DRY Ruby Initialization with Hash Argument

Question

I find myself using hash arguments to constructors quite a bit, especially when writing DSLs for configuration or other bits of API that the end user will be exposed to. Wha

Answer 1

Given your hashes would include ActiveSupport::CoreExtensions::Hash::Slice, there is a very nice solution:

class Example

  PROPERTIES = [:name, :age]

  attr_reader *PROPERTIES  #<-- use the star expansion operator here

  def initialize(args)
    args.slice(PROPERTIES).each {|k,v|  #<-- slice comes from ActiveSupport
      instance_variable_set "@#{k}", v
    } if args.is_a? Hash
  end
end

I would abstract this to a generic module which you could include and which defines a "has_properties" method to set the properties and do the proper initialization (this is untested, take it as pseudo code):

module HasProperties
  def self.has_properties *args
    class_eval { attr_reader *args }
  end

  def self.included base
    base.extend InstanceMethods
  end

  module InstanceMethods
    def initialize(args)
      args.slice(PROPERTIES).each {|k,v|
        instance_variable_set "@#{k}", v
      } if args.is_a? Hash
    end
  end
end