Hook Methods

Hook Methods

Hook methods are messages used during inheritance allowing sub-classes the ability to add value without creating a common anti-pattern. For instance, calling super from a subclass is a strong indicator of a tightly coupled relationship. Changing one may require a change to the other.

Before refactor

class Stroller
  ...
  def initialize
    @wheels = DEFAULT_WHEELS
    @seats = DEFAULT_SEATS
  end
  ...
end

class DoubleStroller < Stroller
  def initialize
    super
    @seats = 2
  end
end

class JoggingStroller < Stroller
  def initialize
    super
    @wheels = 3
  end
end

The double and jogging stroller’s both call super during initialize to inherit the defaults. Then each subclass adds their Stroller specific information by overriding the attributes set via super. The problem here is that every future Stroller type will need to be aware of the super-classes initialize method. A developer may forget to call super and not know to explicitly set the wheels and seats.

The goal is to make these classes more independent from each other. We do so with introducing a hook method.

class Stroller
  attr_accessor :wheels, :seats

  def initialize
    @wheels = DEFAULT_WHEELS
    @seats = DEFAULT_SEATS

    post_initialize
  end

  def post_initialize
  end
  ...
end

class DoubleStroller < Stroller
  def post_initialize
    @seats = 2
  end
end

class JoggingStroller < Stroller
  def post_initialize
    @wheels = 3
  end
end

post_initialize is defined and called by the parent. Stroller’s implementation of it is intentionally empty as it is not required. Any future type of stroller may or may not use post_initialize.

 

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