Create a new ggproto object

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 named members in the ggproto object. These can be functions that become methods of the class or regular objects.
parent, self: Access parent class parent of object self.
x: An object to test.

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.

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).

Working with ggproto classes

The ggproto objects constructed are build on top of environments, which has some ramifications. Environments do not follow the 'copy on modify' semantics one might be accustomed to in regular objects. Instead they have 'modify in place' semantics.

Examples

Adder <- ggproto("Adder",
  x = 0,
  add = function(self, n) {
    self$x <- self$x + n
    self$x
  }
 )
is.ggproto(Adder)
#> [1] TRUE

Adder$add(10)
#> [1] 10
Adder$add(10)
#> [1] 20

Doubler <- ggproto("Doubler", Adder,
  add = function(self, n) {
    ggproto_parent(Adder, self)$add(n * 2)
  }
)
Doubler$x
#> [1] 20
Doubler$add(10)
#> [1] 40