Dependency Inversion Principle in Ruby

I knew that the rule existed but I couldn’t imagine it properly. I wasn’t able to understand “inversion” with codes. That’s why I actually wrote codes about DIP in order to make easier to comprehend it. The codes are quoted from “practical object-oriented design in ruby”.

sample codes

# wheel class depends on gear class
class Gear
  attr_reader :chainring, :cog
  def initialize(chainring, cog)
    @chainring = chainring
    @cog = cog
  end
  def gear_inches(diameter)
    ratio * diameter
  end
  def ratio
    chainring / cog.to_f
  end
end
class Wheel
  attr_reader :rim, :tire, :gear
  def initialize(rim, tire, chainring, cog)
    @rim = rim
    @tire = tire
    @gear = Gear.new(chainring, cog)
  end  
  def diameter
    rim + (tire * 2)
  end
  def gear_inches
    gear.gear_inches(diameter)
  end
end
p Wheel.new(26, 1.5, 52, 11).gear_inches

It is clear that the method in Gear Class is called in Wheel Class. In other words, if Gear Class changes, Wheel class will be affected. Therefore, Wheel Class depends on Gear Class.

On the other hand,

# gear class depends on wheel class
class Gear 
  attr_reader :chainring, :cog, :wheel
  def initialize(chainring, cog, wheel)
    @chainring = chainring
    @cog = cog
    @wheel = wheel
  end  
  def gear_inches
    ratio * wheel.diameter
  end
  def ratio
    chainring / cog.to_f
  end
end
class Wheel
  attr_reader :rim, :tire
  def initialize(rim, tire)
    @rim = rim
    @tire = tire
  end
  def diameter
    @rim + (@tire * 2)
  end
end
p Gear.new(52, 11, Wheel.new(26, 1.5)).gear_inches

in this case, the method of Wheel Class is called in Gear Class. The result will be opposite.

How can we realize DIP?

In the case of the codes above, it is important to decide which class should have gear_inches method and which instance should be injected. If we think that wheel class is rarely changed, we should inject wheel class into gear class. The key to comprehend DIP is to understand which class refers to which instance.

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