rxjs-interop: bidirectional interop

[krulod]

Which @angular/* package(s) are relevant/related to the feature request?

core

Description

Currently, toSignal and toObservable functions always return read-only primitives - Signal and Observable respectively. It would be really handy if passing a writable primitive to interop functions created a writable counterpart of passed value.

Examples:

const someSignal = signal(1)
effect(() => {
  const value = someSignal()
  console.log('signal changed:', value)
})

const someSubject = toObservable() // returned a Subject!
someSubject.next(2) // signal changed: 2

const someSubject = new Subject(1)
someSubject.subscribe(value => console.log('subject changed:', value)

const someSignal = toSignal(someSubject) // returned a WritableSignal!
someSignal.set(2) // => subject changed: 2
someSignal.update(prev => prev * 2) // => subject changed: 4

Proposed solution

Return a WritableSignal from toSignal if argument is next-able. Whenever a value is written to the signal manually, push it to the target observable.

In toObservable, return a subject if argument is a WritableSignal. Whenever a value is pushed to the subject manually, write it to the target signal.

Alternatives considered

Leave everything as is.

[alxhub]

This is a neat idea. Can you elaborate on use cases you've encountered where this API could be beneficial?

[krulod]

My specific use case is creation of an adapter between Angular Signals and Reatom, a signals-like library. In a similiar adapter for Vue which is already available, you can see that it creates a writable Ref-like from writable Reatom atoms.

If there is a hypothetical reatomToObservable which creates Subject-s from AtomMut-s and Observable-s from Atom-s, the discussed feature makes implementation of reatomToSignal really trivial: target => toSignal(reatomToObservable(target)).