Reaction in sub-store won't fire when observable in parent store changes

[omerfaran]

Hello everyone,

This is something I'm struggling with in Mobx, I have a parent store:

export class ParentStore extends StoreBase {
  public subStore: SubStore;

  public myObservable = [];
  public myOtherObservable = [];

  constructor() {
    super();

    this.subStore = new SubStore({
      store: this,
    });

    makeObservable(this, {
      myObservable: observable,
      myOtherObservable: observable,
      combined: computed,
    });

    autorun(() => {
      console.log(this.myObservable, 'I ran, not related to the question but this one will fire');
    });
  }

  get combined() {
    return [...this.myObservable, ...this.myOtherObservable];
  }
}

With an observable called myObservable. In this parent store I create a sub-store called subStore. This is the sub store:

class SubStore extends StoreBase {
  store: ParentStore;

  constructor({ store }: { store: ParentStore }) {
    super();

    this.store = store;

    makeObservable(this, {
      // * Computed
      options: computed,
      willActuallyRun: computed,
    });

    reaction(
      () => this.store.myObservable,
      (myObservable) => {
        console.log(myObservable, 'This will not fire');
      }
    );
  }

  get willActuallyRun() {
    // This will fire
    return this.store.myObservable;
  }

  get options() {
    // This won't fire either
    return this.store.combined;
  }
}

export default SubStore;

I have a reaction to myObservable in the parent store, but it will never fire. On the other hand, the computed willActuallyRun does run when myObservable in the parent store changes, if I subscribe to that sub store in a react component, wrapping it in mobx observer function, and read willActuallyRun - it will update.

I don't understand why the getter willActuallyRun does update when myObservable changes, but the reaction does not fire. Does anyone know, please?

Also, another thing (I'm sorry in advance if it's for another topic) - is it possible for the sub store to subscribe to a computed in the parent store? In this example, I would like the sub store to know when the computed called combined changes in the parent (that computed includes the two observables in the parent).

Thanks a lot in advance everyone! Please tell me if I didn't explain something well enough