shvets / design_patterns_in_ruby

# lazy-initialization.rb
 
# Lazy initialization is the tactic of delaying the creation of an object, 
# the calculation of a value, or some other expensive process until 
# the first time it is needed.
 
class Item
  def operation
    puts "executing operation..."
  end
end
 
class ItemProxy < Item
  def self.creational_block(&creation_block)
    @@creation_block = creation_block
  end
 
  def operation
    @subject.operation
  end
 
  def self.subject
    if (@subject == nil)
      puts "Creating new instance"
    else
      puts "Using existing instance"
    end
 
    @subject || (@subject = @@creation_block.call)
  end
end
 
 
# test
 
ItemProxy.creational_block { Item.new }
 
item1 = ItemProxy.subject
 
item2 = ItemProxy.subject
 
item3 = ItemProxy.subject
 
item1.operation
item2.operation
item3.operation