Create an R6 reference object generator

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Description

R6 objects are essentially environments, structured in a way that makes them look like an object in a more typical object-oriented language than R. They support public and private members, as well as inheritance across different packages.

Usage

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R6Class(classname = NULL, public = list(), private = NULL,
  active = NULL, inherit = NULL, lock_objects = TRUE, class = TRUE,
  portable = TRUE, lock_class = FALSE, cloneable = TRUE,
  parent_env = parent.frame(), lock)

Arguments

classname

Name of the class. The class name is useful primarily for S3 method dispatch.

public

A list of public members, which can be functions (methods) and non-functions (fields).

private

An optional list of private members, which can be functions and non-functions.

active

An optional list of active binding functions.

inherit

A R6ClassGenerator object to inherit from; in other words, a superclass. This is captured as an unevaluated expression which is evaluated in parent_env each time an object is instantiated.

lock_objects

Should the environments of the generated objects be locked? If locked, new members can't be added to the objects.

class

Should a class attribute be added to the object? Default is TRUE. If FALSE, the objects will simply look like environments, which is what they are.

portable

If TRUE (the default), this class will work with inheritance across different packages. Note that when this is enabled, fields and members must be accessed with self$x or private$x; they can't be accessed with just x.

lock_class

If TRUE, it won't be possible to add more members to the generator object with $set. If FALSE (the default), then it will be possible to add more members with $set. The methods $is_locked, $lock, and $unlock can be used to query and change the locked state of the class.

cloneable

If TRUE (the default), the generated objects will have method named $clone, which makes a copy of the object.

parent_env

An environment to use as the parent of newly-created objects.

lock

Deprecated as of version 2.1; use lock_class instead.

Details

An R6 object consists of a public environment, and may also contain a private environment, as well as environments for superclasses. In one sense, the object and the public environment are the same; a reference to the object is identical to a reference to the public environment. But in another sense, the object also consists of the fields, methods, private environment and so on.

The active argument is a list of active binding functions. These functions take one argument. They look like regular variables, but when accessed, a function is called with an optional argument. For example, if obj$x2 is an active binding, then when accessed as obj$x2, it calls the x2() function that was in the active list, with no arguments. However, if a value is assigned to it, as in obj$x2 <- 50, then the function is called with the right-side value as its argument, as in x2(50). See makeActiveBinding for more information.

If the public or private lists contain any items that have reference semantics (for example, an environment), those items will be shared across all instances of the class. To avoid this, add an entry for that item with a NULL initial value, and then in the intialize method, instantiate the object and assign it.

The print method

R6 object generators and R6 objects have a default print method to show them on the screen: they simply list the members and parameters (e.g. lock_objects, portable, etc., see above) of the object.

The default print method of R6 objects can be redefined, by supplying a public print method. (print members that are not functions are ignored.) This method is automatically called whenever the object is printed, e.g. when the object's name is typed at the command prompt, or when print(obj) is called. It can also be called directly via obj$print(). All extra arguments from a print(obj, ...) call are passed on to the obj$print(...) method.

Portable and non-portable classes

When R6 classes are portable (the default), they can be inherited across packages without complication. However, when in portable mode, members must be accessed with self and private, as in self$x and private$y.

When used in non-portable mode, R6 classes behave more like reference classes: inheritance across packages will not work well, and self and private are not necessary for accessing fields.

Cloning objects

R6 objects have a method named clone by default. To disable this, use cloneable=FALSE. Having the clone method present will slightly increase the memory footprint of R6 objects, but since the method will be shared across all R6 objects, the memory use will be negligible.

By default, calling x$clone() on an R6 object will result in a shallow clone. That is, if any fields have reference semantics (environments, R6, or reference class objects), they will not be copied; instead, the clone object will have a field that simply refers to the same object.

To make a deep copy, you can use x$clone(deep=TRUE). With this option, any fields that are R6 objects will also be cloned; however, environments and reference class objects will not be.

