part of conversation: https://www.reddit.com/r/androiddev/comments/7owabf/reactive_apps_architecture_design_asymnetry_that/

ReactiveApiDesign.kt

//bad
interface ShoppingCart {
  fun observe(): Observable<ShoppingCart>
  fun setShoppingCart(cart: ShoppingCart)
}

//good
interface ShoppingCart {
  fun observe(): Observable<ShoppingCart>
  
  //these all update the public state, and return results (failures, successes, relevant info)
  fun clearAllProducts(): ClearProductsResult 
  fun addProduct(): AddProductResult
  fun removeAllProductsOfCategory(category: String): ProductsRemovedResult
  
}