Once you make the broad cuts on the observable state, it is time to tailor it further with the observable decorators. This gives you better control of the observability and improves the performance of the MobX reactivity system. We have seen two different ways of doing this: one with the @decorator syntax and the other using the decorate() API.

It is also possible to add new observable properties on the fly with extendObservable(). In fact, you can even add new actions and computed properties with extendObservable().

Observable State = Core State + Derived State

The core state and the derived state are two aspects of the observable state in MobX. This is easy to model with classes and decorators, as shown in the preceding sections. Once you identify the vocabulary of your feature, they become the class names that encapsulate the observable state. To handle the complexity of the feature, you can break it into smaller classes and compose them in the feature store. These feature stores...