rewire

rewire is a dependency injection library.

It keeps your application code completely free from testing concerns.

And you can bring your own mock (mox is recommended).

Installation

Just add rewire to your list of dependencies in mix.exs:

def deps do
  [
    {:rewire, "~> 0.10", only: :test}
  ]
end

Usage

Given a module such as this:

# this module has a hard-wired dependency on the `English` module
defmodule Conversation do
  @punctuation "!"
  def start(), do: English.greet() <> @punctuation
end

If you define a mox mock EnglishMock you can rewire the dependency in your unit test:

defmodule MyTest do
  use ExUnit.Case, async: true
  import Rewire                                  # (1) activate `rewire`
  import Mox

  rewire Conversation, English: EnglishMock      # (2) rewire `English` to `EnglishMock`

  test "start/0" do
    stub(EnglishMock, :greet, fn -> "g'day" end)
    assert Conversation.start() == "g'day!"      # (3) test using the mock
  end
end

This example uses mox, but rewire is mocking library-agnostic.

You can use multiple rewires and multiple overrides:

  rewire Conversation, English: EnglishMock
  rewire OnlineConversation, Email: EmailMock, Chat: ChatMock

You can also give the alias a different name using as:

  rewire Conversation, English: EnglishMock, as: SmallTalk

Note that the rewire acts like an alias here in terms of scoping.

Alternatively, you can also limit the scope to a dedicated block:

  rewire Conversation, English: EnglishMock do   # (1) only rewired inside the block
    stub(EnglishMock, :greet, fn -> "g'day" end)
    assert Conversation.start() == "g'day!"      # (2) test using the mock
  end

Plus, you can also rewire module attributes.

FAQ

Will it work with async: true?

Yes! Instead of overriding the module globally - like meck - it creates a copy for each test.

Does it work with mox?

It works great with mox since rewire focuses on the injection and doesn't care about where the mock module comes from. rewire and mox are a great pair!

Will that slow down my tests?

Maybe just a little? Conclusive data from a larger code base isn't in yet.

Will test coverage be reported correctly?

Yes!

Will it work with stateful processes?

If the stateful process is started after its module has been rewired, it will work fine. However, if the module is started before - like a Phoenix controller - it won't work since it can't be rewired anymore. rewire is best used for unit tests.

Will it work with Erlang modules?

It is not able to rewire Erlang modules - but you can replace Erlang module references in Elixir modules.

How does it deal with nested modules?

Only the dependencies of the rewired module will be replaced. Any modules defined around the rewired module will be ignored. All references of the rewired module to them will be pointing to the original. You're always able to rewire them separately yourself.

How do I stop mix format from adding parentheses around rewire?

Add this to your .formatter.exs file:

import_deps: [:rewire]

Why do I need this?

I haven't been happy with the existing tradeoffs of injecting dependencies into Elixir modules that allows me to alter their behavior in my unit tests.

For example, if you don't use mox, the best approach known to me is to pass-in dependencies via a function's parameters:

defmodule Conversation do
  def start(mod \\ English), do: mod.greet()
end

The downsides to that approach are:

1. Your application code is now littered with testing concerns.
2. Navigation in your code editor doesn't work as well.
3. Searches for usages of the module are more difficult.
4. The compiler is not able to warn you in case greet/0 doesn't exist on the English module.

If you use mox for your mocking, there's a slightly better approach:

defmodule Conversation do
  def start(), do: english().greet()
  defp english(), do: Application.get(:myapp, :english, English)
end

In this approach we use the app's config to replace a module with a mox mock during testing. This is a little better in my opinion, but still comes with most of the disadvantages described above.

Witchcraft! How does this work??

Simply put, rewire will create a copy of the module to rewire under a new name, replacing all hard-coded module references that should be changed in the process. Plus, it rewrites the test code in the rewire block to use the generated module instead.