chain_of_responsibility

Add this line to your application's Gemfile:

gem "chain_of_responsibility"

And then execute:

$ bundle

Or install it yourself as:

$ gem install chain_of_responsibility

Some time ago I've stumbled upon pretty nice article written by Sergii Makagon about a chain of responsibility pattern which I highly encourage you to read.

The main idea is that we can change nasty if/else statements to a chain of handlers that can decide whether they are applicable or not. Handlers are specified in the certain order and when we get a match we resolve using an appropriate handler.

This gem provides a tiny wrapper to make defining chains nicer. It's pretty minimal so feel free to simply copy the source code and adjust it to your needs if you wish.

A chain is composed of handlers defined in a specific order which are called one by one until there's a handler that can handle the request. If none of the handlers can be used, error will be raised.

Handlers should inherit from ChainOfResponsibility::Handler and define applicable? and resolve methods that can take an arbitrary number of arguments (but both the same).

Example:

module Handlers
  class WelcomeEmailHandler < ChainOfResponsibility::Handler
    def applicable?(user)
      user.registered? && user.confirmed?
    end

    def resolve(user)
      # send email
    end
  end
end

Once you have your specific handlers defined, you can easily compose them.

Example

module Handlers
  class Passing < ChainOfResponsibility::Handler
    def applicable?
      true
    end

    def resolve
      :ok
    end
  end
end

module Handlers
  class Failing < ChainOfResponsibility::Handler
    def applicable?
      false
    end

    def resolve
      # won't be reached
    end
  end
end

class PassingTransaction
  include ChainOfResponsibility

  def_handler Handlers::Failing
  def_handler Handlers::Passing

  def call
    chain.call
  end
end

PassingTransaction.new.call # => :ok

class FailingTransation
  include ChainOfResponsibility

  def_handler Handlers::Failing

  def call
    chain.call
  end
end

FailingTransation.new.call # => ChainOfResponsibility::NoAppropriateHandlerFoundError

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Bug reports and pull requests are welcome on GitHub at https://github.com/KacperPucek/chain_of_responsibility. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.

The gem is available as open source under the terms of the MIT License.