POODR: Chapter 5 - Reducing Costs With Duck Typing

Chapter_5.rb

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# Overlooking Ducks

# 1 -------------------BAD

class Trip
  attr_reader :bicycles, :customers, :vehicle

  def prepare(mechanic)
    # no depedency on mecahnic
    # create a dependency that the object must respond to #prepare_bicycles
    mechanic.prepare_bicycles(bicycles)
  end
  # ...
end

class Mechanic

  def prepare_bicycles(bicycles)
    bicycles.each { |bicycle| prepare_bicycle(bicycle) }
  end

  def prepare_bicycle(bicycle)
    # ...
  end

end

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# Compounding The Problem

# 2 -------------------WORST
class Trip
  attr_reader :bicycles, :customers, :vehicle

  def prepare(preparers)
    # Method now needs to check each object for its class
    # In scaling an app, this case statement grows
    preparers.each do | preparer |
      case preparer
      when Mechanic
        preparer.prepare_bicycles(bicycle)
      when TripCoordinator
        prepare.buy_food(customers)
      when Driver
        prepare.gas_up(vehicle)
        prepare.fill_water_tank(vehicle)
      end
    end
  end
end

class TripCoordinator
  def buy_food(customers)
    # ...
  end
end

class Driver
  def gas_up(vehicle)
    # ...
  end

  def fill_water_tank(vehicle)
    # ...
  end
end

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# Finding the Duck

# 3 -------------------BEST

class Trip
  attr_reader :bicycles, :customers, :vehicle

  # prepare ultimately has a single purpose, its arguments arrive wishing to
  # collaborate to accomplish a single goal.
  def prepare(preparers)
    # passes self as a parameter
    preparers.each { |preparer| preparer.prepare_trip(self) }
  end

end

class Mechanic
  def prepare_trip(trip)
    trip.bicycles.each { |bicycle| prepare_bicycle(bicycle) }
  end
  # ...
end

class TripCoordinator
  def prepare_trip(trip)
    buy_food(trip.customers)
  end
  # ...
end

class Driver
  def prepare_trip(trip)
    vehicle = trip.vehicle
    gas_up(vehicle)
    fill_water_tank(vehicle)
  end
  # ...
end


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# Recognizing Hidden Ducks

# 4 -------------------BAD

# A. CASE STATEMENTS (SEE ABOVE)

# B. #kind_of? and #is_a?
def prepare(preparer)
  if preparer.kind_of?(Mechanic)
    preparer.prepare_bicycles(bicycle)
  elsif preparer.kind_of?(TripCoordinator)
    preparer.buy_food(customers)
  elsif preparer.kind_of?(Driver)
    preparer.gas_up(vehicle)
    preparer.fill_water_tank(vehicle)
  end
end

# C. #responds_to?
def prepare(preparer)
  if preparer.respond_to?(:prepare_bicycles)
    preparer.prepare_bicycles(bicycle)
  elsif preparer.respond_to?(:buy_food)
    preparer.buy_food(customers)
  elsif preparer.respond_to?(:gas_up)
    preparer.gas_up(vehicle)
    preparer.fill_water_tank(vehicle)
  end
end