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.
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
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
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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