• atomicobject.github.com/diy

DIY (Dependency Injection in YAML) is a simple dependency injection library which focuses on declarative composition of objects through constructor injection.

• gem install diy

Author a YAML file that describes your objects and how they fit together. This means you’re building a Hash whose keys are the object names, and whose values are Hashes that define the object.

The following context defines an automobile engine:

context.yml:

---
engine:
  compose: throttle, block
throttle:
  compose: cable, pedal
block:
cable:
pedal:

In your code, use DIY to load the YAML, then access its parts:

context = DIY::Context.from_file('context.yml')
context[:engine]  => <Engine:0x81eb0>

This approach assumes:

• You’ve got classes for Engine, Throttle, Block, Cable and Pedal

• They’re defined in engine.rb, throttle.rb, etc

• The library files are in your load-path

• Engine and Throttle both have a constructor that accepts a Hash. The Hash will contain keys ‘throttle’, ‘block’ (for Engine) and ‘cable, ’pedal’ (for Throttle) and the values will be references to their respective objects.

• Block, Cable and Pedal all have default constructors that accept no arguments

Sample code for Engine’s constructor:

class Engine
  def initialize(components)
    @throttle = components['throttle']
    @block = components['block']
  end
end

Writing code like that is repetetive; that’s why we created the Constructor gem, which lets you specify object components using the “constructor” class method:

Using constructor, you can write Engine like this:

class Engine
  constructor :throttle, :block
end

If your object has a lot of components (or they have big names) you can specify an array of component names as opposed to a comma-separated list:

engine:
  compose:
  - throttle
  - block

Sometimes you won’t be able to rely on DIY’s basic assumptions about class names and library files.

• You can specify the ‘class’ option

• You can specify the ‘library’ option. If you do not, the library is inferred from the class name. (Eg, My::Train::Station will be sought in “my/train/station.rb”

  engine:

  class: FourHorse::Base
  library: general_engines/base
  compose: throttle, block

If the Hash coming into your constructor needs to have some keys that do not exactly match the official object names, you can specify them one-by-one:

engine:
  the_throttle: throttle
  the_block: block

Non-singletons are named objects that provide a new instance every time you ask for them. By default, DIY considers all objects to be singletons. To override, use the “singleton” setting and set it to false:

foo:
  singleton: false

Sub-contexts are useful for creating isolated object networks that may need to be instantiated zero or many times in your application. Objects defined in subcontexts can reference “upward” to their surroundings, as well as objects in the subcontext itself.

If you wanted to be able to make more than one Engine from the preceding examples, you might try:

---
epa_regulations:

+automotive_plant:
  engine:
    compose: block, throttle, epa_regulations
  block:
  throttle:

Each time you delve into the automotive_plant, you get a solar system of the defined objects. In this context, the objects are singleton-like. The next time you invoke the subcontext, however, you’ll be working with a fresh set of objects… another solar system with the same layout, so to speak.

Subcontexts are not initialized until you call upon them, which you do using the “within” method:

context = DIY::Context.from_file('context.yml')
context.within('automotive_plant') do |plant|
  puts plant[:engine]
end

Occasionally you will have a class at your disposal that you’d like to provide directly as components to other objects (as opposed to getting instances of that class, you want to reference the class itself, eg, to use its factory methods). Enter the “use_class_directly” flag:

---
customer_order_finder:
  class: CustomerOrder
  use_class_directly: true

This can be handy in Rails when you’d like to use some of class methods on an ActiveRecord subclass, but you’d like to avoid direct ActiveRecord class usage in your code. In this case, the customer_order_finder is actually the CustomerOrder class, and so, it has methods like “find” and “destroy_all”.

If you find yourself writing context entries like this:

---
engine:
  class: Car::Parts::Engine
throttle:
  class: Car::Parts::Block
cable:
  class: Car::Parts::Cable

You can set the “assumed” module for a group of objects like this:

---
using_namespace Car Parts:
  engine:

  throttle:

  block:

Normally, DIY will “require” the library for an object just before it instantiates the object. If this is not desired (in Rails, auto-require can lead to library double-load issues), you can deactivate auto-require. There is a global default setting (handled in code) and a per-object override (handled in the context YAML):

DIY::Context.auto_require = false

---
engine:
  auto_require: false

It is possible to create factories automatically with DIY:

---
car_dealer:
  compose: car_factory

car_factory:
  builds: car

Then you can use the factory to easily build objects:

context = DIY::Context.from_file('context.yml')
context[:car_factory].create  => <Car:0x81eb0>

This introduces the concept of first class methods. An object can now be constructed with a method object bound to a particular object in the diy context.

---
trinket_builder:

method build_trinket:
    object: trinket_builder
    method: build

• David Crosby (crosby@atomicobject.com)

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