Ruby Initialization Chain Module

The Ruby Initialization Chain module comes from my current project where I was using the Initialization Chain pattern. The Initialization Chain module allows you to call setter methods that set a value and return the instance. This chaining of method calls allows you to initialize an object with one line of code.

foo = Foo.new.set_bar("baz).set_cat("dog")

If you were to define the setter methods, they would all follow the same basic pattern

def bar=(value)
  @bar = value
end

def set_bar(value)
  bar=value
end

The Initialization Chain module assumes you will follow this pattern and removes the need to define them.

Code and Tests:

module InitializationChain
  alias :pre_init_chain_method_missing :method_missing
  def method_missing(sym, *args, &block)
    if matches_set_and_writer_exists?(sym)
      self.send convert_set_to_setter(sym), *args, &block
      return self
    end
    pre_init_chain_method_missing(sym, *args, &block)
  end

  def matches_set_and_writer_exists?(sym)
    matches_set?(sym) && writer_exists?(sym)
  end

  def match_regex
    /^set_/
  end

  def convert_set_to_setter(sym)
    "#{sym.to_s.sub(match_regex,'')}=".to_sym
  end

  def writer_exists?(sym)
    self.respond_to?(convert_set_to_setter(sym))
  end

  def matches_set?(sym)
    sym.to_s =~ match_regex
  end
end

class Foo
  include InitializationChain

  attr_accessor :bar

  def cat
    @cat
  end

  def cat=(value)
    @cat=value
  end
end

require 'test/unit'

class InitializationChainTest < Test::Unit::TestCase
  def test_matches_set
    assert(Foo.new.matches_set?(:set_foo))
  end

  def test_writer_exists
    assert(Foo.new.writer_exists?(:bar))
  end

  def test_add_dynamic_set_for_attr
    foo = Foo.new.set_bar("baz")
    assert_equal "baz", foo.bar
  end

  def test_add_dynamic_set_for_defined_setter
    foo = Foo.new.set_cat("dog")
    assert_equal "dog", foo.cat
  end
end