Rewire is a libary for replacing hard-wired dependencies of the module your unit testing. This keeps your production code free from any unit testing-specific concerns.

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

Alternatively, you can also rewire a module inside a 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.