Behaves is a gem that helps you maintain contracts between different classes. This is especially useful for dealing for adapter patterns by making sure that all of your adapters define the required behaviors.

For example, you can specify that class Dog and class Cat should both behave the same as Animal, or that your ApiClientMock should behave the same as the original ApiClient (more explanation below)

The idea for Behaves stemmed from my research into adapter pattern in Ruby and José Valim's article on Mocks and explicit contracts.

Monthly Downloads: 181
Programming language: Ruby
License: MIT License
Latest version: v0.3.0

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Behaves is a gem that helps you define behaviors between classes. Say goodbye to runtime error when defining behaviors.

Behaves is especially useful for dealing with adapter patterns by making sure that all of your adapters define the required behaviors. See usage below for more examples.

Detailed explanations in the sections below.


Add this line to your application's Gemfile:

gem 'behaves'


This is how you define behaviors with behaves.

First, define required methods on the Behavior Object with the implements method, which take a list of methods.

class Animal
  extend Behaves

  implements :speak, :eat

Then, you can turn any object (the Behaving Object) to behave like the Behavior Object by using the behaves_like method, which takes a Behavior Object.

class Dog
  extend Behaves

  behaves_like Animal

Voilà, that's all it takes to define behaviors! Now if Dog does not implement speak and eat, your code will then throw error on file load, instead of at runtime.

- NotImplementedError: Expected `Dog` to behave like `Animal`, but `speak, eat` are not implemented.

This is in stark contrast to defining behaviors with inheritance. Let's take a look.

Inheritance-based behaviors

# Inheritance - potential runtime error.
class Animal
  def speak
    raise NotImplementedError, "Animals need to be able to speak!"

  def eat
    raise NotImplementedError, "Animals need to be able to eat!"

class Dog < Animal
  def speak

1) It is unclear that Dog has a certain set of behaviors to adhere to.

2) Notice how Dog does not implement #eat? Inheritance-based behaviors have no guarantee that Dog adheres to a certain set of behaviors, which means you can run into runtime errors like this.

corgi = Dog.new
# => NotImplementedError, "Animals need to be able to eat!"

3) Another problem is you have now defined Animal#speak and Animal#eat, two stub methods of which they do nothing but raise an undesirable NotImplementedError.

The power of Behaves does not stop here either.



Behaves allow you to define multiple behavior for a single behaving object. This is not possible with inheritance.

class Predator
  extend Behaves

  implements :hunt

class Prey
  extend Behaves

  implements :run, :hide

class Shark
  extend Behaves

  # Shark is both a `Predator` and a `Prey`
  behaves_like Predator
  behaves_like Prey

Inject Behaviors

When someone decides to use behaves to define behaviors, they in turn lose the ability to utilize some other aspect of inheritance, one of it being inheriting methods.

So, Behaves now ship with a feature called inject_behaviors for that need!

class Dad
  extend Behaves

  implements :speak, :eat

  inject_behaviors do
    def traits; "Dad's traits!"; end

class Child
  extend Behaves

  behaves_like Dad

  def speak; "BABA"; end
  def eat; "NOM NOM"; end

# Child.new.traits #=> "Dad's traits!"

This extends to more than just method implementation too, you can do anything you want! That's because the code inside inject_behaviors run in the context of the Behaving Object, also self inside injected_behaviors refers to the Behaving Object.

Do note that if you use this extensively, you might be better off using inheritance, since this will create more Method objects than inheritance.

Private Behaviors

Private behaviors can be defined like so:

class Interface
  extend Behaves

  implements :foo
  implements :bar, private: true

class Implementor
  extend Behaves

  behaves_like Interface

  def foo


  def bar


If you do not want to type extend Behaves every time, you can monkey patch Behaves onto Object class, like so:

Object.send(:extend, Behaves)


The idea for Behaves stemmed from my research into adapter pattern in Ruby and José Valim's article on Mocks and explicit contracts.

I found that the current idiom to achieve behaviors in Ruby is through inheritence, and then subsequently defining 'required' methods, which does nothing except raising a NotImplementedError. This approach is fragile, as it does not guarantee behaviors, runs the risk of runtime errors, and has an opaque implementation.

Thus with this comes the birth of Behaves.

Also referring to the article by José Valim, I really liked the idea of being able to use Mock as a noun. However, while the idea sounds good, you've now introduced a new problem in your codebase -- your Mock and your original Object might deviate from their implementation later on. Not a good design if it breaks. Elixir has @behaviors & @callback built in to keep them in sync. Behaves is inspired by that.


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/edisonywh/behaves. 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.

*Note that all licence references and agreements mentioned in the Behaves README section above are relevant to that project's source code only.