ggplot2 (version 3.3.6)

ggproto: Create a new ggproto object

Description

Construct a new object with ggproto(), test with is.ggproto(), and access parent methods/fields with ggproto_parent().

Usage

ggproto(`_class` = NULL, `_inherit` = NULL, ...)
ggproto_parent(parent, self)
is.ggproto(x)

Arguments

_class

Class name to assign to the object. This is stored as the class attribute of the object. This is optional: if NULL (the default), no class name will be added to the object.

_inherit

ggproto object to inherit from. If NULL, don't inherit from any object.

...

A list of members in the ggproto object.

parent, self

Access parent class parent of object self.

x

An object to test.

Calling methods

ggproto methods can take an optional self argument: if it is present, it is a regular method; if it's absent, it's a "static" method (i.e. it doesn't use any fields).

Imagine you have a ggproto object Adder, which has a method addx = function(self, n) n + self$x. Then, to call this function, you would use Adder$addx(10) -- the self is passed in automatically by the wrapper function. self be located anywhere in the function signature, although customarily it comes first.

Calling methods in a parent

To explicitly call a methods in a parent, use ggproto_parent(Parent, self).

Details

ggproto implements a protype based OO system which blurs the lines between classes and instances. It is inspired by the proto package, but it has some important differences. Notably, it cleanly supports cross-package inheritance, and has faster performance.

In most cases, creating a new OO system to be used by a single package is not a good idea. However, it was the least-bad solution for ggplot2 because it required the fewest changes to an already complex code base.

Examples

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)

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