What is the functionality of onBecomeObserved when property (2nd arg) is omitted?

[FirstWhack]

The relevant documentation doesn't seem to explain this, though we can see from the relevant code that the listener can actually be passed as the second argument omitting property entirely.

I am wondering what is the functionality of using onBecomeObserved like so:

    onBecomeObserved(this, () => console.log('became observed');
    onBecomeUnobserved(this, () => console.log('no longer observed');

[urugator]

Depends on what this is. If observable object it will probably throw:

mobx/packages/mobx/src/types/type-utils.ts

Line 42 in d233eba

return die(26)

But it can be observable.box(5), observable.computed(() => {}), createAtom(), observable([]), observable(new Set()), observable(new Map()).
In case of observable(new Map()) it reacts to subscriptions related only to the keys (addition/removal of key).

[FirstWhack]

@urugator great insights.

Still not sure I understand as we usually pass a Class to the first argument (and that class is not observable).

Am I correct in assuming you can actually pass a reference to the class property (or in general a reference to any observable) if omitting the second argument?

E.g.

class MyClass {
  myProperty = true;
  constructor() {
    makeObservable(this, {
      myProperty: observable
    };
    onBecomeObserved(this.myProperty, () => console.log('myProperty became observed'));
  }
}

[urugator]

You can pass reference to any observable (not just object), but this.myProperty is not a reference to observable, it's boolean. You can't access the observable backing the property directly, therefore you need onBecomeObserved(obj, key), isObservableProp(obj, key) etc...

Actually you can via getAtom, so you can do onBecomeObserved(getAtom(this, 'myProperty')), but lets keep it simple...

[FirstWhack]

Oh...... Ohhhhhh!!

Thank you so much. This makes sense to me now.