Design Pattern: Decorator Pattern - Ruby Solution

decorator_pattern.rb

## ConcreteComponent
class SimpleWriter 

  def initialize(path)
    @file = File.open(path, 'w')
  end

  def write_line(line)
    @file.print(line)
    @file.print('\n')
  end

  def pos
    @file.pos
  end

  def rewind
    @file.rewind
  end

  def close
    @file.close
  end

end

## Decorator
class WriterDecorator

  def initialize(real_writer)
    @real_writer = real_writer
  end

  def write_line(line)
    @real_writer.write_line(line)
  end

  def pos
    @real_writer.pos
  end

  def rewind
    @real_writer.rewind
  end

  def close
    @real_writer.close
  end
end

## Component
class NumberingWriter < WriterDecorator

  def initialize(real_writer)
    super(real_writer)
    @line_number = 1
  end

  def write_line(line)
    @real_writer.write_line("#{@line_number}: #{line}")
    @line_number += 1
  end

end

## Component
class CheckSummingWriter < WriterDecorator

  attr_reader :check_sum

  def initialize(real_writer)
    super(real_writer)
    @check_sum = 0
  end

  def write_line(line)
    line.each_byte { |byte| @check_sum = (@check_sum + byte) % 256 }
    @check_sum = @check_sum.to_s
    @check_sum += "\n" [0] % 256
    @real_writer.write_line(line)
  end
end

## Component
class TimeStampingWriter < WriterDecorator

  def initialize(real_writer)
    super(real_writer)
  end

  def write_line(line)
    @real_writer.write_line("#{Time.now}: #{line}")
  end
end

writer = CheckSummingWriter.new(TimeStampingWriter.new(NumberingWriter.new(SimpleWriter.new('final.txt'))))
writer.write_line('Hello Out There')