Command (object behavioral pattern)

GoF definition (p. 233):

"Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations."

The key to this pattern is that it decouples the object that invokes an operation (the 'invoker') from the object having the knowledge to perform it (the 'command'). In this respect it is similar to the strategy pattern.

Famously, this pattern can allow for undoable operations. If a command provides a means to reverse its operations (e.g. an 'unexecute' method as well as 'execute'), command objects can be stored in a history list and 'undone' and 'redone' with this methods.

Pros:

• Decouples the object that invokes the operation from the one that knows how to perform it.
• You can assemble commands into a composite command.
• You can support undoable operations.

Example

Abstract Command [defines interface]

class Command
  def execute
    raise NotImplementedError
  end

  def unexecute
    raise NotImplementedError
  end
end

Concrete Commands

class CreateFile < Command
  def initialize(path, data)
    @path = path
    @data = data
  end

  def execute
    file = File.open(@path, 'w')
    file.write(@data)
    file.close
  end

  def unexecute
    File.delete(@path)
  end
end

Composite Commands

class CompositeCommand < Command
  def initialize(commands=[])
    @commands = commands
  end

  def execute
    @commands.each { |command| command.execute }
  end

  def unexecute
    @commands.each { |command| command.unexecute }
  end

  def add_command(command)
    @commands << command
  end
end

Implementation

create_some_files = CompositeCommand.new
create_some_files.add_command(CreateFile.new('./EN.txt', 'hello'))
create_some_files.add_command(CreateFile.new('./FR.txt', 'bonjour'))
create_some_files.add_command(CreateFile.new('./ES.txt', 'hola'))

create_some_files.execute
puts "\nLOCAL DIRECTORY AFTER FILES CREATED:"
puts "===================================="
puts Dir.entries('.')

create_some_files.unexecute
puts "\nLOCAL DIRECTORY AFTER FILES DELETED:"
puts "===================================="
puts Dir.entries('.')