Dependency injection in ruby

@huydx

what is depedency injection

(DI)

dependency: a part of your code "use" another part

dependency injection: how you inject a part of your code in another part

Some examples

example class Train attr_accessor :food_service, :drink_service

def initialize(food_service, drink_service) @food_service = food_service @drink_service = drink_service end

def serve(customer = Customer.new) customer.eat(food_service.make_food) customer.drink(drink_service.make_drink) customer.sleep end end

example class Train attr_accessor :food_service, :drink_service

def initialize(food_service, drink_service) @food_service = food_service @drink_service = drink_service end

def serve(customer = Customer.new) customer.eat(food_service.make_food) customer.drink(drink_service.make_drink) customer.sleep end end

Train depends on food service and drink service

Train depends on customer

You already use it everyday!

Some "ruby" ways:

- constructor (initialize) - parameter passing - module include / extend - inheritance

Disadvantages

- constructor (initialize)+ parameter passing:

api changes -> parameter changes

Disadvantages

- module include / extend

poison method call space, sometimes you don't know what is included??!!

Disadvantages

- inheritance

create once, stick forever!! Parent API/interface needed to be carefully design

Another DI approach (not ruby way):

- Use DI framework - define dependencies in configuration file - found https://github.com/dsawardekar/encase

Point: is it really needed??

Point is: why do you need to care about DI?

Some points:

- Testability - Loosely coupled - Code extensibility