Command in Ruby
Command is behavioral design pattern that converts requests or simple operations into objects.
The conversion allows deferred or remote execution of commands, storing command history, etc.
Complexity:
Popularity:
Usage examples: The Command pattern is pretty common in Ruby code. Most often it’s used as an alternative for callbacks to parameterizing UI elements with actions. It’s also used for queueing tasks, tracking operations history, etc.
Identification: The Command pattern is recognizable by behavioral methods in an abstract/interface type (sender) which invokes a method in an implementation of a different abstract/interface type (receiver) which has been encapsulated by the command implementation during its creation. Command classes are usually limited to specific actions.
Conceptual Example
This example illustrates the structure of the Command design pattern. It focuses on answering these questions:
- What classes does it consist of?
- What roles do these classes play?
- In what way the elements of the pattern are related?
main.rb: Conceptual example
# The Command interface declares a method for executing a command. class Command # @abstract def execute raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'" end end # Some commands can implement simple operations on their own. class SimpleCommand < Command # @param [String] payload def initialize(payload) @payload = payload end def execute puts "SimpleCommand: See, I can do simple things like printing (#{@payload})" end end # However, some commands can delegate more complex operations to other objects, # called "receivers". class ComplexCommand < Command # Complex commands can accept one or several receiver objects along with any # context data via the constructor. def initialize(receiver, a, b) @receiver = receiver @a = a @b = b end # Commands can delegate to any methods of a receiver. def execute print 'ComplexCommand: Complex stuff should be done by a receiver object' @receiver.do_something(@a) @receiver.do_something_else(@b) end end # The Receiver classes contain some important business logic. They know how to # perform all kinds of operations, associated with carrying out a request. In # fact, any class may serve as a Receiver. class Receiver # @param [String] a def do_something(a) print "\nReceiver: Working on (#{a}.)" end # @param [String] b def do_something_else(b) print "\nReceiver: Also working on (#{b}.)" end end # The Invoker is associated with one or several commands. It sends a request to # the command. class Invoker # Initialize commands. # @param [Command] command def on_start=(command) @on_start = command end # @param [Command] command def on_finish=(command) @on_finish = command end # The Invoker does not depend on concrete command or receiver classes. The # Invoker passes a request to a receiver indirectly, by executing a command. def do_something_important puts 'Invoker: Does anybody want something done before I begin?' @on_start.execute if @on_start.is_a? Command puts 'Invoker: ...doing something really important...' puts 'Invoker: Does anybody want something done after I finish?' @on_finish.execute if @on_finish.is_a? Command end end # The client code can parameterize an invoker with any commands. invoker = Invoker.new invoker.on_start = SimpleCommand.new('Say Hi!') receiver = Receiver.new invoker.on_finish = ComplexCommand.new(receiver, 'Send email', 'Save report') invoker.do_something_important output.txt: Execution result
Invoker: Does anybody want something done before I begin? SimpleCommand: See, I can do simple things like printing (Say Hi!) Invoker: ...doing something really important... Invoker: Does anybody want something done after I finish? ComplexCommand: Complex stuff should be done by a receiver object Receiver: Working on (Send email.) Receiver: Also working on (Save report.) 
