Module | Multiton |
In: |
lib/more/facets/multiton.rb
|
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
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 Klass.multiton_id(*args, &block) # ... end
Which should return a hash key used to identify the object being constructed as (not) unique.