If you want different deep copying behavior, you can supply your own private method called deep_clone. This method will be called for each field in the object, with two arguments: name, which is the name of the field, and value, which is the value. Whatever the method returns will be used as the value for the field in the new clone object. You can write a deep_clone method that makes copies of specific fields, whether they are environments, R6 objects, or reference class objects.

S3 details

Normally the public environment will have two classes: the one supplied in the classname argument, and "R6". It is possible to get the public environment with no classes, by using class=FALSE. This will result in faster access speeds by avoiding class-based dispatch of $. The benefit is is negligible in most cases.

If a class is a subclass of another, the object will have as its classes the classname, the superclass's classname, and "R6"

The primary difference in behavior when class=FALSE is that, without a class attribute, it won't be possible to use S3 methods with the objects. So, for example, pretty printing (with print.R6Class) won't be used.

Examples

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# A queue ---------------------------------------------------------
Queue <- R6Class("Queue",
  public = list(
    initialize = function(...) {
      for (item in list(...)) {
        self$add(item)
      }
    },
    add = function(x) {
      private$queue <- c(private$queue, list(x))
      invisible(self)
    },
    remove = function() {
      if (private$length() == 0) return(NULL)
      # Can use private$queue for explicit access
      head <- private$queue[[1]]
      private$queue <- private$queue[-1]
      head
    }
  ),
  private = list(
    queue = list(),
    length = function() base::length(private$queue)
  )
)

q <- Queue$new(5, 6, "foo")

# Add and remove items
q$add("something")
q$add("another thing")
q$add(17)
q$remove()
#> [1] 5
q$remove()
#> [1] 6

# Private members can't be accessed directly
q$queue
#> NULL
# q$length()
#> Error: attempt to apply non-function

# add() returns self, so it can be chained
q$add(10)$add(11)$add(12)

# remove() returns the value removed, so it's not chainable
q$remove()
#> [1] "foo"
q$remove()
#> [1] "something"
q$remove()
#> [1] "another thing"
q$remove()
#> [1] 17


# Active bindings -------------------------------------------------
Numbers <- R6Class("Numbers",
  public = list(
    x = 100
  ),
  active = list(
    x2 = function(value) {
      if (missing(value)) return(self$x * 2)
      else self$x <- value/2
    },
    rand = function() rnorm(1)
  )
)

n <- Numbers$new()
n$x
#> [1] 100
n$x2
#> [1] 200
n$x2 <- 1000
n$x
#> [1] 500

# If the function takes no arguments, it's not possible to use it with <-:
n$rand
#> [1] 0.2648
n$rand
#> [1] 2.171
# n$rand <- 3
#> Error: unused argument (quote(3))


# Inheritance -----------------------------------------------------
# Note that this isn't very efficient - it's just for illustrating inheritance.
HistoryQueue <- R6Class("HistoryQueue",
  inherit = Queue,
  public = list(
    show = function() {
      cat("Next item is at index", private$head_idx + 1, "\n")
      for (i in seq_along(private$queue)) {
        cat(i, ": ", private$queue[[i]], "\n", sep = "")
      }
    },
    remove = function() {
      if (private$length() - private$head_idx == 0) return(NULL)
      private$head_idx <<- private$head_idx + 1
      private$queue[[private$head_idx]]
    }
  ),
  private = list(
    head_idx = 0
  )
)

hq <- HistoryQueue$new(5, 6, "foo")
hq$show()
#> Next item is at index 1
#> 1: 5
#> 2: 6
#> 3: foo
hq$remove()
#> [1] 5
hq$show()
#> Next item is at index 2
#> 1: 5
#> 2: 6
#> 3: foo
hq$remove()
#> [1] 6



# Calling superclass methods with super$ --------------------------
CountingQueue <- R6Class("CountingQueue",
  inherit = Queue,
  public = list(
    add = function(x) {
      private$total <<- private$total + 1
      super$add(x)
    },
    get_total = function() private$total
  ),
  private = list(
    total = 0
  )
)

cq <- CountingQueue$new("x", "y")
cq$get_total()
#> [1] 2
cq$add("z")
cq$remove()
#> [1] "x"
cq$remove()
#> [1] "y"
cq$get_total()
#> [1] 3


# Non-portable classes --------------------------------------------
# By default, R6 classes are portable, which means they can be inherited
# across different packages. Portable classes require using self$ and
# private$ to access members.
# When used in non-portable mode, members can be accessed without self$,
# and assignments can be made with <<-.

