Construct a new object with ggproto(), test with is.ggproto(),
and access parent methods/fields with ggproto_parent().
ggproto(`_class` = NULL, `_inherit` = NULL, ...)
ggproto_parent(parent, self)
is.ggproto(x)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.
ggproto object to inherit from. If NULL, don't
inherit from any object.
A list of members in the ggproto object.
Access parent class parent of object self.
An object to test.
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.
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.
To explicitly call a methods in a parent, use
ggproto_parent(Parent, self).
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