R6 and parent exposure

About R6 and parent exposure.

Default inheritance

By default, R6 objects can only herit from their direct parent. But you may need to create an inheritance which goes along the family tree.

For example, we need to create a grand-child class PersonGrandChild, which inherits from a child class PersonChild, which inherits from the parent Person. The idea is to have a grand-child class with predifined and fixed first_name, last_name, and use with initialize the method from its grand-parent. The naive approach would be to use $super$super

library(R6)
Person <- R6Class("Person", 
                    public = list(
                      first_name = NULL, 
                      last_name = NULL, 
                      initialize = function(first, last){
                        self$first_name <- first
                        self$last_name <- last
                      }
                    )
)
PersonChild <- R6Class("PersonChild", 
                       inherit = Person)
PersonGrandChild <- R6Class("PersonGrandChild",
                            inherit = PersonChild, 
                            public = list(
                              initialize = function(){
                                super$super$initialize(first_name = "Colin", last_name = "Fay", age = 37, job = "R developper")
                              }  
                            )
)
try(PersonGrandChild$new())
Error in .subset2(public_bind_env, "initialize")(...) : 
  tentative d'appliquer un objet qui n'est pas une fonction

But that throws an error, because you can’t use $super$super: an R6 object doesn’t, by defaut, expose his parents to his children. A way to get around this feature is to use active binding. For that, you can create a method super_ inside a class, which simply returns super. Then, use $super$super_ inside the child class :

PersonChild$set("active", "super_", function() super)
PersonGrandChildExposed <- R6Class("PersonGrandChildExposed",
                                   inherit = PersonChild, 
                                   public = list(
                                     initialize = function(){
                                       super$super_$initialize(first = "Colin", last = "Fay")
                                     }  
                                   )
)

PersonGrandChildExposed$new()
<PersonGrandChildExposed>
  Inherits from: <PersonChild>
  Public:
    clone: function (deep = FALSE) 
    first_name: Colin
    initialize: function () 
    last_name: Fay
    super_: active binding

Works like a charm ;)