🎉 Hooray! After 3 years of work, I've finally released the ebook on design patterns! Check it out »
Chain of Responsibility

Chain of Responsibility in Ruby

Chain of Responsibility is behavioral design pattern that allows passing request along the chain of potential handlers until one of them handles request.

The pattern allows multiple objects to handle the request without coupling sender class to the concrete classes of the receivers. The chain can be composed dynamically at runtime with any handler that follows a standard handler interface.

Learn more about Chain of Responsibility

Usage of the pattern in Ruby

Complexity:

Popularity:

Usage examples: The Chain of Responsibility pattern isn’t a frequent guest in a Ruby program since it’s only relevant when code operates with chains of objects.

Identification: The pattern is recognizable by behavioral methods of one group of objects indirectly call the same methods in other objects, while all the objects follow the common interface.

Conceptual Example

This example illustrates the structure of the Chain of Responsibility 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 Handler interface declares a method for building the chain of handlers. It
# also declares a method for executing a request.
class Handler
  # @abstract
  #
  # @param [Handler] handler
  def next_handler=(handler)
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end

  # @abstract
  #
  # @param [String] request
  #
  # @return [String, nil]
  def handle(request)
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end
end

# The default chaining behavior can be implemented inside a base handler class.
class AbstractHandler < Handler
  # @return [Handler]
  attr_writer :next_handler

  # @param [Handler] handler
  #
  # @return [Handler]
  def next_handler(handler)
    @next_handler = handler
    # Returning a handler from here will let us link handlers in a convenient
    # way like this:
    # monkey.next_handler(squirrel).next_handler(dog)
    handler
  end

  # @abstract
  #
  # @param [String] request
  #
  # @return [String, nil]
  def handle(request)
    return @next_handler.handle(request) if @next_handler

    nil
  end
end

# All Concrete Handlers either handle a request or pass it to the next handler
# in the chain.
class MonkeyHandler < AbstractHandler
  # @param [String] request
  #
  # @return [String, nil]
  def handle(request)
    if request == 'Banana'
      "Monkey: I'll eat the #{request}"
    else
      super(request)
    end
  end
end

class SquirrelHandler < AbstractHandler
  # @param [String] request
  #
  # @return [String, nil]
  def handle(request)
    if request == 'Nut'
      "Squirrel: I'll eat the #{request}"
    else
      super(request)
    end
  end
end

class DogHandler < AbstractHandler
  # @param [String] request
  #
  # @return [String, nil]
  def handle(request)
    if request == 'MeatBall'
      "Dog: I'll eat the #{request}"
    else
      super(request)
    end
  end
end

# The client code is usually suited to work with a single handler. In most
# cases, it is not even aware that the handler is part of a chain.
def client_code(handler)
  ['Nut', 'Banana', 'Cup of coffee'].each do |food|
    puts "\nClient: Who wants a #{food}?"
    result = handler.handle(food)
    if result
      print "  #{result}"
    else
      print "  #{food} was left untouched."
    end
  end
end

monkey = MonkeyHandler.new
squirrel = SquirrelHandler.new
dog = DogHandler.new

monkey.next_handler(squirrel).next_handler(dog)

# The client should be able to send a request to any handler, not just the first
# one in the chain.
puts 'Chain: Monkey > Squirrel > Dog'
client_code(monkey)
puts "\n\n"

puts 'Subchain: Squirrel > Dog'
client_code(squirrel)

output.txt: Execution result

Chain: Monkey > Squirrel > Dog

Client: Who wants a Nut?
  Squirrel: I'll eat the Nut
Client: Who wants a Banana?
  Monkey: I'll eat the Banana
Client: Who wants a Cup of coffee?
  Cup of coffee was left untouched.

Subchain: Squirrel > Dog

Client: Who wants a Nut?
  Squirrel: I'll eat the Nut
Client: Who wants a Banana?
  Banana was left untouched.
Client: Who wants a Cup of coffee?
  Cup of coffee was left untouched.

Chain of Responsibility in Other Languages

Design Patterns: Chain of Responsibility in Java Design Patterns: Chain of Responsibility in C# Design Patterns: Chain of Responsibility in PHP Design Patterns: Chain of Responsibility in Python Design Patterns: Chain of Responsibility in Swift Design Patterns: Chain of Responsibility in TypeScript