dizzy

Dependency Injection that won't make your head spin!

Purpose

dizzy provides a declarative mechanism for assembling complex objects. Its main advantages include:

Objects are only initialized when they are needed.
Objects are only assembled after all of their dependencies have already been initialized.

5-minute Tour

Typically, you would use dizzy when you have dozens or hundreds of objects that need to be assembled. But, for the sake of example, let's just assemble 2 objects. First, let's define a Person class:

class Person
  attr_accessor :name, :friend

  def initialize(name = nil)
    @name = name
  end
end

We'd like bob and joe to be instances of Person. Furthermore, we'd like them both to be friends. So:

require 'dizzy'

class IocContainer
  extend Dizzy::DSL

  # This will define the IocContainer#bob method
  di :bob do
    # Constructor dependency injection occurs here.
    init do
      Person.new "Bob"
    end

    # Setter dependency injection occurs here.
    wire do |instance|
      # Get the value from IocContainer#joe
      instance.friend = joe
    end
  end


  di :joe do
    init do
      Person.new "Joe"
    end

    wire do |instance|
      instance.friend = bob
    end
  end

end

container = IocContainer.new

# Prints "Joe"
puts container.bob.friend.name


# Prints "Bob"
puts container.joe.friend.name

Holy cow! That seems like a TON of work just to assemble 2 objects! Is all of that really necessary? Fortunately, the answer is usually "No". In many cases, an object's init logic is uninteresting. It just creates an instance of a class with a few hard-coded parameters. So, you can simplify things by using the di_wire method:

require 'dizzy'

class IocContainer
  extend Dizzy::DSL


  di_wire :bob, Person, "Bob" do |bob|
    bob.friend = joe
  end


  di_wire :joe, Person, "Joe" do |joe|
    joe.friend = bob
  end

end

container = IocContainer.new

# Prints "Joe"
puts container.bob.friend.name


# Prints "Bob"
puts container.joe.friend.name