This gem provide class attributes for mruby in a way that does not break the Principle of Least Surprise when used with inheritance.

Problem

This gem solves the very old Ruby problem, in which class attributes are shared withing the whole hierarchy of classes. See this example:

class Vehicle
  @@wheels = 0
  def self.inspect_wheels
    puts "#{self}: #{@@wheels}"
  end
end
Vehicle.inspect_wheels
class Car < Vehicle
  @@wheels = 4
end
Car.inspect_wheels
Vehicle.inspect_wheels

The output of this code would be:

Vehicle: 0
Car: 4
Vehicle: 4

Note that the value for Vehicle class changes just because the class Car, inheriting from Vehicle, was defined. This leads to very subtle bugs and is why class attributes are generally discouraged.

Solution

The gem ports the solution created by Hanami web framework for Ruby. This is the counterpart code using ClassAttribute:

class Vehicle
  include ClassAttribute
  class_attribute :wheels
  self.wheels = 0
  def self.inspect_wheels
    puts "#{self}: #{self.wheels}"
  end
end
Vehicle.inspect_wheels
class Car < Vehicle
  self.wheels = 4
end
Car.inspect_wheels
Vehicle.inspect_wheels

In this case the output is as expected:

Vehicle: 0
Car: 4
Vehicle: 0

License

The gem is distributed on a MIT license.

Acknowledgements

The Hanami team, especially Luca Guidi, for writing the original implementation of the class attribute.