composite pattern

composite_pattern.rb

# use the Composite pattern when you are trying to build a hierarchy
# three moving parts: base class/interface (component), leaf classes, composite classes

class Task
  attr_reader :name
  
  def initialize(name)
    @name = name
  end
  
  def get_time_required
    0.0
  end
end

class CompositeTask < Task
  def initialize(name)
    super(name)
    @sub_tasks = []
  end
  
  def add_sub_task(task)
    @sub_tasks << task
  end
  alias_method :<<, :add_sub_task
  
  def remove_sub_task(task)
    @sub_tasks.delete(task)
  end
  
  def get_time_required
    @sub_tasks.inject(0) do |task, time|
      time + task.get_time_required
    end
  end
end

class AddDryIngredientsTask < Task
  def initialize
    super("Add dry ingredients")
  end
  
  def get_time_required
    1.0
  end
end

class MakeBatterTask < CompositeTask
  def initialize
    super('Make batter')
    add_sub_task( AddDryIngredientsTask.new )
    add_sub_task( AddLiquidTask.new )
    add_sub_task( MixTask.new )
  end
end


# traverse the tree from leaves to the root:
# add a parent reference
class Task 
  def initialize(name)
    ***
    @parent = nil
  end
end

class CompositeTask < Task
  def add_sub_task(task)
    @sub_tasks << task
    task.parent = self
  end
  
  def remove_sub_task(task)
    @sub_tasks.delete(task)
    task.parent = nil
  end
end

# get the ultimate parent
while task
  task = task.parent
end



# the usual error of Composite Pattern is that assuming all of the child components
# are leaf objects and not other composites.

# the right way to handle this situation is to define total_num_of_tasks method in the 
# component class:

class Task
  def total_num_of_tasks
    1
  end
end

class CompositeTask < Task
  def total_num_of_tasks
    @sub_tasks.inject(0) do |task, sum|
      sum + task.total_num_of_tasks
    end
  end
end