Control the Power

Active bindings can also be used to set private fields. In this case, the binding function should accept a single argument, named "value".

The pattern for creating a read/write active binding is as follows.

thing_factory <- R6Class(
  "Thing",
  private = list(
    ..a_field = "a value"
  ),
  active = list(
    a_field = function(value) {
      if(missing(value)) {
        private$..a_field
      } else {
        assert_is_a_string(value) # or another assertion
        private$..a_field <- value
      }
    }
  )
)

Values are assigned as though the binding was a data variable, not a function.

a_thing <- thing_factory$new()
a_thing$a_field <- "a new value" # not a_thing$a_field("a new value")

This exercise is part of the course

Object-Oriented Programming with S3 and R6 in R

View Course

Exercise instructions

A microwave oven class has been partially defined for you.

Extend the definition of the microwave oven class to include an active list element.
Add an active binding to the active element control the power level.
- The function should be called power_level_watts.
- It should accept a single argument named value.
- The private variable to get/set is called ..power_level_watts.
- Use assert_is_a_number() (docs) to check that value is a single number.
- Use assert_all_are_in_closed_range() (docs) to check that value is between 0 and ..power_rating_watts.
Create a microwave oven object, and assign it to a_microwave_oven.
Get the power level.
Try to set the the power level to the value "400", as a string.
Try to set the power level to 1600.
Set the power level to 400.

Hands-on interactive exercise

Have a go at this exercise by completing this sample code.

# Add a binding for power rating
microwave_oven_factory <- R6Class(
  "MicrowaveOven",
  private = list(
    ..power_rating_watts = 800,
    ..power_level_watts = 800
  ),
  # Add active list containing an active binding
  ___ = ___(
    ___ = ___(___) {
      if(missing(___)) {
        # Return the private value
        ___
      } else {
        # Assert that value is a number
        ___
        # Assert that value is in a closed range from 0 to power rating
        ___
        # Set the private power level to value
        ___ <- ___
      }
    }
  )
)

# Make a microwave 
a_microwave_oven <- ___

# Get the power level
___

# Try to set the power level to "400"
___ <- ___

# Try to set the power level to 1600 watts
___ <- ___

# Set the power level to 400 watts
___ <- ___

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