Multiton¶ ↑

Multiton design pattern ensures only one object is allocated for a given state.

The 'multiton' pattern is similar to a singleton, but instead of only one instance, there are several similar instances. It is useful when you want to avoid constructing objects many times because of some huge expense (connecting to a database for example), require a set of similar but not identical objects, and cannot easily control how many times a contructor may be called.

class SomeMultitonClass
  include Multiton
  attr :arg
  def initialize(arg)
    @arg = arg
  end
end

a = SomeMultitonClass.new(4)
b = SomeMultitonClass.new(4)   # a and b are same object
c = SomeMultitonClass.new(2)   # c is a different object

How It Works¶ ↑

A pool of objects is searched for a previously cached object, if one is not found we construct one and cache it in the pool based on class and the args given to the contructor.

A limitation of this approach is that it is impossible to detect if different blocks were given to a contructor (if it takes a block). So it is the constructor arguments only which determine the uniqueness of an object. To workaround this, define the class method ::multiton_id.

def SomeMultitonClass.multiton_id(*args, &block)
  # ...
end

Which should return a hash key used to identify the object being constructed as (not) unique.