the-builder-pattern

The Builder Pattern

1. Takes construction logic and encapsulates it in a class of its own.
2. The Builder class takes charge of assembling all the components of a complex object.
3. Each builder has an interface that lets you specify the configuration of your new object step by step.
4. Sort of like a multipart .new method, where objects are created in an extended process instead of all in one shot.

class Computer
  attr_accessor :display
  attr_accessor :motherboard
  attr_reader :drives

  def initialize(display = :crt, motherboard = Motherboard.new, drives = [])
    @motherboard = motherboard
    @drives = drives
    @display = display
  end
end

class CPU
  # Common CPU stuff...
end

class BasicCPU < CPU
  # Lots of not very fast CPU-related stuff
end

class TurboCPU < CPU
  # Lots of very fast CPU-related stuff
end

class Motherboard
  attr_accessor :cpu
  attr_accessor :memory_size

  def initialize(cpu = BasicCPU.new, memory_size = 1000)
    @cpu = cpu
    @memory_size = memory_size
  end
end

class Drive
  attr_reader :type
  attr_reader :size
  attr_reader :writable

  def initialize(type, size, writable)
    @type = type
    @size = size
    @writable = writable
  end
end

class ComputerBuilder
  attr_reader :computer

  def initialize
    @computer = Computer.new
  end

  def turbo(has_turbo_cpu = true)
    @computer.motherboard.cpu = TurboCPU.new
  end

  def display=(display)
    @computer.display = display
  end

  def memory_size=(size_in_mb)
    @computer.motherboard.memory_size = size_in_mb
  end

  def add_cd(writer = false)
    @computer.drives << Drive.new(:cd, 760, writer)
  end

  def add_dvd(writer = false)
    @computer.drives << Drive.new(:dvd, 4000, writer)
  end

  def add_hard_disk(size_in_mb)
    @computer.drives << Drive.new(:hard_disk, size_in_mb, true)
  end
end

builder = ComputerBuilder.new
builder.turbo
builder.add_cd(true)
builder.add_dvd
builder.add_hard_disk(100000)

computer = builder.computer

Polymorphic Builders

class DesktopComputer < Computer
  # Lots of interesting desktop details omitted...
end

class LaptopComputer < Computer
  def initialize(motherboard = Motherboard.new, drives = [])
    super(lcd, motherboard, drives)
  end

  # Lots of interesting laptop details omitted...
end

class ComputerBuilder
  attr_reader :computer

  def turbo(has_turbo_cpu = true)
    @computer.motherboard.cpu = TurboCPU.new
  end

  def memory_size=(size_in_mb)
    @computer.motherboard.memory_size = size_in_mb
  end
end

class DesktopBuilder < ComputerBuilder
  def initialize
    @computer = DesktopComputer.new
  end

  def display=(display)
    @display = display
  end

  def memory_size=(size_in_mb)
    @computer.motherboard.memory_size = size_in_mb
  end

  def add_cd(writer = false)
    @computer.drives << Drive.new(:cd, 760, writer)
  end

  def add_dvd(writer = false)
    @computer.drives << Drive.new(:dvd, 4000, writer)
  end

  def add_hard_disk(size_in_mb)
    @computer.drives << Drive.new(:hard_disk, size_in_mb, true)
  end
end

class LaptopBuilder < ComputerBuilder
  def initialize
    @computer = LaptopComputer.new
  end

  def display=(display)
    raise "Laptop display must be lcd" unless display == :lcd
  end

  def add_cd(writer = false)
    @computer.drives << LaptopDrive.new(:cd, 760, writer)
  end

  def add_dvd(writer = false)
    @computer.drives << LaptopDrive.new(:dvd, 4000, writer)
  end

  def add_hard_disk(size_in_mb)
    @computer.drives << LaptopDrive.new(:hard_disk, size_in_mb, true)
  end
end

Sane Objects

def computer
  raise "Not enough memory" if @computer.motherboard.memory_size < 250
  raise "Too many drives" if @computer.drives.size > 4
  hard_disk = @computer.drives.find { |drive| drive.type == :hard_disk }
  raise "No hard disk" unless hard_disk
  @computer
end