This function will be called after an instance is built. User could override this function in derived class.
Examples
BASE$..init..
#> function (...)
#> {
#> ..mro_current.. <- "BASE"
#> return(invisible(self))
#> }
#> <environment: 0x125c124e8>
# Inherit from BASE
TEST <- new_class(BASE, class_name = "TEST")
# Override the `..init..` method
register_method(TEST, ..init.. = function(a) {self$x <- a})
# Build a `TEST` instance
test <- TEST$instantiate(a = 2)
test$x
#> [1] 2