Delx

Defdelegate on steroids! An Elixir library to make function delegation testable.

Prerequisites

• Erlang 20 or greater
• Elixir 1.8 or greater

Installation

If available in Hex, the package can be installed by adding delx to your list of dependencies in mix.exs:

def deps do
  [
    {:delx, "~> 2.1.0"}
  ]
end

Usage

Check out the full docs at https://hexdocs.pm/delx.

Let's say you have the following module.

defmodule Greeter.StringGreeter do
  def hello(name) do
    "Hello, #{name}!"
  end
end

You can delegate functions calls to another module by using the Delx module and calling the defdel/2 macro in the module body. It has the same API as Elixir's own Kernel.defdelegate/2 macro.

defmodule Greeter do
  use Delx, otp_app: :greeter

  defdel hello(name), to: Greeter.StringGreeter
end

Greeter.hello("Tobi")
# => "Hello, Tobi!"

Testing

One great benefit of Delx is that you can test delegation without invoking the actual implementation of the delegation target, thus eliminating all side effects.

Built-In Assertions

Delx brings it's own test assertions.

All you need to do is to activate delegation stubbing for your test environment by putting the following line in your config/test.exs:

config :greeter, Delx, stub: true

Then in your tests, you can import Delx.TestAssertions and use the assert_delegate/2 and refute_delegate/2 assertions.

defmodule GreeterTest do
  use ExUnit.Case

  import Delx.TestAssertions

  describe "hello/1" do
    test "delegate to Greeter.StringGreeter" do
      assert_delegate {Greeter, :hello, 1}, to: Greeter.StringGreeter
    end
  end
end

Note that once you activate stubbing all delegated functions do not return anymore but instead raise the Delx.StubbedDelegationError. If you really want to call the original implementation, you have to avoid any calls of delegated functions.

With Mox

If you are using Mox in your application you have another possibility to test delegated functions.

Register a mock for the Delx.Delegator behavior to your test/test_helper.exs (or wherever you define your mocks):

Mox.defmock(Delx.Delegator.Mock, for: Delx.Delegator)

Then, in your config/test.exs you have to set the mock as delegator module for your app.

config :my_app, Delx, delegator: Delx.Delegator.Mock

Please make sure not to use the :stub option and a :delegator option at the same time as this may lead to unexpected behavior.

Now you are able to expect calls to delegated functions:

defmodule GreeterTest do
  use ExUnit.Case

  import Mox

  setup :verify_on_exit!

  describe "hello/1" do
    test "delegate to Greeter.StringGreeter" do
      expect(
        Delx.Delegator.Mock,
        :apply,
        fn {Greeter, :hello},
           {Greeter.StringGreeter, :hello},
           ["Tobi"] ->
          :ok
        end
      )

      Greeter.hello("Tobi")
    end
  end
end

For more information on how to implement your own delegator, refer to the docs of the Delx.Delegator behavior.

Note that the configuration is only applied at compile time, so you are unable to stub or replace the delegator module at runtime.