signal and myValue$ as a stream. In that case, if there is no $ as a postfix, we'll have to add it.

[jdegand]

And convert .next to .set?

[ShacharHarshuv]

Yes, sorry. Added!

[eneajaho]

Hi, something is coming 🙌

Let's wait a little bit more 😬

[ShacharHarshuv]

Do you mean you are working on it, that Angular's core team is working on it, or that you think it's premature to add such a feature now?

[eneajaho]

I've been working on a prototype for myself to do these kinds of migration, but nothing ready yet.

[ShacharHarshuv]

Cool! I also thought that a next step could be converting something like that:

combineLatest({
  a: toObservable(this.a),
  b: toObservable(this.b),
})
.pipe(
  map(({a, b}) => this.a() + this.b()),
  distinctUntilChanged(), // ... and other operators that don't matter with signals like share etc
);

to

computed(() => this.a() + this.b());

Might be harder than I think but it feels like it could be possible.

[kbrilla]

I would argue that this could be applyFix type of migration for VS code, but should not be used with ng migrate; Converting all BehaviorSubject signal may lead to errors, as there may by something along the line of pipe(pairwise()) somewhere in the code and using toObservable() would break this behaviour as instead, beacause of lazy nature of signals instead of abcd we would get only d delivered to pipe;

https://stackblitz.com/edit/stackblitz-starters-qnm6xx?file=src%2Fmain.ts (look at console to see difrence)
Basicly:

Only BehaviorSubject prints all values, while signal, and toSignal/toObservable prints only last values that were recived in CD cycle because they relay on effect().

With pairWise we would get completle diffrent values [ab], [bc], [cd], [de], [ef], [fg] compared to [dg] only

[ShacharHarshuv]

Yeah they behave differently. Though I feel like in practice if you use a BehaviorSubject you probably only care about the last value.