NP <- R6Class("NP",
  portable = FALSE,
  public = list(
    x = NA,
    getx = function() x,
    setx = function(value) x <<- value
  )
)

np <- NP$new()
np$setx(10)
np$getx()
#> [1] 10

# Setting new values ----------------------------------------------
# It is possible to add new members to the class after it has been created,
# by using the $set() method on the generator.

Simple <- R6Class("Simple",
  public = list(
    x = 1,
    getx = function() self$x
  )
)

Simple$set("public", "getx2", function() self$x*2)

# Use overwrite = TRUE to overwrite existing values
Simple$set("public", "x", 10, overwrite = TRUE)

s <- Simple$new()
s$x
s$getx2()


# Cloning objects -------------------------------------------------
a <- Queue$new(5, 6)
a$remove()
#> [1] 5

# Clone a. New object gets a's state.
b <- a$clone()

# Can add to each queue separately now.
a$add(10)
b$add(20)

a$remove()
#> [1] 6
a$remove()
#> [1] 10

b$remove()
#> [1] 6
b$remove()
#> [1] 20


# Deep clones -----------------------------------------------------

Simple <- R6Class("Simple",
 public = list(
   x = NULL,
   initialize = function(val) self$x <- val
 )
)

Cloner <- R6Class("Cloner",
  public = list(
    s = NULL,
    y = 1,
    initialize = function() self$s <- Simple$new(1)
  )
)

a <- Cloner$new()
b <- a$clone()
c <- a$clone(deep = TRUE)

# Modify a
a$s$x <- 2
a$y <- 2

# b is a shallow clone. b$s is the same as a$s because they are R6 objects.
b$s$x
#> [1] 2
# But a$y and b$y are different, because y is just a value.
b$y
#> [1] 1

# c is a deep clone, so c$s is not the same as a$s.
c$s$x
#> [1] 1
c$y
#> [1] 1


# Deep clones with custom deep_clone method -----------------------

CustomCloner <- R6Class("CustomCloner",
  public = list(
    e = NULL,
    s1 = NULL,
    s2 = NULL,
    s3 = NULL,
    initialize = function() {
      self$e <- new.env(parent = emptyenv())
      self$e$x <- 1
      self$s1 <- Simple$new(1)
      self$s2 <- Simple$new(1)
      self$s3 <- Simple$new(1)
    }
  ),
  private = list(
    # When x$clone(deep=TRUE) is called, the deep_clone gets invoked once for
    # each field, with the name and value.
    deep_clone = function(name, value) {
      if (name == "e") {
        # e1 is an environment, so use this quick way of copying
        list2env(as.list.environment(value, all.names = TRUE),
                 parent = emptyenv())

      } else if (name %in% c("s1", "s2")) {
        # s1 and s2 are R6 objects which we can clone
        value$clone()

      } else {
        # For everything else, just return it. This results in a shallow
        # copy of s3.
        value
      }
    }
  )
)

a <- CustomCloner$new()
b <- a$clone(deep = TRUE)

# Change some values in a's fields
a$e$x <- 2
a$s1$x <- 3
a$s2$x <- 4
a$s3$x <- 5

# b has copies of e, s1, and s2, but shares the same s3
b$e$x
#> [1] 1
b$s1$x
#> [1] 1
b$s2$x
#> [1] 1
b$s3$x
#> [1] 5


# Debugging -------------------------------------------------------
## Not run: 
# This will enable debugging the getx() method for objects of the 'Simple'
# class that are instantiated in the future.
Simple$debug("getx")
s <- Simple$new()
s$getx()

# Disable debugging for future instances:
Simple$undebug("getx")
s <- Simple$new()
s$getx()

# To enable and disable debugging for a method in a single instance of an
# R6 object (this will not affect other objects):
s <- Simple$new()
debug(s$getx)
s$getx()
undebug(s$getx)

## End(Not run)

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