ggproto objects: ggproto

Mix.install([
  {:giggly, path: Path.join(__DIR__, "..")},
  {:kino, "~> 0.19"}
  ])

About this notebook

This notebook follows the examples on ggplot2's reference page for ggproto(), which covers ggproto_parent() too, in the order the page gives them. Each example shows ggplot2's code first and then Giggly's. An example Giggly cannot draw yet keeps its ggplot2 code and names what it is waiting for, so the page is also a list of what remains.

ggproto is the object system ggplot2 is built with. ggproto() makes an object holding fields and methods, which may inherit from another object, across packages too; is_ggproto() tests for one, and ggproto_parent() calls a method of the parent from a method that overrides it. A method that takes self is given the object it is called on. Objects are built on R environments, so they change in place rather than being copied. Not in Giggly yet: ggproto().

An object with a field and a method

Adder <- ggproto("Adder",
  x = 0,
  add = function(self, n) {
    self$x <- self$x + n
    self$x
  }
 )
is_ggproto(Adder)

Adder$add(10)
Adder$add(10)

Adder holds a field, x, and a method, add(), which adds n to the object's x and returns it; is_ggproto() returns TRUE. The two calls return 10 and then 20, since the object changes in place. Not in Giggly yet: ggproto().

Calling the parent's method

Doubler <- ggproto("Doubler", Adder,
  add = function(self, n) {
    ggproto_parent(Adder, self)$add(n * 2)
  }
)
Doubler$x
Doubler$add(10)

Doubler inherits from Adder, so Doubler$x reads Adder's x, 20. Its own add() doubles n and hands it to Adder's add() through ggproto_parent(), which adds it to Doubler's x, so Doubler$add(10) returns 40. Not in Giggly yet: ggproto() and ggproto_parent().