Question

Does anyone have a good example in Ruby of using a Composite of Commands? It is a design pattern hybrid that I have seen mentioned in various Design Patterns literature which sounds quite powerful, but have not been able to find any interesting use cases or code.

Answer 1

Inspired by the general idea and the sample pattern implementations in this blog post, here’s a stab at what it might look like:

class CompositeCommand
  def initialize(description, command, undo)
    @description=description; @command=command; @undo=undo
    @children = []
  end
  def add_child(child); @children << child; self; end
  def execute
    @command.call() if @command && @command.is_a?(Proc)
    @children.each {|child| child.execute}
  end
  def undo
    @children.reverse.each {|child| child.undo}
    @undo.call() if @undo && @undo.is_a?(Proc)
  end
end

And sample usage using the application of a software installer program:

class CreateFiles < CompositeCommand
  def initialize(name)
    cmd = Proc.new { puts "OK: #{name} files created" }
    undo = Proc.new { puts "OK: #{name} files removed" }
    super("Creating #{name} Files", cmd, undo)
  end
end

class SoftwareInstaller
  def initialize; @commands=[]; end
  def add_command(cmd); @commands << cmd; self; end
  def install; @commands.each(&:execute); self; end
  def uninstall; @commands.reverse.each(&:undo); self end
end

installer = SoftwareInstaller.new
installer.add_command(
  CreateFiles.new('Binary').add_child(
    CreateFiles.new('Library')).add_child(
    CreateFiles.new('Executable')))
installer.add_command(
  CreateFiles.new('Settings').add_child(
    CreateFiles.new('Configuration')).add_child(
    CreateFiles.new('Preferences')).add_child(
    CreateFiles.new('Help')))
installer.install # => Runs all commands recursively
installer.uninstall