RxSwift: read-only property and read-write property wrapper

RxSwift: read-only property and read-write property wrapper

Utilize Swift 5.1 property wrapper to have elegant read-only property with read-write internal semantic

BehaviorRelay and RxProperty

Any Swift developer who worked with RxSwift knows that Observables and Subjects lack the ability to store the last value. Before RxSwift 5.0.0 there was the Variable _generic class for this purpose, now it has been substituted with the _BehaviorRelay which technically is not even part of RxSwift, but RxRelay module.

A developer who just starting to use RxSwift may be confused, what’s the difference between PublishSubjectBehaviorSubjectPublishRelay, and BehaviorRelay.

  • Publish vs _Behavior. _The first doesn’t store the last value, while the last - does.
  • Subject vs Relay. The last doesn’t through an error and can’t be terminated, while the first - can.

As you may already know, RxSwift is used to glue components in the app: a ViewModel with a ViewController in MVVM, an Interactor with Services in RIBs, a Middleware with a Store in Redux.

Normally, a PublishSubject _is used to propagate an event, while _BehaviorRelay to share some value or a state. The common interface for a ViewModel looks like:

There is a slight problem with this ViewModel declaration though: its state is modifiable outside. Because even it is only a get property, ._accept()_method, which modifies the value, is available. This totally breaks one of the fundamental rules of OOP - encapsulation.

Someone may propose to use Observable in the protocol instead of BehaviorRelay, i.e. something like that:

mvvm rxswift ios-app-development ios property-wrapper

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