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A lightweight dependency injection framework to help wire up objects and modularise larger ruby codebases
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README.md

Build Status Gem Version

Wirer

Lightweight Ruby-style dependency injection to help wire up objects and modularise larger ruby codebases.

Wirer allows you to combine objects in a container and then to wire up those objects by stating dependencies on other objects. Objects can be identified by class and by 'features'.

Wirer doesn't force you to make your code wirer aware. You can use it as much or as little as you want.

This readme along with yard docs are also hosted at http://public.playlouder.com/doc/wirer.

A Basic Example

require 'wirer'

# You add objects to, and get objects out of, an instance of Wirer::Container
ctr = Wirer::Container.new

# first, let's store a bit of configuration in the container
ctr.add(:db_cnx_string) { "sqlite:tmp.db" }

# now something that wants that bit of configuration
class Database
  # some sort of database abstraction library goes here...
  def initialize(cnx_string) ; end
end

# Add Database to container, stating our class and a dependency on the
# :db_cnx_string feature
ctr.add :db, Database, :cnx_string => :db_cnx_string do |deps|
  Database.new(deps[:cnx_string])
end

# lets define an object that has a dependency on the database
# using the `Wirer::Service` base class
class Users < Wirer::Service

  # declare a dependency on an object of class `Database`.  This also defines an
  # instance method, `database`, that we can use to access the object
  dependency :database, Database

  def all
    database[:users].all
  end
end

# Adding a `Wirer::Service` object to the container is a lot simpler, no need to
# declare dependencies again here.
ctr.add :users, Users

# Query the container for a `Users` object...
users = ctr[Users]

# this will fail, because the `Database` class is incomplete, but hopefully you
# get the picture at this point.
puts users.all

See README.rb for a boatload of usage examples for declaring dependencies and adding objects to the container.

You're doing it wrong, you don't need dependency injection with ruby!

We've found this to be false. YMMV. We originally went down the god object path using merb and sequel model, but as the software grew it became a nightmare to test and maintain. Screeds upon screeds of mixins and flakey DSL magic. Lots of singletons, and liable to break in mysterious ways when refactoring. We also started building in more variation in to how the application could be deployed, so we needed a way of plugging together different components, i.e. customer A authorises this way, customer B that way. So we began breaking up our codebase in to smaller pieces, and as soon as we started doing that we needed some way to do either service location or dependency injection. Enter wirer.

There are plenty of other {http://dotnetslackers.com/articles/designpatterns/InversionOfControlAndDependencyInjectionWithCastleWindsorContainerPart1.aspx good discussions} on the web about the benefits of using dependency injection / inversion of control, but I guess the main ones for us at MSP were:

  • Easier to write unit tests that are actually unit tests and not integration tests.
  • Encourages a nice modular and OO design.
  • A standardised way of configuring system components, such as having pluggable behaviour, optional functionality etc.

Dependencies, Classes and Features

Wirer exists to wire together different objects in your system using the principles of Inversion of Control. In the simplest case (and this will probably be what you use 95% of the time) you will wire things up by Class, i.e. Class Foo has a dependency on class Bar. Class Bar has a dependency on module Baz.

You can also wire things up using features - you can advertise what feautures your object provides, and on the other end depend on a set of features. You come up with some conventions for how the combinations of features should be interpreted, and then use these to differentiate between the different objects in the container. You can do some useful stuff with it (I promise), such as:

  • Distinguishing between objects of the same Class, i.e. {:class => Logger, :features => [:http_logger]} alongside {:class => Logger, :features => [:user_activity_logger]}.
  • Using the special case default feature, i.e. {:class => Logger, :default => true}. If such a default is specified, this will be picked in the case of a tie when resolving dependencies.
  • Combining a class with some kind of function, i.e. {:features => [[:persists, User]]} (yes, that is an array within an array). The persistence gem uses this convention to find other repositories.
  • Describe the object without caring about type, i.e. {:features => [[:config, :max_user_count]]} # an integer with {:features => [[:config, :user_api_url]]} # a URL object

On the other, you can declare a dependency using a combination of classes and features:

  • By just a class, i.e. dependency :logger, Logger. In the case that multiple Loggers are available, the default, if specified, will be given.
  • With a strict dependency on a particular feature, i.e. dependency :database, Sequel::Database, :reporting_database, where you want an instance of a Sequel::Database that provides the reporting_database feature.
  • By class, preferring a certain feature if available, i.e. dependency :logger, Logger, :prefer => :fast_logger. Wirer will give you a fast_logger if available, but will fallback to any other that is available.
  • You can also optionally depend on an object, i.e. dependency :custom_logger, CustomLogger, :optional => true
  • And allow wirer to return you multiple matches, i.e. dependency :validators, Validator, :multiple => true

Adding objects to the container

When you add an object to a container, what you are really doing is defining a factory that wirer can use to get at an object when it is needed.

Objects can be added to the container using the add_* methods (most of these examples are using the verbose method {Wirer::Container#add_new_factory}:

  • the class of the object. This class used to construct the object directly, i.e with the_class.new
    • ctr.add_new_factory :class => SomeClass
  • an optional constructor block, used by the factory for constructing the object in lieu of a class.
    • ctr.add_new_factory {|deps| SomeClass.new }
  • a convenient method name to access it from the container directly (i.e ctr.some_service). This name also becomes a feature of the object.
    • ctr.add :some_service, SomeClass
  • a set of features to associate with an object
    • ctr.add_new_factory :features => [:foo_feature, :bar_feature]
    • ctr.add_new_factory :features => [:foo, [:feature_group, :bar]]
  • a set of dependencies that this object needs
    • ctr.add_new_factory(:class => SomeClass, :dependencies => {:foo => Foo}) # depend on an object of a particular class
    • ctr.add_new_factory(:dependencies => {:foo => foo_feature}) # depend on an object with a particular feature
    • ctr.add_new_factory(:dependencies => {:foo => [FooClass, :foo, :features]}) # fruit salad

Using the {Wirer::Factory::ClassDSL}

Stating dependencies when adding objects to the container can be a bit clunky, so you can use the {Wirer::Factory::ClassDSL class DSL} to state the depedencies on the dependant. The {Wirer::Factory::ClassDSL} can be used by including it directly, extending {Wirer::Service}, or by calling {Class.wireable} when defining your class.

class FooClass
  wireable # mixes in ClassDSL

  dependency :bar, BarClass
end
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