My first experiment with a simple IoC container in Ruby. To run this sample do this: $ ruby ioc.rb

classes.rb

# defining various components of the system
# notice the classes know nothing about the container
# as they require the dependencies to operate, they should be passed into the constructor
class Nails
  def to_s
    "nails"
  end
end

class Glue
  def to_s
    "glue"
  end
end

class PowerSaw
  def initialize(power_source)
    @power_source = power_source
  end

  def to_s
    "#{@power_source} power saw"
  end
end

class HandSaw
  def to_s
    "hand saw"
  end
end

class Electricity
  def to_s
    "electrical"
  end
end

class Carpenter
  def initialize(tool, fixings)
    @tool = tool
    @fixings = fixings
  end

  def build_something
    puts "Building something in wood with #{@fixings} and #{@tool}"
  end
end

container.rb

class Container
  attr_reader :registry

  def self.create_container
    @container = self.new
    yield @container
  end

  def self.resolve(key)
    klass = @container.registry[key]
    constructor_params = klass.instance_method(:initialize).parameters
    required_dependancies = constructor_params.select{|p| p.first == :req}
                                              .map{|p| resolve(p.last)}
    klass.new(*required_dependancies)   # add some options for lifecycle etc
  end

  def initialize
    @registry = {}
  end

  def register(key, klass)
    @registry[key] = klass
  end
end

ioc.rb

require './container'
require './classes'

# register the components of the system with the container
# as we can't use types to inject the correct dependency we rely on the convention
# of the constructor parameter names matching the keys registered
# this is where you swap implementations to change behaviour without touching your classes
Container.create_container do |c|
  c.register :craftsman, Carpenter
  c.register :tool, PowerSaw  
  c.register :fixings, Nails  
  c.register :power_source, Electricity
end

# resolve the root object, all dependencies will be instantiated and injected
craftsman = Container.resolve(:craftsman)
craftsman.build_something

# $ ruby ioc.rb
#   => Building something in wood with nails and electrical power saw