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R Reference Methods

A reference class method is a function stored inside the class definition. A method reads and writes the fields of its own object through .self.

A method that adds one

bump adds one to count through .self. Two calls move the count from 0 to 2.

Counter <- suppressMessages(setRefClass("Counter", fields = list(count = "numeric"), methods = list(
  bump = function() {
    .self$count <- .self$count + 1
  }
)))
c1 <- Counter$new(count = 0)
c1$bump()
c1$bump()
print(c1$count)

A method with an argument

add takes an amount and adds it to count. The count climbs from 10 to 15 and then to 18.

Counter <- suppressMessages(setRefClass("Counter", fields = list(count = "numeric"), methods = list(
  add = function(amount) {
    .self$count <- .self$count + amount
  }
)))
c1 <- Counter$new(count = 10)
c1$add(5)
print(c1$count)
c1$add(3)
print(c1$count)

One method calls another

bump_twice calls bump twice through .self. One later bump call takes the count to 3.

Counter <- suppressMessages(setRefClass("Counter", fields = list(count = "numeric"), methods = list(
  bump = function() {
    .self$count <- .self$count + 1
  },
  bump_twice = function() {
    .self$bump()
    .self$bump()
  }
)))
c1 <- Counter$new(count = 0)
c1$bump_twice()
print(c1$count)
c1$bump()
print(c1$count)

A reset method

reset writes 0 into count through .self. The prints show the count fall back to 0 and climb again.

Counter <- suppressMessages(setRefClass("Counter", fields = list(count = "numeric"), methods = list(
  bump = function() {
    .self$count <- .self$count + 1
  },
  reset = function() {
    .self$count <- 0
  }
)))
c1 <- Counter$new(count = 8)
c1$bump()
print(c1$count)
c1$reset()
print(c1$count)
c1$bump()
print(c1$count)

Example

Counter <- suppressMessages(setRefClass("Counter", fields = list(count = "numeric"), methods = list(
  add = function(amount) {
    .self$count <- .self$count + amount
  },
  add_twice = function(amount) {
    .self$add(amount)
    .self$add(amount)
  }
)))
c1 <- Counter$new(count = 1)
c1$add(4)
print(c1$count)
c1$add_twice(10)
print(c1$count)
print(c1)

Key points

  • The methods value inside setRefClass() holds a list of named functions.
  • Each method runs through the object with $, as in c1$bump().
  • .self stands for the object the method is running on.
  • A method writes a field through .self, as in .self$count.
  • A method calls another method of the same object through .self as well.
  • Field changes made by a method stay with the object after the call ends.

In this example, add takes the count from 1 to 5. add_twice then calls add twice through .self and the count reaches 25, as the field dump confirms.

Try it Yourself

Edit the code, then press Run.

Running is simulated · rdrr.io R Snippets

Exercise

Inside a method, what does .self$count refer to?