example of singleton pattern

singleton

## Singleton pattern
Singleton <- setRefClass("Singleton",
                             fields=list(
                               Class="ANY",
                               instance="ANY"
                               ),
                            methods=list(
                              initialize=function(...) {
                                "Override this by defining Class"
                                instance <<- NULL
                                callSuper(...)
                               },
                               get_instance=function(...) {
                                 "Get a unique instance of the class defined in the subclasses initialize method"
                                 if(is.null(instance)) 
                                   instance <<- Class$new(...)
                                 instance

                               }
                               ))



SomeSingleObject <- setRefClass("SomeSingleObject",
                                contains="Singleton",
                                fields=c("property"), # fake, just to keep down warnings
                                methods=list(
                                  initialize=function(...) {
                                    Class <<- setRefClass("Class",
                                                          fields=list(
                                                            property="ANY"
                                                            ),
                                                          methods=list(
                                                            initialize=function(...) {
                                                              initFields()
                                                              callSuper(...)
                                                            },
                                                            set_property=function(value) {
                                                              property <<- value
                                                            },
                                                            get_property=function() property
                                                            )
                                                          )

                                    callSuper(...)
                                  }
                                  ))$new()



a <- SomeSingleObject$get_instance()
b <- SomeSingleObject$get_instance() ## sam object
identical(a, b)
a$set_property("some value")
print(b$get_property())